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IntroductionCapacitors are components that store electricity and electrical energy (potential energy) and play an important role in circuits such as tuning, bypassing coupling, and filtering. Capacitors are connected in parallel to increase capacity, and capacitors are connected in series to decrease capacity. When the capacitor is connected in series in the circuit, it can prevent the sudden change of voltage and absorb the overvoltage in the peak state. The series resistance plays a damping role, and the resistance consumes the energy of the overvoltage, thereby suppressing the oscillation of the circuit. When the capacitor is connected in parallel, the parallel resistor can absorb the electric energy of the capacitor, prevent the discharge current of the capacitor from being too large, and avoid damaging the devices (such as thyristors) connected in parallel with it. This is a very comprehensive article including the calculation formulas, circuits, and related common problems of series capacitors and parallel capacitors.Capacitors in Series & Parallel - Electronics BasicsCatalogIntroductionCatalogI What are the Capacitors in Series and Parallel?II Calculation Methods of Capacitance of a Series/Parallel Network  2.1 The Series and Parallel Combination  2.2 Voltage Division  2.3 How to Divide the Voltage When Capacitors are Connected in Series?  2.4 What is the Voltage Division Formula When Connecting 2 Capacitors in Series?III The Equivalent Method of Series or Parallel Connection of Capacitors with Different Rated Voltages and CapacitiesIV Comparison Table of Capacitors in Series and Parallel  4.1 Calculation Comparison of Capacitors in Series and Parallel  4.2 Correspondence Between Magnetic Circuit and Electric Circuit  4.3 Basic Physical Quantities of Magnetic Field and Magnetic CircuitV Frequently Asked Questions about Capacitors in Series and ParallelVI Electrolytic Capacitors in Series  6.1 Function and Purpose of 2 Electrolytic Capacitors in Anti-phase Series  6.2 Is the Electrolytic Capacitor in Series a Non-polarised Capacitor?VII QuizVIII FAQI What are the Capacitors in Series and Parallel?1.1 Parallel Connection of CapacitorsWe can describe the capacitors in parallel as a "water tank", but the water tank stores water, and the capacitor stores electric charges. If multiple capacitors are connected in parallel, they can naturally store more charge.(1) The equivalent capacitance after parallel connection is equal to the sum of the capacitance of each capacitance;(2) The voltage at both ends of each capacitor after parallel connection is equal;The withstand voltage after parallel connection is equal to the smallest capacitor voltage, and the equivalent capacitance is C1+C2, as shown in the figure below.Figure1. Parallel Connection of Capacitors1.2 Series Connection of Capacitors(1) The equivalent capacitance capacity after series connection is equal to the sum of the reciprocal of each capacitance;(2) The capacitance of each capacitor after series connection is equal;(3) The withstand voltage after series connection is equal to the sum of each capacitor voltage.After the capacitor is connected in series, it is equivalent to increase the distance between the two poles. The more the number in series, the smaller the capacitance, but the higher the withstand voltage. In actual circuit design, we generally rarely use capacitors in series, but capacitors in parallel are often used. Sometimes the capacity of a single capacitor is not enough, and one more is added.Figure2. Series Connection of CapacitorsII Calculation Methods of Capacitance of a Series/Parallel Network2.1 The Series and Parallel Combination(1) How to calculate the series capacitance of a capacitor?Suppose there are n capacitors connected in series. The series combination of these n capacitors is connected across a voltage source of V volts. Let us consider that the voltages across capacitors 1, 2, 3...n are V 1, V 2, V 3... Vn, respectively. The capacitances of capacitors 1, 2, 3 ... n are C 1, V 2, V 3 ... C n farad. Since all capacitors are connected in series, each of them will get the same charge, ie it is Q Coulomb. Now we know that the charge at both ends of the capacitor is only the product of the potential difference between the two ends of the capacitor and its capacitance value.Since the series combination of these capacitors is connected across the source of the voltage V volts, replacing the series combination n of multiple capacitors.If we consider a single equivalent capacitor of C,Now we get from equations 1 and 2,Therefore, when multiple capacitors are connected in series, the reciprocal of the equivalent capacitance of the system is given by the arithmetic sum of the reciprocal of their respective capacitances. (2) How to calculate the capacitance in parallel circuits?Suppose there are n capacitors connected in parallel. The parallel combination of these n capacitors is connected across the V volt voltage source. Since the capacitors are connected in parallel to the same voltage source, the charge of each capacitor is different and depends on their respective capacitance values. Let us consider that the charges of capacitors 1, 2, 3...n are Q 1, Q 2, Q 3,..., Q n coulombs, respectively. The capacitances of capacitors 1, 2, 3,..., n are C_1, C_2, C_3,... C_n coulombs respectively. It is now known that a charging capacitor is just the product of the voltage across the capacitor and its capacitance value. therefore,Now instead of connecting multiple capacitors in parallel, if we connect a single equivalent capacitor with capacitance C across the voltage source, then the total charge at both ends of the equivalent capacitor,Since all capacitors are connected in parallelWe can get from equations 1 and 2,Therefore, when multiple capacitors are connected in parallel, the capacitance of the system is given by the arithmetic sum of their respective capacitances.Figure3. (a) Three capacitors are connected in parallel. Each capacitor is connected directly to the battery.              (b) The charge on the equivalent capacitor is the sum of the charges on the individual capacitors. (3) Other related calculation formulasWhen the capacitor is connected in parallel, the area of the electrode is increased, and the capacitance is increased. The total capacity when connected in parallel is the sum of each capacity. When the capacitors are connected in series, the resistance value of the capacitor should be smaller than the insulation resistance of the capacitor in parallel to make the voltage distribution on each capacitor even, so as not to damage the capacitor due to uneven voltage distribution. The series and parallel calculations of capacitors are just the opposite of the series and parallel calculations of resistors.Voltage is the voltage during charging. The relationship between capacity and current, voltage is similar to power and is related to load.When voltage and capacity are quantitative, the smaller the load resistance, the larger the current and the shorter the time.When the voltage and load are quantitative, the larger the capacity, the longer the current and the longer the time.But in the actual discharge circuit, the general load is unchanged, the voltage of the capacitor is gradually reduced, and the current is gradually reduced. (1) Electric capacity (uf) = current (mA)/15Current limiting resistance (Ω)=310/maximum allowable surge currentDischarge resistance (KΩ)=500/capacitance (uf) (2) Calculation method C=15×IC is the capacitance of the capacitor, the unit is microfarad; the i device is the working current, the unit is ampere.For example, if the resistance of a bulb is 0.6 amps, the capacitance should be 15×0.6=9 microfarads, and a 9 microfarad capacitor in series is sufficient. (3) Empirical formula, 1uF output 50mA (if it is linear, a 10000F super capacitor can reach a surge current of 500 megaamps) (4) The calculation of the half-wave rectification method should provide about 30mA current per uF capacitance, which is a reference on the 50Hz220V line in China.The current is doubled in full-wave rectification, that is, 60mA current can be provided per uF.Formula: R*C≥(3～5)*T/2, you need to know the frequency of the lowest signal in the ripple component (that is, the maximum T), and then determine the value of C. ● Capacitor capacityCapacitor capacity indicates the size of electric energy that can be stored. The obstructive effect of capacitors on AC signals is called capacitive reactance. The capacitive reactance is related to the frequency and capacitance of the AC signal. The capacitive reactance XC=1/2πf c (f represents the frequency of the AC signal, and C represents the capacitance of the capacitor). ● The capacity unit and withstand voltage of the capacitor.The basic unit of capacitance is F (farad), and other units include: millifarad (mF), microfarad (uF), nanofarad (nF), picofarad (pF). Since the capacity of the unit F is too large, we generally see units of μF, nF, and pF. Conversion relationship: 1F=1000000μF, 1μF=1000nF=1000000pF. Each capacitor has its withstand voltage value, denoted by V. Generally, the nominal withstand voltage of the electrodeless capacitor is relatively high: 63V, 100V, 160V, 250V, 400V, 600V, 1000V, etc. The withstand voltage of polar capacitors is relatively low. Generally, the nominal withstand voltage values ​​are: 4V, 6.3V, 10V, 16V, 25V, 35V, 50V, 63V, 80V, 100V, 220V, 400V, etc. Power capacitor calculation: such as a three-phase capacitor bank with a nominal voltage of 690v and a capacity of 15kvar. Used in 600v circuit, delta connection, the actual effective capacity is: s=15kvar*600*600/(690*690)=11.34kvar. That is: the capacity and voltage are proportional to the square.2.2 Voltage DivisionDue to the large capacity of large capacitors, the volume is generally large, and they are usually made by multi-layer winding, which leads to a relatively large distributed inductance of large capacitors (also called equivalent series inductance, or ESL for short). The impedance of the inductor to the high frequency signal is very large, so the high frequency performance of the large capacitor is not good. Some small-capacity capacitors are just the opposite. Because of their small capacity, the volume can be made small (shortening the lead wire reduces the ESL, because a piece of wire can also be regarded as an inductance), and flat capacitors are often used Structure, such a small capacity capacitor has a small ESL so that it has a good high frequency performance, but due to the small capacity, the impedance to low frequency signals is large. So, if we want to pass the low frequency and high frequency signals well, we use a large capacitor and then a small capacitor.The commonly used small capacitor is 0.1uF CBB capacitor is better (ceramic capacitor is also OK), when the frequency is higher, you can also connect smaller capacitors in parallel, such as a few pF, hundreds of pF. In digital circuits, a 0.1uF capacitor is generally connected to the ground in parallel to the power pin of each chip (this capacitor is called a decoupling capacitor, of course, it can also be understood as a power filter capacitor, the closer the chip is, the better), because The signal in these places is mainly high-frequency signal, and it is enough to use a smaller capacitor to filter. The impedance of an ideal capacitor decreases as the frequency increases (R = 1/jwc), but an ideal capacitor does not exist. Due to the distributed inductance effect of the capacitor pins, the capacitor is no longer a simple capacitor in the high frequency range. , It should be regarded as a series high-frequency equivalent circuit of capacitance and inductance. When the frequency is higher than its resonance frequency, the impedance shows the characteristic of increasing with the increase of frequency, which is the inductance characteristic. At this time, the capacitance is like An inductance. On the contrary, inductors have the same characteristics. Large capacitors in parallel with small capacitors are widely used in power supply filtering. The fundamental reason is the self-resonance characteristics of the capacitor. The combination of large and small capacitors can well suppress low-frequency to high-frequency power interference signals. Small capacitors filter high frequencies (high self-resonant frequency), and large capacitors filter low frequencies (low self-resonant frequency). The two complement each other. ● Series voltage divider ratio: V1 = C2/(C1 + C2)*V...the larger the capacitance, the smaller the voltage divided, which is the same under AC and DC conditions● Parallel shunt ratio: I1 = C1/(C1 + C2)*I...The larger the capacitance, the larger the current that passes. Of course, this is under AC conditions.Explanation: When two or more capacitors are connected in series, it is equivalent to lengthening the insulation distance, because only the two polar plates on the two sides work, and because the capacitance is inversely proportional to the distance, the distance increases and the capacitance decreases; two or two When the above capacitors are connected in parallel, the area equivalent to the plate increases, and because the capacitance is proportional to the area, the area increases and the capacitance increases. ● Capacitors in series: After the capacitors are connected in series, the capacity decreases and the withstand voltage increases. Formula: 1\C1+1\C2=1\C If two 50uf are connected in series, it becomes 25uf.● Withstand voltage = add the withstand voltage values ​​of two capacitors. If two 100V withstand voltages are connected in series, it becomes 200V.● The formula for calculating the capacity of the series circuit of the capacitor C: 1/C=1/C+1/C2+1/C3+.+1/CnC is the total capacitance value of the capacitor series circuit, C1, C2, C3, Cn are the capacitance values of each capacitor in the capacitor parallel circuit, that is, the reciprocal of the total capacitance of the series circuit is equal to the sum of the reciprocal of the capacitance of each capacitor in the series circuit.Figure4. Capacitors in Series and Parallel2.3 How to Divide the Voltage When Capacitors are Connected in Series?For example 4V voltage source, two capacitors of 0.5F and 1F in series. If it is a DC voltage source, according to the characteristics of capacitor series voltage division introduced in middle school physics:(1) The total voltage across the capacitor series circuit is equal to the sum of the divided voltages across the capacitors. That is, U= U1+ U2+ U3+…+Un.(2) When capacitors are connected in series, the voltage distributed on each capacitor is inversely proportional to its capacitance. That is, Un = Q / Cn (because in the capacitor series circuit, the amount of charge carried on each capacitor is equal, so the larger the capacitor, the lower the voltage, and the smaller the capacitor, the higher the voltage. .)Then the voltage source of 4V, the voltage on the two capacitors of 0.5F and 1F are 8/3V and 4/3V respectively 2. If it is an AC voltage source, from the impedance of the capacitor Xc=1/jωC, we can see |Xc| and C In inverse proportion, the same result can be obtained by using |Xc| as a resistor to calculate the voltage divider.2.4 What is the Voltage Division Formula When Connecting 2 Capacitors in Series?This is a theoretical calculation problem. It is necessary to assume that the withstand voltage value of the capacitor has no margin, that is, a capacitor of 200pF is breakdown when it exceeds 500V; a capacitor of 300pF is breakdown when it exceeds 900V.After adding 1000V voltage, the 200pF capacitor will withstand 600V voltage. Regardless of the capacitor's withstand voltage margin, the 200pF capacitor will break down; at this time, 1000V will all be added to the 300pF capacitor, which exceeds its withstand voltage, so it will breakdown. Calculation formula:If there are M capacitors connected in series, the actual voltage value Un of any capacitor Cn is:Un=U*C/Cn Among them: U is the total voltage; C is the total capacity of M capacitors in series.For two capacitors in series, the formula evolves into:Assuming that the total voltage is U, the voltages on C1 and C2 are U1 and U2 respectively, thenU1=C2*U/(C1+C2)U2=C1*U/(C1+C2)III The Equivalent Method of Series or Parallel Connection of Capacitors with Different Rated Voltages and CapacitiesThe equivalent method of using capacitors with the same rated voltage in series or in parallel is relatively simple and commonly used.Several capacitors with different rated voltages and different capacities are connected in series or in parallel, and the equivalent methods are different. Now give examples to illustrate. There are three capacitors C1: 220µF /10V C2: 100µF/25V C3: 10µF/100VCalculate their parallel and series equivalent values ​​respectively. (1) Parallel equivalent method1) Equivalent capacitanceC and = C1 + C2 + C3= 220µF + 100µF + 10µF/= 330µF2) Equivalent withstand voltageU parallel = U1 = 10V (take the minimum withstand voltage value U1) (2) Series equivalent method1) Equivalent capacitance1/C string ==1/C1 + 1/C2 + 1/C3= 1/220 + 1/100 + 1/10= 252/2200C string == 2200/252≈ 8 (µF)2) Equivalent withstand voltage ● Compare the Q value of each capacitorQ1= C1 X U1 Q2=C2 X U2 Q3=C3 X U3= 220 X 10 =100 X 25 =10 X 100=2200 (C) =2500 (C) =1000 (C)Q = Q3 =1000 (C) (take the minimum power value Q3) ● Find the actual allowable withstand voltage value of each capacitorU1 (actual) = Q/C1 U2 (actual) = Q/C2 U3 (actual) = Q/C3= 1000/220 = 1000/100 = 1000/10≈4.5(V) = 10 (V) =100 (V)3) U string = U1 (actual) + U2 (actual) + U3 (actual)≈4.5 + 10 + 100≈114.5(V)Figure5. Equivalent CapacitanceIV Comparison Table of Capacitors in Series and Parallel4.1 Calculation Comparison of Capacitors in Series and Parallel4.2 Correspondence Between Magnetic Circuit and Electric Circuit4.3 Basic Physical Quantities of Magnetic Field and Magnetic CircuitV Frequently Asked Questions about Capacitors in Series and Parallel(1) Do capacitors charge faster in parallel or series?If two capacitors with the same capacity are connected in parallel or in series in the same circuit, the capacitor in series will charge faster, because the capacity of the capacitor is reduced by half after the capacitor is connected in series, and the charging time becomes shorter. The capacity of the capacitor after parallel connection is doubled, and the charging time will be longer for the same charging circuit. (2) The electric charge of each capacitor in the series circuit is equal. Why is the electric charge of each capacitor equal to the electric charge of the equivalent capacitor?Capacitor voltage: U=Q/CQ=I*tSo U=(I*t)/CWhen the capacitors connected in series are connected to the power supply, the capacitors start to charge. The current flowing through each capacitor is the same. As time goes by, the voltage of each capacitor increases. However, due to the different voltage rise rates of C, the sum of the voltage of each capacitor is equal to the power supply. When the voltage is applied, charging stops and the current is zero. Analyze this process: the current flowing through each capacitor during the entire charging process is the same, and the elapsed time is the same, so the current of each capacitor is the same over time, so the amount of charge is the same and equal to the capacity of the capacitor.Figure6. A Charging State of Three Capacitors in Parallel(3) Are the filter capacitors in the power amplifier power supply connected in parallel?The filter capacitor of the power amplifier power supply is set to eliminate some of the AC components contained in the rectification from AC to DC (the purpose is to improve the audio quality), so all capacitors with larger capacity are selected, generally using electrolysis above tens of microfarads Capacitor. The parallel connection of capacitors is the addition of the capacity of each capacitor, usually forming a standard type 1 filter circuit: "capacitor-resistor (or inductance)-capacitor". If the capacitors are connected in series, the capacity will decrease, it will only increase the cost and occupy more space, meaningless. The power supply line filter capacitor of the amplifier circuit of the power amplifier is generally grouped in parallel. Depending on the design of the power supply, the single power supply circuit may also be directly connected in parallel, or divided into two groups. The two groups are separated by power inductors or resistors into two filter circuits to form a pie-type filter circuit; if it is a dual power supply circuit, , It is generally divided into two groups as for the two groups of power lines. The easiest way to increase the filter capacitor of the power amplifier is to see the positive and negative poles and the rated withstand voltage. Connecting them in parallel can improve the stability of the DC voltage and improve the low-frequency characteristics of the amplifier, making the low frequency of the speaker sound more full and round. Capacitors are generally used in parallel, and capacitors of different capacities filter noise at different frequencies. Large-capacity capacitors can only be realized by electrolytic capacitors. Electrolytic capacitors have positive and negative polarity and are very loud when connected reversely.VI Electrolytic Capacitors in Series6.1 Function and Purpose of 2 Electrolytic Capacitors in Anti-phase SeriesIn some circuit designs, it is seen that two electrolytic capacitors are connected in series in the reverse phase. The capacity of the two components should be equal and the withstand voltage is the same. In AC circuits, the leakage current can be reduced. Just use a non-polar capacitor to get a large-capacity non-polar capacitor. . Large-capacity non-polar capacitors are more expensive. The electrolytic capacitor has a large capacity and is cheap, but it has a polarity, and the two are connected in reverse series. It is non-polar. It can only be used in very low voltage applications (up to 1-2V). The voltage is slightly higher. When the capacitor is used in the opposite direction, the leakage will be large. The accumulated effect will cause the electrolytic capacitor to heat up and eventually cause the capacitor to explode. Electrolytic capacitors are used in DC circuits. So its series connection should be the negative pole of the first one and the positive pole of the second (just like dry batteries in series). But in the circuit, there is indeed a case where the negative poles of two electrolytic capacitors are connected to the negative pole (inverted series), and the two positive poles are used. This is because it is used in an AC circuit (in a circuit where DC and AC coexist), There is no guarantee that the potential of one pole is always higher than the other pole), so that when the capacitor is under reverse voltage, serious leakage current will be generated. At this time, non-polar capacitors should be used, but non-polar capacitors are expensive and expensive. The volume is large, so some people use two electrolytic capacitors to "reverse series".  Its working state is that when there is alternating current, one of them is in the reverse state. Due to its serious leakage, the voltage drop across it is very small. Almost all of the voltage falls on the positive capacitor, and when the other half cycle of the alternating current, the state of the two capacitors will be exchanged, so these two capacitors are used as one, and the total capacitance is equal to any one of them. The total withstands voltage value is equal to 2 times of any capacitor.6.2 Is the Electrolytic Capacitor in Series a Non-polarised Capacitor?Of course, two electrolytic capacitors in parallel will not work. If two electrolytic capacitors are connected in series, it will still not work without applying a proper bias voltage. Applying a bias voltage is quite complicated, especially when both ends of the capacitor (two in series) are not grounded (the bias voltage must be floating). Considering the complexity of applying the bias voltage, it is better not to use this method: connect the negative poles of the two capacitors, and connect the two capacitors in parallel with a high-current diode. The positive of the diode is connected to the negative of the capacitor, and the negative is connected to the positive of the capacitor. Parallel connection of course still has polarity. If reverse parallel connection, it is non-polar, but it is non-polar. Reverse series connection is also not advisable. If you do a test, you will find that there must be a capacitor that withstands the backpressure. If the voltage is large, it will blow up. Unless special measures are taken, the voltage is always applied to the capacitor with the positive voltage. on. Two electrolytic capacitors of the same capacity can be connected in series, but a diode must be connected in anti-parallel to prevent the reverse breakdown of the electrolytic capacitor. After adding a diode, it is okay if it is used for filtering, but it is definitely not good for blocking DC. Because the electrolytic capacitor is only charged and not discharged. Two identical electrolytic capacitors connected in reverse series can replace non-polar capacitors with the same capacity. The dielectric loss of the electrolytic capacitor is very large, and it must be connected to the AC circuit after the voltage is greatly reduced. Otherwise, it is either burned or fried.VII Quiz● QuestionA network of five capacitors of C is connected to a 100 V supply, as shown below figure. Determine(a) the equivalent capacitance of the network(b) the charge on each capacitor. ● SolutionIn the given network, the top three Capacitance is in series, So equivalent capacitance of the top part1C1=1C+1C+1CC1=C3Similarly, the lower two Capacitance is in series, So equivalent capacitance of lower part1C2=1C+1CC2=C2 Now both C1 and C2 are in parallel, so equivalent Capacitance of the NetworkCeq=C1+C2=C3+C2=5C3Now Charge on top part will beQ1=C1V=CV3Now Charge on lower part will beQ2=C2V=CV2  VIII FAQ1. How do you solve capacitors in series and parallel?To calculate the total overall capacitance of a number of capacitors connected in this way you add up the individual capacitances using the following formula: CTotal = C1 + C2 + C3 and so on Example: To calculate the total capacitance for these three capacitors in parallel. 2. How do you know if a capacitor is in series or parallel?In your circuit current, all of the current going to one capacitor must also go to the other. Therefore they are in series. Hope this helps. If two (two-terminal) circuit elements are series-connected, they have identical (not just equal) currents through. 3. What is a capacitor in parallel?Capacitors are connected together in parallel when both of their terminals are connected to each terminal of another capacitor. The voltage ( Vc ) connected across all the capacitors that are connected in parallel is the same. 4. Can you put two capacitors in series?If two or more capacitors are connected in series, the overall effect is that of a single (equivalent) capacitor having the sum total of the plate spacings of the individual capacitors. With resistors, series connections result in additive values while parallel connections result in diminished values. 5. How are capacitors connected in series?Here are the rules for calculating capacitances in series: If the capacitors are of equal value, you're in luck. All you must do is divide the value of one of the individual capacitors by the number of capacitors. For example, the total capacitance of two, 100 μF capacitors is 50 μF. 6. Why capacitor is connected in parallel?Capacitors are devices used to store electrical energy in the form of electrical charges. By connecting several capacitors in parallel, the resulting circuit is able to store more energy since the equivalent capacitance is the sum of individual capacitances of all capacitors involved. 7. Do capacitors in series increase voltage?Capacitors connected in series will have lower total capacitance than any single one in the circuit. This series circuit offers a higher total voltage rating. The voltage drop across each capacitor adds up to the total applied voltage. This is why series capacitors are generally avoided in power circuits. 8. Do capacitors in series or parallel store more energy?The energy stored in a capacitor is a function of the voltage across the capacitor. The voltage will be higher when they are in parallel, so the parallel connection stores the most energy. 9. Why voltage is different in a series combination of capacitors?In a series combination, since the charge stored is the same as the same charge flows through all the capacitors, the potential difference across each will be different. 10. When capacitors are wired in parallel what must be the same for the two capacitors?The charge in the two capacitors is different. Capacitors connected in parallel are connected to the same start and end points of the input and output that's why they have the same potential difference.  

IntroductionA relay is an electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. Because of unique characteristics, it is widely used in many fields. How does a relay work? What the functions of relays, let’s check the following details.How Does A Relay Work?CatalogIntroductionⅠ Working Principle  1.1 Operation Example: Relay as a Switch  1.2 Working Principle of Different RelaysⅡ What is the Function of a Relay?  2.1 Summary  2.2 Types of RelayⅢ Relay Applications  3.1 Automotive Field  3.2 Household Appliance  3.3 Industrial Relays  3.4 Example Analysis: JYB-714 Liquid Level RelayⅣ Relay Selection RulesⅤ One Question Related to DC RelayⅥ Frequently Asked Questions about How Relay WorksⅠ Working PrincipleThe relay is generally composed of an iron core, coil, armature, contact reed and so on. As long as both ends of the coil having a voltage, a certain current will flow through the coil, which will produce electromagnetic effects. Under the action of the electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby the movable contact and the static contact (normally opened terminal) are in the state of pull-in.Figure 1. Relay StructureWhen the coil is power-off, the electromagnetic attraction will disappear. The armature will move back to its original position under the reaction force of the spring, making the movable contact and the static contact (normally closed terminal) contract together. Under the actions of pull-in and release, achieve the purpose of conducting and cutting off in the circuit. 1.1 Operation Example: Relay as a SwitchThe following figure is the circuit diagram of the relay controlling the light. The relay has normally open contacts and normally closed contacts. The movable contact is a common terminal. This is a DC relay powered by a battery. When the coil of the relay is powered by a DC power supply, the coil with the iron core will generate the corresponding magnetic field to adsorb the armature, and the movable contact will move from the normally closed contact side to the normally open contact side, which is equal to the normally open contact being pulled in. We can see that the start/stop button, battery, and relay coil form a control loop. As long as this loop is closed, the current will flow through the coil and a magnetic field will be generated. The normally open contact, the lamp, and the control power supply (the other battery in the picture) form a loop. When the normally opened contact is closed, the loop is closed and the current will flow from the positive of control power supply to the bulb, passing through the closed normally opened contact to the negative pole, so that the light will on.Figure 2. A relay as a SwitchWhen the start/stop button disconnects, the coil has no current. So that the armature will not be attracted by the magnetic force, and will be reset by the spring. So that the other end of the moving contact will go from the normally opened contact to the normally closed contact. The circuit of the bulb is forcibly disconnected and does not turn on.Figure 3. Relay Controls a Light1.2 Working Principle of Different Relays1) Electromagnetic RelayIt works by using the suction force generated by the circuit in the input circuit between the electromagnet core and the armature.2) Solid State RelayElectronic components have their functions without mechanical moving parts, and the input and output are isolated.3) Temperature RelayIt will act when the outside temperature reaches a given value.4) Reed RelayUsing the reed action sealed in the tube, open, close, or switch the circuit with the function of the electric contact reed and the armature magnetic circuit.5) Time RelayWhen the input signal is added or removed, the output part needs to be delayed or time-limited before closing or opening its controlled circuit until the specified time.6) High-frequency Relay It is used to switch high-frequency and radiofrequency lines with minimal loss.7) Polarized RelayA polarized magnetic field and a control current are combined to act by the magnetic field generated by the control coil. Ⅱ What is the Function of a Relay?2.1 SummaryRelay is an automatic switching element with isolation function, which is widely used for remote control, telemetry, communication, automatic control, electronic equipment, etc. It is the most important control element in the circuits.Relays generally have a sensing mechanism (input part) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.); there is a mechanism (input part) that can realize "switch on" and "switch off" to the controlled circuit. Between the input end and the output end of the relay, there is also an intermediate mechanism for coupling isolation of the input quantity, functional processing and driving the output part (driving part). As a control element, relays have the following functions:1) Expanding control rangeFor example, when the control signal of a multi-contact relay reaches a certain value, multiple circuits can be switched, disconnected, and connected at the same time from different forms of contact groups.2) AmplificationFor example, using a very small control quantity can control a large power circuit, such as sensitive relays, intermediate relays and so on.3) Integrated signalFor example, when multiple control signals are input to a multi-winding relay in a prescribed form, they will be relatively integrated to achieve a predetermined control effect.4) Automatic control, remote control, and monitoringFor example, the relay on the automatic device and other electrical appliances can form a program control circuit to realize automatic operation. 2.2 Types of RelayIntermediate RelayIt is the function of converting and transmitting the control signal. That is, its input signal is the power off/on the signal of the coil, and the output signal is the contact action of the intermediate relay. In essence, it belongs to one of the voltage relays and has the characteristics of multi-contacts (six pairs or even more). The contact can withstand a large current (rated current is 5A~10A), and its action is more sensitive (response time less than 0.05s). Voltage RelayIts main principle uses the voltage signal, and determines the action of the contact according to the coil voltage, in addition, the coil needs to be connected in parallel with the load during circuit design. The voltage relay can be divided into AC and DC type according to the coil voltage, and can be divided into overvoltage and Undervoltage according to the operating voltage. Therefore, their functions are also different. As for the overvoltage relay, when the coil voltage is within the rated value range, the armature will not make any pull-in action. On the contrary, the action will execute if the coil voltage is exceeded. The AC overvoltage relay plays the role of overvoltage protection in the circuit. When the coil voltage reaches or exceeds the rated value of the coil, the armature will make a pull-in action, and the coil voltage will be lower than the rated value. The Undervoltage relay mainly plays a role of Undervoltage protection in the circuit when the armature is released immediately. Current RelayIt works according to the current signal and determines the contact action according to the current of the coil. The current relay coil needs to be connected in series with the load when having the installation. According to the coil current, it can be divided into two types AC and DC. According to the action current, it can be divided into overcurrent and undercurrent types.Since the load current will pass through the coil when having overcurrent, the coil rated current (that is, the setting current) is usually chosen to be equal to the maximum load current. When the load current is not higher than the setting value, the armature will not act. On the contrary, if it exceeds, a pull-in action will occur. The main function of the overcurrent relay is to play overcurrent protection in the circuit, especially in some occasions where impulsive over-current occurs. Because it has a good protective effect.  The principle of the undercurrent relay is that when the current in the coil reaches or exceeds the operating current value, the armature will perform a pull-in action. On the contrary, the armature will be released immediately when the coil current is less than the operating current value. In a normal state, if the load current exceeds the working current of the coil, the armature will also perform pull-in. When the load current drops below the coil current, the armature will be released. Time RelayIt belongs to a relay that starts with the input signal (that is when the coil is powered on or off) and will output the signal (contact closed or disconnected) after a preset delay in advance. Time relays are generally used in relatively low voltage or current circuits to turn on and off higher voltage or current, just as an electric switch device in the circuit used for automatic control.Figure 4. Electrical Relay Symbol (SPST/SPDT/DPST/DPDT)Ⅲ Relay ApplicationsRelays are employed in a wide range of fields, and their environmental conditions and technical requirements vary greatly. What’s more, in the same application field there are different requirements. Here are some examples and a brief description.3.1 Automotive FieldThe automotive industry is increasingly using relays. The more common relays are: starting relays to start motors, horn relays, open circuit relay of motor or generator, regulating relays for charging voltage and current, flashing relays, control relays fro light brightness,control relays for air conditioning, and so on. The power supply in the car now mostly uses 12V, and the coil voltage is mostly set to be 12V. Due to the battery power supply, the voltage is unstable.  The environmental conditions are not good, for example, the suction voltage is less than 60VH (rated working voltage), and the overvoltage of the coil is required to 1.5VH. What’s more, the power consumption of the coil is relatively large, generally 1.6~2W, and the temperature rise is relatively high. Their environmental requirements are also quite harsh: the ambient temperature range is -40℃～100℃; the relay used in the engine box must be able to withstand the damage of sand, dust, water, salt, and oil; vibration and shock are undoubtedly affecting normal operation. 3.2 Household Appliance1) Air conditioning relays are mainly used to control compressor motors, fan motors and cooling pump motors to have control functions. Owing to the moment when the load starts, a large inrush current appears, which is about 6 times the full-load operating current. It takes a long time for the compressor motor to reach full speed (the power of the home appliance compressor motor is generally 1 to 3 horsepower, where the fan motor and cooling pump motor are 1/4 to 2 horsepower.), which is a serious threat to the relay contacts to eliminate as much as possible the contact bounce when the relay is sucked.  Because the relay is required to release fast, minimize contact bounce as much as possible. The safety requirements are also strict and must be recognized by a safety certification agency. For example, as for product environmental conditions, the ambient temperature requires -40 to 55℃, relative humidity up to 40%, 90RH, and have rainwater infiltration. Because weight and size are not important indicators, the relay is required to be robust and impact resistant.2) Relay used in washing machines, microwave ovens, electric heaters, etc. Relay contact load: the large load can reach 220V, 5000W heater (or 1 horsepower motor), and the small load can be as small as driving solenoids load, other relay coils load, indicator light load, etc. The expected life span of the relay is required to reach 5 to 10 years. That is to say, the electrical life of the relay is required to reach 105 times to 2×105 times. Ambient temperature: -40 to 55°C (85°C for microwave ovens and electric heaters); relative humidity 20 to 95%/RH. 3.3 Industrial RelaysIn industrial control, the main control function is completed by the universal AC relay. The relay is usually driven by a button or limit switch. It is also used in traffic signal controllers, temperature controllers, etc. The contacts of the relay can control solenoid valves, larger start motors, and indicator lights.  What’s more, the field of digital control has expanded the application of relays. Copy milling and coordinate boring are operated by data programming, and the signals are sent to the machine tool controller, memory unit and other logic elements to control 2 to 5 axes of the coordinate servo motor. With this mechanical control method, it is easy to control drilling machines, hexagonal lathes, ordinary lathes and automatic profiling machines.The digital control system requires the relay to have the ability to adapt to low-level signals, medium sensitivity, fast action and high switching reliability. The environmental conditions for the installation of industrial machinery must be considered. For instance, operating industrial machinery and surrounding equipment always transmit some shocks and vibrations to the control cabinet, and they also have the influence of splashing cutting coolant. So that these unfavorable environmental conditions must still be considered when selecting and designing relays. With strict safety requirements, high requirements are needed for electrical insulation, voltage resistance, and flame retardants. 3.4 Example Analysis: JYB-714 Liquid Level RelayLiquid level relay is a kind of relay that uses liquid level to control the circuit. To be specific, this is a relay with electronic circuits inside. Based on the conductivity of the liquid, when the liquid level reaches a certain height, the relay will act to cut off the power; when the liquid level is lower than a certain position, turn on the power to make the pump work.  To achieve the role of automated control, this control is composed of sensors and control actuators. According to the conductivity of water, but it is poor and cannot directly drive the relay. Therefore, there must be an electronic circuit to amplify the current to drive the relay to work. So the sensor of the liquid level controller is generally a wire. The line is divided into three types, high and low, and the middle line. The high is the water level overflow point to control the water level freely, in addition, the water will stop to fill in automatically. At the low water level, the low point is the automatic water filling point. Where the middle is constant contact.JYB-714 Liquid Level Relay①, ⑧ are the working power connecting terminals of the relay. ① is connected to L1, ⑧ is connected to N.②, ③, and ④ output the automatic control signal, and the working voltage of the output terminal is AC220V. ③ is the output signal common end, the level control signal of the water supply pump is output between ② and ③, and the drainage pump level control signal is output between ③ and ④.  ⑤, ⑥, ⑦ are the wiring terminals corresponding to the liquid level electrodes A, B, C in the pool.  ⑤ is connected to the high water level electrode A, ⑥ is connected to the low water level electrode B, and ⑦ connect to the lowest common electrode C. Note that in the experiment, the water inlet electrode uses a copper hard insulated wire of 1 to 1.5mm2, and the water inlet end is stripped of 5mm insulation. In addition, the safety voltage between the liquid level electrode terminals is DC24V. Tech Note1) Drainage type liquid level relay instructions"High" is the upper limit liquid level control point of the pool. When the water level rises to a high level, the water contacts the probe (electrode), and the controller automatically turns on the pump and starts to drain."Middle" is the lower limit liquid level control point of the pool. When the water level drops below the midpoint level, the water and the probe (electrode) are out of contact, and the controller automatically turns off the pump and stops draining."Low" is the ground line of the pool, the lowest point of the pool. 2) The difference between water-supply type liquid level relay and drainage-type liquid level relay:Water-supply type liquid level relay works in water shortage and stops when the water is full.The drainage-type relay works when water is full and stops in a water shortage.Figure 5. A RelayⅣ Relay Selection RulesTo use the relay well, the correct selection is very important. First of all, you must get a thorough understanding of characteristics and requirements of the controlled object, and have careful consideration. The principle, purpose, technical parameters, structural characteristics, specifications and models of the selected relays should be analyzed. On this basis, the relay should be correctly selected according to the actual situation and specific conditions of the project.1. The necessary conditions① The power supply voltage of the control circuit, the maximum current that can be provided.② Voltage and current in the controlled circuit. ③ Contact: When selecting a relay, on the one hand, you should consider whether the control circuit can provide enough working current, otherwise the pull-in of the relay is unstable. When the pull-in and release time of the relay cannot meet the requirements, the time constant of the coil loop can be changed to solve the problem. On the other hand, there is the elimination of electric sparks. Due to the small on-off current of the relay contacts, there will be no arc between the contacts, but "spark discharge" will occur. This is due to the presence of inductance in the contact circuit, and an overvoltage will appear on the inductance when it is disconnected. Together with power supply voltage on the contact gap, so that the contact gap will break down and discharge. Because of energy limitation, only spark discharge will generate. The alternating energy conversion between the capacitance and inductance existing between the contacts makes the spark looming and becoming a high-frequency signal. In addition, spark discharge will cause damage to the contacts, resulting in short service life. 2. After consulting the relevant materials to determine the conditions of use, you can find the relevant materials to find out the specific relay. If you already have a relay on hand, you can check whether it can be used according to the datasheet, and finally consider whether the size is appropriate. 3. Pay attention to the size of the appliance. If it is employed in general electrical appliances, in addition to the cabinet volume, small relays mainly consider the circuit board installation layout. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be used. 4. Rated load and service life are reference values, which will vary greatly according to different environmental factors, load properties and types. So it is better to confirm in actual or simulate actual use. 5. Try to use rectangular wave control for DC relays, and use sine wave control for AC relays. 6. In order to maintain the performance of the relay, please be careful not to drop the relay or subject it to strong shocks. 7. Do not use the relay in an environment with much dust and harmful gas. Harmful gases include gas containing sulfur, silicon, nitrogen oxides, etc. 8. As for the magnetic latching relay, it should be placed in the action or reset position as needed before use. 9. For polarized relays, please pay attention to the polarity of the coil voltage. 10. The relay is a heat-resistant component. High temperature can speed up the aging of the internal plastic and insulating materials of the relay. Contacts are oxidized and corroded, making it difficult to extinguish the arc. The technical parameters of the electrical components decay and the reliability reduces. So that good ventilation conditions should be maintained.And meanwhile, the low temperature cannot be ignored. Low temperature can aggravate the cold adhesion of the contacts and expose the contact surface. Many manufacturers’ relays indicate that the minimum temperature is -25°C, but high-voltage switches are also used in extreme cold. So it is recommended to leave the room when selecting the model to avoid the relay being unreliable due to low temperature. If circumstances permit, add heaters in the high cold area to ensure that the relay operates reliably and ensure the stability of the entire system. 11. Under the condition of low air pressure, the heat dissipation condition of the relay goes bad,  and the temperature of the coil rises, which changes the given pull-in and release parameters of the relay, affecting the normal operation of the relay. The low air pressure can also reduce the insulation resistance of the relay. It is difficult to extinguish the arc and is easy to melt the contacts and affect the reliability of the relay. It can be used normally at an altitude of fewer than 2000 meters, and it needs capacitance derated used at an altitude of more than 2000 meters. 12. Reduce the impact of mechanical stress on the relay. Mechanical force mainly refers to stress such as vibration, impact, and collision on the control system. The self-vibration of the circuit breaker in the high-voltage switch and the vibration caused by the opening and closing operations has a greater impact on the relay. An intermediate relay with a balanced armature mechanism should be selected. Electromagnetic relays have cantilevered beam structure, the natural frequency is low, oscillation and impact will cause resonance, resulting in the relay contact pressure to drop and contact instant disconnection or contact vibration, which will affect the reliability of the relay. It suggests that vibration measures should be taken to prevent resonance. Ⅴ One Question Related to DC Relay5.1 QuestionHow Does a DC relay work?5.2 AnswerA DC relay uses a single coil of wire wound around the iron core to make the electromagnet. When the DC coil is energized, the magnetism generated in the core is steady because the DC just keeps going. The steady magnetism keeps the lever attracted as long as the DC is flowing. Ⅵ Frequently Asked Questions about How Relay Works1. What is a relay and how it works?A relay is an electrically operated switch. They commonly use an electromagnet (coil) to operate their internal mechanical switching mechanism (contacts). When a relay contact is open, this will switch power ON for a circuit when the coil is activated. 2. Why relay is used?The switch may have any number of contacts in multiple contact forms, such as making contacts, break contacts or combinations thereof. Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. 3. How do you know if a relay is working?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 4. What is the main function of the relay?Relays are electric switches that use electromagnetism to convert small electrical stimuli into larger currents. These conversions occur when electrical inputs activate electromagnets to either form or break existing circuits. 5. What is the difference between relay and switch?The main difference between Relay and Switch is that the Relay is an electrically operated switch and Switch is an electrical component that can break an electrical circuit. ... Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. 6. Are Relays AC or DC?A Dc relay coil has a resistance that limits the dc current. An AC coil relies on its impedance for governing the current. An AC relay will remain contact closed due to mechanical inertia and a little mechanical hysteresis and, the fact that an alternating north and south pole both attract the relay armature. 7. How a relay works in a car?Although there are various relay designs, the ones most commonly found in low voltage auto and marine applications are electro-mechanical relays that work by activating an electromagnet to pull a set of contacts to make or break a circuit. These are used extensively throughout vehicle electrical systems. 8. What happens when a relay fails?If the ignition relay shorts burns out or otherwise fails while the engine is operating it will cut off power to the fuel pump and ignition system. ... In some instances of a faulty relay the vehicle will be able to restart once the relay cools off, only to stall out once again after the relay overheats. 9. Does a relay need constant power?The answer to that one is No. Relays have a finite lifetime in terms of how many times they can open and close. And limit to how much current they can handle. But keeping a relay constantly energized does not wear it out. 10. Why is a relay better than a switch?Relays are a better choice for switching large currents (> 5A). Relays can switch many contacts at once. Disadvantages of relays: • Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. Recommended ReadingBasic Knowledge of Relay Electronics Tutorial with VideoThe Role of the Relay and Its Working PrincipleThe Types of Common Relay and How to Choose Relay?

IntroductionSimply put, an oscillator is a device that can convert DC power into AC power without external signal excitation. The so-called "oscillation" implies alternating current. This article will mainly explain the circuits of different sine wave oscillators, including their working principles, how to realize their functions, circuit composition and comparison of advantages and disadvantages of different forms of circuits. This article uses a large number of circuit diagrams and formulas to explain in detail, which can help you understand in a better way.Basics of oscillators and their different typesCatalogIntroductionCatalogI The Principle of Feedback Oscillator1.1 How Does the Feedback Oscillator Work?1.2 Equilibrium Conditions1.3 Starting Conditions of the Oscillator1.4 Stable Conditions of the Oscillator1.5 General Composition of Sine Wave OscillatorII LC Oscillator Circuit2.1 The Composition Principle of the Oscillator2.2 Capacitive Feedback Oscillator2.3 Inductive feedback oscillatorIII RC Oscillator Circuit3.1 Brief Introduction of RC Oscillator and its Circuit3.2 RC Phase Shift Oscillator3.3 Wien Bridge OscillatorIV Quartz Crystal Oscillator Circuit4.1 What is a Quartz Crystal Oscillator?4.2 Quartz Crystal4.3 Quartz Crystal Oscillator CircuitV Non-sine Wave Generating Circuit5.1 What is a Non-sine Wave Generating Circuit?5.2 Rectangular Wave Generator5.3 Triangle Wave and Sawtooth Wave Signal GeneratorVI QuizⅦ FAQI The Principle of Feedback Oscillator1.1 How Does the Feedback Oscillator Work?The feedback oscillator is when the power is turned on, the various electrical disturbance signals in the loop are selected by the frequency selection network, and the signal of a certain frequency is fed back to the input terminal, and then the cycle of amplification → feedback → amplification → feedback , The amplitude of the signal increases continuously, and the oscillation is established from small to large. As the signal amplitude increases, the amplifier will enter a non-linear state, and the gain will decrease. When the feedback voltage is exactly equal to the input voltage, the oscillation amplitude will no longer increase and enter a balanced state. As can be seen from the figure below, the feedback oscillator is a closed loop composed of an amplifier and a feedback network. The amplifier is usually a tuned amplifier with an oscillation circuit as a load. The feedback network is generally a linear network composed of passive components.Figure1. Block Diagram of Feedback OscillatorIn order to generate self-oscillation, there must be positive feedback, that is, the signal fed back to the input terminal and the signal at the input terminal of the amplifier have the same phase. For the above figure, suppose the voltage amplification factor of the amplifier is K(s), the voltage feedback coefficient of the feedback network is F(s), and the closed-loop voltage amplification factor is Ku(s), thenby We can writeIf a certain frequency ω1=ω, then T(jω1) is equal to 1. From the above formula, we can see that Ku(jω) will tend to infinity. This shows that there is no external signal, and self-excited to produce signal output, namely self-oscillation. Therefore, the condition of self-oscillation is that the loop gain is 1.Definition:1.2 Equilibrium ConditionsThe equilibrium condition of the oscillator isthis can also be expressed asIf a certain frequency ω1=ω, then T(jω1) is equal to 1. From the above formula, we can see that Ku(jω) will tend to infinity. This shows that there is no external signal, and self-excited to produce signal output, namely self-oscillation. Therefore, the condition of self-oscillation is that the loop gain is 1. The above two formulas are the amplitude and the phase equilibrium conditions respectively. Equilibrium conditions are also called "two conditions for maintaining self-oscillation". The amplitude balance condition determines the amplitude of the oscillator output signal, and the phase equilibrium condition determines the frequency of the oscillator output signal. But it must be pointed out that the loop can only meet the phase equilibrium condition at a certain frequency (f), which is the resonant frequency (f0) of the loop.1.3 Starting Conditions of the OscillatorWhen the oscillator is in actual application, there should be no external signal Us(s) shown in Figure 1. The initial source of oscillation is electrical signals such as electrical shock and various thermal noises that inevitably exist when the oscillator is switched on.It can be seen from the establishment process of the oscillation that in order to make the oscillator start-up, the feedback voltage Uf and the input voltage Ui should be in phase at the beginning of the oscillation (that is, positive feedback); Uf>Ui should be required in amplitude, that is:Vibration conditions: φA+φF=2nπ(n=0,1,2,•••)AF>1 Simply put, as we know, the condition of unity gain must be met to make the oscillation continue. But to start the oscillation, the voltage gain of the positive feedback loop must be greater than 1, so that the amplitude of the output voltage can reach the required potential. Then the gain must be reduced to 1, so that the output voltage can be maintained at the required potential and the oscillation phenomenon can continue. Transition from |T(jω)|>1 to |T(jω)|=1 when the oscillator is workingThe amplifier must work in the linear amplification region of the transistor when amplifying small signals.When the oscillation is started, the amplifier works in the linear region. At this time, the output of the amplifier increases linearly with the increase of the input signal; as the amplitude of the input signal increases, the amplifier gradually enters the saturation or cut-off region from the amplification region, and enters a nonlinear state. The closed-loop gain of will decrease with the increase of the input signal, as shown in the figure below:Figure2. Graphical Representation of Amplitude ConditionsWhen the loop gain drops to |T(jω)|=1, the growth process of the amplitude will stop, and the oscillator will reach a balanced state and perform constant amplitude oscillation. It can be seen that the transition of the oscillator from amplified oscillation to steady amplitude oscillation is realized by the non-linear characteristics of the amplifier. When the oscillating circuit is powered on, the current of the transistor increases abruptly from zero, and the sudden change current contains a wide spectrum component. The start-up process of the circuit is very short! As long as the circuit satisfies the starting conditions, after the oscillator is powered on, there will be an output signal with stable amplitude at the output.Figure3. Circuit Start-up Process1.4 Stable Conditions of the OscillatorIf the loop gain characteristic has two equilibrium points A and B, among them, point A is stable and point B is unstable.Figure4. Stable Conditions of the OscillatorIt can be seen from the above discussion that in order to stabilize the equilibrium point, |T(ω0)| must have a negative slope change near UiA.Stable conditions are divided into amplitude stable conditions and phase stable conditionsTo make the amplitude stable, the oscillator must have the ability to prevent amplitude changes at its equilibrium point. Then the amplitude stability condition should beSince the feedback network is a linear network, which means the size of the feedback coefficient does not change with the input signal, so the amplitude stability condition can be written asThe phase stability depends on the increase of ω, and the decrease of , meaning that the phase characteristic of the parallel oscillator circuit ensures the phase stability.Therefore, the phase stability condition is. The higher the Q value of the loop,  the larger of the value of , and the better phase stability.1.5 General Composition of Sine Wave Oscillator(1) Amplifying circuit-realize energy control.(2) Positive feedback network-meets the conditions for starting vibration.(3) Frequency selection network-only one frequency satisfies the oscillation condition to obtain a sine wave output of a single frequency. Commonly used frequency selection networks include RC frequency selection and LC frequency selection(4) Amplitude stabilization link-makes the circuit easy to start and oscillate stably, with little waveform distortion. Examples of oscillation circuits:High frequency resonant amplifier and sine wave oscillatorFigure5. High Frequency Small Signal Resonant AmplifierFigure6. Mutual Inductance Coupled OscillatorII LC Oscillator CircuitLC oscillators can be divided into three types: mutual inductance coupled oscillators, inductive feedback oscillators and capacitive feedback oscillators according to their different feedback networks.This section focuses on different types of feedback LC oscillators and three-point oscillators.2.1 The Composition Principle of the OscillatorFigure7. General Form of Three-terminal OscillatorThe basic circuit is the so-called three-terminal (also called three-point) oscillator, which means the circuit formed by connecting the three terminals of the LC loop and the three electrodes of the transistor respectively, as shown in the figure. The three-terminal LC oscillator is a feedback type LC oscillator. In order to obtain positive feedback, the feedback circuit must make the instantaneous polarity of the transistor's AC voltage meet a certain phase relationship: when Vbe is negative, Vce should be positive, namely, Vbe and Vce are in reverse phase, and Veb and Vce are in phase. Only when the reactance Xce and Xeb have the same properties can they be guaranteed to be in phase. When the resistance of the loop element is very small, its influence can be ignored, and the influence of the input impedance and output impedance of the transistor can also be ignored. To maintain oscillation, the circuit must meet the requirements. Otherwise, the loop resonance condition cannot be met.The criteria for the three-terminal oscillator circuit to meet the phase equilibrium condition is as follows:(1) The reactance properties of Xce and Xeb are the same, but the reactance properties of Xcb are opposite. That is, ce and be are the same resisting piece, and cb resisting piece.(2) The oscillation frequency should satisfy 1Xo+Xcl-XolBased on this criterion, it can be quickly judged whether the oscillating circuit composition is reasonable or not.Three-terminal LC oscillatorThe three-terminal LC oscillation circuit is often used, and its operating frequency is about a few MHz to a few hundred MHz. The frequency stability is also higher than that of the transformer coupled oscillation circuit, which is about 10-3~10-4. After some frequency stabilization measures, it can be higher. There are many types of three-terminal LC oscillators, mainly: Inductance three-terminal type, also known as Hartley oscillator; capacitor three-terminal type, also known as Colpitts oscillator; series type improved capacitor three-terminal type, also known as Clapp parallel type improved capacitor three-terminal type , Also known as Sellier oscillator. The three-terminal oscillator has two basic circuits, as shown in the figure below.Figure8. Capacitive Feedback Oscillator and Inductive Feedback Oscillator2.2 Capacitive Feedback OscillatorCapacitive feedback three-terminal oscillator is an electronic component, also called Colpitts oscillator, which is a kind of self-excited oscillator. It is composed of a series capacitor, an inductance circuit and a positive feedback amplifier. It is named because the three end points of the two series capacitors of the oscillating circuit are connected to the three pins of the oscillating tube respectively.Figure9. Capacitive Feedback OscillatorAs shown in the figure, use C2 to feed back part of the voltage of the resonant tank to the base. The three end points of the LC resonant tank are respectively connected to the three electrodes of the transistor, so it is called a capacitive feedback three-terminal oscillator, or Colpitts oscillator. The vector analysis method can be used to prove that the circuit meets the phase balance condition. As long as the ratio of C1 and C2 is appropriately selected and the amplifier has enough amplification, the circuit can oscillate.Figure10. Capacitive Feedback OscillatorAdvantages of Colpitts circuit:(1) Good oscillation waveform;(2) The frequency stability of the circuit is high. If the capacitance of the circuit is increased appropriately, the influence of unstable factors on the oscillation frequency can be reduced;(3) The operating frequency of the three-terminal circuit of the capacitor can be made higher. The output and input capacitors of the oscillator tube can be directly used as the oscillation capacitor of the loop, and the operating frequency can reach a very high frequency range of tens of MHz to hundreds of МHz. Disadvantages of Colpitts circuit:When adjusting C1 and C2 to change the oscillation frequency, the feedback coefficient will also change, which will affect the starting conditions and working status. But as long as a variable capacitor is connected to both ends of L and C1 and C2 are fixed capacitors, the feedback coefficient will not be affected when the frequency is adjusted.2.3 Inductive feedback oscillator(1) Circuit compositionIn order to overcome the shortcomings that the transformer primary coil and the secondary coil in the transformer feedback oscillation circuit are not tightly coupled, the N1 and N2 of the transformer feedback oscillation circuit can be combined into one coil. As shown in the figure below, in order to strengthen the resonance effect, the capacitor C is connected across the entire coil. This is the inductive feedback oscillator circuit, or Harley oscillator.Figure11. Inductive Feedback Oscillator Circuit(2) Working principle✿The circuit includes four parts: amplifier circuit, frequency selection network, feedback network and non-linear element (transistor), and the amplifier circuit can work normally.✿Use the instantaneous polarity method to judge whether the circuit meets the phase condition of sine wave oscillation: disconnect the feedback, add the input voltage with frequency f0, give its polarity, and judge that the polarity of the feedback voltage obtained from N2 is the same as the input voltage , So the circuit satisfies the phase condition of sine wave oscillation.✿As long as the circuit parameters are selected properly, the circuit can meet the amplitude condition and produce sine wave oscillation. The following figure shows the AC path of the inductive feedback oscillation circuit. The three ends of the primary coil are connected to the three poles of the transistor, so the inductive feedback oscillation circuit is called an inductive three-point circuit.Figure12. AC Path of Inductive Feedback Oscillator Circuit(3) Advantages and disadvantagesThe coupling between N2 and N1 in the inductance feedback oscillation circuit is tight, the amplitude is large, and it is easy to oscillate; when C uses a variable capacitor, a wide adjustment range of oscillation frequency can be obtained, and the highest oscillation frequency can reach tens of MHz. Since the feedback voltage is taken from inductance, it has greater reactance to high-frequency signals, and the feedback signal contains more high-order harmonic components, and the output voltage waveform is not good. The following introduces two improved capacitor three-terminal oscillation circuits:Clap oscillator:The following figure (a) is the principle circuit of the Krapper oscillator, and (b) is its AC equivalent circuit. Its characteristic is that a capacitor C3 is added to the inductance branch of the aforementioned capacitive three-point oscillating resonant tank. Its value is relatively small, requiring C3<< C1, C3<< C2.Figure13. Clapp OscillatorRegardless of the influence of the capacitance between the poles, the total capacitance CΣ of the resonant circuit is the series connection of C1, C2 and C3, namely. Thus, the oscillation frequency is .The condition for the above formula to be true is that C1 and C2 must be selected relatively large. It can be seen that the influence of C1 and C2 on the oscillation frequency is significantly reduced, so the influence of the capacitance between the transistors connected in parallel with C1 and C2 is also very large. It is smaller, and the stability of the oscillation frequency is improved. Sellier oscillator:Figure14. Sellier Oscillator so the oscillation frequency:L is the inductance of the inductance coil of the resonant amplifier circuit; C is the total capacitance of the resonant circuit. In the LC resonance circuit, the inductance L(H)/capacitance C(F)=105~106, which can achieve better results.III RC Oscillator Circuit3.1 Brief Introduction of RC Oscillator and its Circuit● What is an RC oscillator?(1) The sine wave oscillator has no input signal and is a positive feedback amplifier with a frequency selection network. If resistors and capacitors are used to form a frequency selection network, it is called an RC oscillator, which is generally used to generate 1Hz-1MHz low-frequency signals. The frequency selection effect of the RC frequency selection network is not as good as that of the LC resonant circuit, so the waveform and stability of the RC oscillator are worse than that of the LC oscillator.(2) RC oscillator can be divided into sine wave oscillator and non-sine wave oscillator according to whether the output wave type is sine wave.(3) There are many kinds of RC oscillation circuits: bridge type, phase shift type, double T type, the most commonly used is bridge type oscillation circuit, namely RC series-parallel frequency selection network. ● Features of RC oscillator(1) RC phase-shift oscillator features: simple, poor frequency selection, unstable amplitude, inconvenient frequency adjustment, generally used in occasions with fixed frequency and low stability requirements. Frequency range: several hertz-tens of kilohertz(2) RC series-parallel network oscillator features: it can easily and continuously change the oscillation frequency, it is convenient to add negative feedback to stabilize the amplitude, and it is easy to get a good oscillation waveform.(3) Double T frequency selective network oscillator characteristics: good frequency selection characteristics, difficult frequency modulation, suitable for generating single frequency oscillation. ● RC oscillator circuitThe oscillating circuit composed of RC frequency selection network is called RC oscillating circuit, which is suitable for low-frequency oscillation, and is generally used to generate low-frequency signals of 1Hz~1MHz. The circuit is composed of four parts: amplifier circuit, frequency selection network, positive feedback network, and amplitude stabilization link. The main advantage is simple structure, economic and convenient. According to the different forms of the RC frequency selection network, the RC oscillator circuit can be divided into an RC lead (or lag) phase shift oscillator circuit and a Wien circuit oscillator circuit. For RC oscillator circuits, increasing the resistance R can reduce the oscillation frequency, and increasing the resistance does not need to increase the cost. The frequency of the sine wave generated by the commonly used LC oscillator circuit is relatively high. If a sine wave with a lower frequency is to be generated, the oscillation circuit must have a larger inductance and capacitance. This will not only cause the components to be bulky, heavy and inconvenient to install, but also difficult to manufacture. high cost.  Therefore, the sinusoidal oscillation circuit below 200kHz generally adopts an RC oscillation circuit with a lower oscillation frequency.3.2 RC Phase Shift OscillatorThe phase shift oscillator is an oscillator composed of an advanced phase shift or a lag phase shift circuit as a frequency selection network and an inverting amplifier. It has the advantages of simple circuit, economy and convenience, but the effect of frequency selection is poor, the amplitude is not stable enough, and the frequency adjustment is inconvenient. Therefore, it is generally used for occasions with fixed frequency and low stability requirements. Its oscillation frequency is:Figure15. RC Phase Shift Oscillator Schematic Diagram3.3 Wien Bridge OscillatorThe RC series-parallel frequency selection network and amplifier can be combined to form an RC oscillator circuit, and the amplifier part can be an integrated operational amplifier. As shown in the figure, the RC series-parallel frequency selection network is connected between the output of the operational amplifier and the non-inverting input to form positive feedback. Rt and R1 are connected between the output of the operational amplifier and the inverting input to form negative feedback. . The positive feedback circuit and the negative feedback circuit constitute a Wien bridge circuit, and the input and output ends of the operational amplifier are respectively connected across the diagonal of the bridge. Therefore, this kind of oscillation circuit is called a Wien bridge oscillation circuit.Figure16. Wien Bridge OscillatorThe oscillating signal is input from the non-inverting terminal, so a non-inverting amplifier is formed. The output voltage is in phase with the input voltage, and the closed-loop voltage amplification factor is equal to: When the RC series-parallel frequency selection network is ω=ω0=1/RC, Fu=1/3, εf=0, so as long as |Au|=1+(Rt/R1)>3, that is, Rt>2R1, oscillation The circuit can meet the self-excited oscillation amplitude and phase start-up conditions to produce self-excited oscillation, the oscillation frequency f0=1/2πRC. Using double adjustable potentiometer or double adjustable capacitor can easily adjust the oscillation frequency. In the commonly used RC oscillator circuit, the high stability capacitor is generally used to switch the frequency band (coarse frequency adjustment), and then the double variable potentiometer is used to fine-tune the frequency.IV Quartz Crystal Oscillator Circuit4.1 What is a Quartz Crystal Oscillator?Quartz crystal oscillator refers to a device made on the principle that the crystal resonates due to the piezoelectric effect when the frequency of the electrical signal is equal to the natural frequency of the quartz crystal. It is a key component of crystal oscillators and narrow-band filters.Although the appearance, size and frequency of the quartz crystal oscillator are different, the structure principle is basically the same. In order to improve the stable and reliable operation of the quartz crystal, the shell components of the quartz crystal oscillator will be sealed and evacuated. Or fill with nitrogen.4.2 Quartz Crystal(1) StructureFigure17. Structure of the Quartz Crystal(2) Basic characteristicsApplying an electric field between the plates→mechanical deformation of the crystalMechanical force is applied between the plates → the crystal generates an electric fieldPiezoelectric effect: alternating voltage → mechanical vibration → alternating voltageWhen the alternating voltage frequency = natural frequency, the amplitude is the largest → piezoelectric resonanceThe natural frequency of mechanical vibration is related to the size of the wafer, and the stability is high.4.3 Quartz Crystal Oscillator CircuitThe high quality factor of quartz crystal is used to form an LC oscillator circuit.(1) Parallel Type quartz crystal oscillatorFigure18. Parallel Type Quartz Crystal Oscillator Quartz crystal works between fs and fp, which is quite a large inductance, and forms a capacitive three-point oscillator with C1 and C2. Because the Q value of the quartz crystal is very high, which can reach more than several thousand, the circuit can obtain high oscillation frequency stability.Figure19. Frequency Characteristics of Parallel Quartz Crystal Oscillator (2) Series type quartz crystal oscillatorFigure20. Series Type Quartz Crystal Oscillator Circuit The quartz crystal works at fs, which is resistive and has the smallest impedance, the strongest positive feedback, and zero phase shift, which meets the phase balance condition of oscillation.For frequencies other than fs, the impedance of the quartz crystal increases, and the phase shift is not zero, then the oscillation condition is not met, and the circuit does not oscillate.Figure21. Frequency Characteristics of Series Quartz Crystal OscillatorV Non-sine Wave Generating Circuit5.1 What is a Non-sine Wave Generating Circuit?It is composed of an integrating circuit and a hysteresis comparator circuit. The role of the integrator circuit is to produce a transient process. The hysteresis comparator acts as a switch, that is, the steady state is destroyed by the continuous closing of the switch, and a transient process is generated.Commonly used non-sine wave generating circuits include rectangular wave generating circuits, triangular wave generating circuits and sawtooth wave generating circuits, etc. They are often used as signal sources in pulse and digital systems.5.2 Rectangular Wave GeneratorIt is composed of hysteresis comparison circuit and RC timing circuit. The output has no steady state and there are two transient states; if the output is high level, it is defined as the first transient state, and the output is low level as the second transient state.Basic components:(1) Switching circuit: The output has only two situations of high level and low level, called two states; therefore, a voltage comparator is used.(2) Feedback network: self-control, when the output is in a certain state, it breeds the condition of turning into another state. Feedback should be introduced.(3) Delay link: Make the two states maintain a certain period of time and determine the oscillation frequency. Use RC circuit to achieve.Circuit composition:Figure22. Rectangular Wave Generating Circuit5.3 Triangle Wave and Sawtooth Wave Signal GeneratorThe circuit structure of the triangle wave generator: hysteresis comparator + inverting integratorworking principle:Figure23. Circuit of Triangle Wave Generator Sawtooth wave generator: change the forward and reverse charging time constant of the integrator, thereby changing the duty cycle.Figure24. Circuit Diagram of Sawtooth Generator uo1=+UZ, D is cut off, charging time constant: R4C.uo1=-UZ, D is on, charging time constant: (R6∥R4)C≈R6C.Figure25. The Waveform of the Sawtooth GeneratorVI QuizLC resonant circuits are used in:a) RF and ultrasonic oscillators.b) AF and ultrasonic oscillators.c) LF sweep oscillators.d) Variable frequency crystal oscillators. Answer: aⅦ FAQ1. What are the uses of an oscillator?Oscillators have very high precision and accuracy usually <100 ppm variation at a max. So with this kind of accuracy, the oscillator can be used as a clock of the microcontroller (the clock is the most essential part of the microcontroller, without an accurate clock, the functionality will be erratic because we cannot expect the output at the same time we have designed it for). All the quartz-based watches use an Oscillator (32kHz quartz crystal) to keep the time. SO if your timekeeping devices are running with an oscillator, you can imagine the accuracy.Oscillators can additionally be used to generate waveforms required in test benches. In almost all the applications where timing and synchronization are very essential, an oscillator finds its place. For example, in communication systems (whichever electronic communication you consider let it be a telephone, lan, anything) an oscillator is the heart.An oscillator can be used in power systems to generate accurate power waveforms. 2. What is the function of a local oscillator?A local oscillator is used in transmitters and receivers to add or subtract an amount to a basic carrier and modulation. Very handy, as it’s easiest to build up a carrier and modulate it at a low frequency, like 445Kc or 10.7MHz, and then add to it a “LO” to get it up to the FM band or the Gigahertz Wifi or radar bands. The same thing on receive, you first use a 'LO' to subtract down to an intermediate frequency where it’s so much easier to amplify and filter. 3. How do you make a crystal oscillator?The simplest is to use a logic inverter. Put a crystal between the input and output. Some schematics show some caps from the crystal legs to the ground and some show a series resistor. 4. Which type of circuit is used in the oscillator?Types of Oscillators: Harmonic Oscillators & Crystal Oscillators. Harmonic or linear oscillators produce a sinusoidal output where a signal increases and decreases at a predictable level over time. Two basic types are RC, or resistor/capacitor circuits, as well as LC, or inductor-capacitor circuits. 5. What is an Oscillator and the types of oscillators?An oscillator is a type of circuit that controls the repetitive discharge of a signal, and there are two main types of the oscillator; a relaxation, or a harmonic oscillator. This signal is often used in devices that require a measured, continual motion that can be used for some other purpose. 6. What is the basic principle of an oscillator?There are many types of electronic oscillators, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback. 7. What is the working of the oscillator?Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments (like calculators) and complex computers and peripherals etc. 8. How does an oscillator work without input?An oscillator circuit uses a vacuum tube or a transistor to generate an AC output. ... For continuously generating output without the requirement of any input from the preceding stage, a feedback circuit is used. From the above block diagram, the oscillator circuit produces oscillations that are further amplified by the amplifier. 9. What causes oscillation?If a constant force such as gravity is added to the system, the point of equilibrium is shifted. ... In the spring-mass system, oscillations occur because, at the static equilibrium displacement, the mass has kinetic energy which is converted into potential energy stored in the spring at the extremes of its path. 10. What is the difference between oscillator and inverter?The oscillator is a generalized term for an active circuit that produces a periodic waveform. The inverter is a specialized term for a system that contains an oscillator and produces large amounts of power(such as AC) from a source (like a DC battery). The oscillator has no input and produces an oscillating wave as output. 

IntroductionA relay is an electronic control device, which has a control system (also called an input loop) and a controlled system (also called an output loop). It is often used in automatic control circuits. In fact, it is an automated switch using a smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, and converter in the circuit. Relay has the features of fast response speed, stable work, long service life and small size. In order to ensure that these performances can be better played, the test and maintenance of the relay (solid state relay) are particularly important. This paper will introduce main relay test parameters, how to test a relay, an example of an automotive relay test.Testing a RelayCatalogIntroductionⅠ Understanding Relays  1.1 Relay ParametersⅡ How to Test A Relay?  2.1 General Test Ideas  2.2 Types of Relay TestⅢ Relay for Life: Automotive Relay Test  3.1 Automotive Relay  3.2 Common Faults of Automotive Relays  3.3 Detection Method  3.4 Specific OperationⅣ One Question Related to Relay Test and Going Further  4.1 Question  4.2 AnswerⅤ Frequently Asked Questions about Relay TestⅠ Understanding Relays1.1 Relay ParametersMain relay parameters include rated working voltage, rated working current, coil resistance, contact load, etc.1) Rated working voltage refers to the voltage required by the coil when the relay is working normally. For DC relays it refers to DC voltage (Figure a), and for AC relays it refers to AC voltage (Figure b). Relays of the same type often have multiple assessed operating voltages for circuit requirements, and the specification number is added to the end of the component to distinguish.Figure 1. Relay Symbol2) The rated working current refers to the current required by the coil when the relay is working normally.Coil resistance refers to the DC resistance of the relay coil. When selecting a relay, you must ensure that it is rated working voltage and rated working current meet the requirements.Figure 2. Rated Working Current3) Contact load refers to the load capacity of the relay contact, also known as the contact capacity. For example, the contact load of the jzx-10m relay is:  DC 28v×2a or AC 115v×1a. When used, the voltage and current passing through the relay contact should not exceed the rated value, otherwise, the contact will be burned out and the relay will be damaged. A load of multiple sets of contacts of a relay is generally the same.Figure 3. Contact LoadRecommended Reading: Basic Knowledge of Relay Electronics Tutorial with Video    The Role of the Relay and Its Working Principle Ⅱ How to Test A Relay?Relays are widely used in power protection, automation, sport, remote control, measurement and communication devices, so it is very important to check and maintain the normal operation of relays. There are many types of relays. Therefore, the inspection of relays cannot only be judged by measuring the resistance value of the coil. It is necessary to adopt multiple detection methods according to different relay types.2.1 General Test Ideas1) measuring contact resistanceApply the specified working voltage to the relay coil, and use a multimeter to detect the on-off condition of the contact at the “R×1k” gear. When the power is not applied, the normally open contact does not work, and the normally closed contact conducts. When the power is turned on, you should be able to hear the pick-up sound of the relay. At this time, the normally open contact is conducting and the normally closed contact is opposite, and the switching contact should be switched accordingly. Otherwise, the relay is damaged. For multi-group contact relays, if some of the contacts are damaged, the remaining contacts can still be used.Figure 4. Relay Test 2) measuring coil resistanceThe resistance value of the relay coil can be measured with the multimeter at R×10Ω gear, so as to determine whether the coil is open. 3) measuring of pull-in voltage and currentUse an adjustable regulated power supply to input a set of voltage to the relay, and connect an ammeter in the power supply circuit to monitor. Increase the power supply voltage slowly, and when you hear the pull-in sound of the relay, write down the voltage and current. In order to be accurate, you can try several times to get the average value. 4) measuring the release voltage and currentSame test connection like the above. When the relay pulls in, then gradually reduce the supply voltage. When you hear the relay release sound again, write down the voltage and current at this time. Try several times to get the average release voltage and release current. Under normal circumstances, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it can't be used normally, which will affect the circuit stability resulting in abnormal operation. 2.2 Types of Relay TestElectromagnetic Relay TestFigure 5. Electromagnetic RelayThe multimeter is placed in the “R×100” or “R×1k” gear, and the two test leads (regardless of positive and negative) are connected to the two pins of the relay coil (shown in Figure 5). The indication of the multimeter should basically match the coil resistance of the relay. If the resistance value is obviously too small, it means that the coil is short-circuited locally; if the resistance value is 0, it means that there is a short circuit between the two coil pins; if the resistance value is infinite, it means that the coil is open or the pins are disconnected. Reed Relay TestReed relay is also one of the most commonly used relays. It consists of a reed switch and a coil, as shown in Figure 6. The reed switch is made by sealing two non-interconnected ferromagnetic metal strips in a glass tube, and the reed switch is placed in the coil. When the current passes through the coil, the magnetic field generated by the coil magnetizes the metal strips in the reed pipe, and the two metal strips attract due to opposite polarities to connect the controlled circuit. Several reed pipes can be placed in the coil, and they will act simultaneously under the action of the coil's magnetic field.Figure 6. Reed Relay Reed relay has a pair of coil pins and several pairs of reed switch pins, and there are corresponding marks on the shell for identification.Figure 7. Reed Relay Reed relays can also use a multimeter to detect their coils and contacts, and the detection method is the same as that of electromagnetic relays.Figure 8. Reed Relay Solid State Relay (SSR) TestThe input end can be tested with a multimeter. The multimeter is placed in the "R×10k" gear, the black test lead (the positive electrode of the battery in the meter) is connected to the positive electrode of the SSR input terminal, and the red test lead (that is, the negative electrode of the battery in the meter) is connected to the negative electrode of the input terminal of SSR. The hands should deflect more than halfway (Figure 9). Re-testing after swapping the two test leads, the hands should not move. If the needle deflects to the top or does not move regardless of the forward or reverse voltage access, the solid-state relay has been damaged.Figure 9. Solid State Relay  SSR You can also make a test circuit according to Figure 10. When the control voltage of the SSR input terminal is turned on, the light-emitting VD is on; when the control voltage of the SSR input terminal is cut off, the light-emitting diode VD is off.Figure 10. SSR Thermal Relay Test1) heating elements detectionThe heating element is composed of an electric heating wire or electric heating sheet, and its resistance is very small (close to 0Ω). The detection is shown in Figure 11. The normal resistance of the three groups of heating elements should be close to 0Ω. If the resistance is infinite (the digital multimeter displays the symbol "1" or "OL" for exceeding the range), the heating element is open.Figure 11. ① 200Ω gear is selected.② The red and black probes are respectively connected to the two ends of a heating element.③ The resistance is close to 0Ω, indicating that the resistor as a heating element is normal. 2) contact detectionThermal relays generally have a normally closed contact and a normally open contact. This detection includes working and non-working conditions. The first picture is the detection of the normally closed contact resistance when it is not in operation. Normally it should be close to 0Ω. Then the detection is taken in the opposite condition. Move the test rod, as shown in the second picture, simulates the over-current heating and bending of the heating element to make the contact action. The normally closed contact becomes an open circuit, and the resistance is infinite.Figure 12. ① 200Ω gear is selected.② The red and black probes are connected to both ends of the normally closed contact.③ The resistance is close to 0 Ω, indicating that the normally closed contact is closed.④ Move the test rod by hand.⑤ The out-of-range symbol "1" is displayed to indicate that the normally closed contact is open. Intermediate Relay TestThe electrical part of the intermediate relay is composed of coils and contacts, both of which use the resistance gear of a multimeter.1) The contact is detected when the control coil is not powered. Contacts include normally open contacts and normally closed contacts. When the control coil is power off, the normally open contacts are open and the resistance is infinite, at this time, the normally closed contacts are closed and the resistance is close to 0Ω. The above-mentioned detection of the normally open contact is shown in the figure below.Figure 13.① 200Ω gear is selected.② The red and black probes are connected to both ends of normally open contact.③ The out-of-range symbol "1" is displayed to indicate that the normally open contact is open. 2) Control coil detection of the intermediate relay is shown in Figure 14. Generally, the greater the rated current of the contact, the smaller the resistance of the control coil. This is because the greater the rated current of the contact, the larger the volume of the contact. Only a small control coil resistance (thicker line diameter) can flow through a larger current to produce a stronger magnetic field suction contact.Figure 14. ① 200Ω gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "6.60" indicates that the resistance of the control coil is 6.6kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then use a multimeter to detect the resistance of the normally open and normally closed contacts. The normally open contact should be closed and the resistance should be close to 0Ω; the normally closed contact should be open and the resistance is infinite. Time Relay TestThe detection of time relay mainly includes contact normal state detection, coil detection and coil energization detection.1) Normal-state detection of contacts. It refers to the detection of the resistance of the contact when the control coil is not energized. The normally open contact is open and the resistance is infinite, while the normally closed contact is closed, and the resistance is close to 0Ω. Normal detection processes are shown in the figure below.Figure 15. ① 200Ω gear is selected for the gear switch.② The red and black lead is connected with two pins of a normally closed contact.③ The resistance is close to 0Ω, indicating that the normally closed contact is closed. 2) Detection of control coil. It is shown in Figure 16.Figure 16. ① 20kΩ gear is selected for the gear switch.② Connect the red and black lead to the two pins of the control coil.③ The display of "4.93" indicates that the resistance of the control coil is 4.93kΩ.3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then check whether the contact status has changed according to the characteristics of types of the time relay. For example, for a delay time relay, after a period of time delay, check whether the delay contact is closed (resistance is close to 0Ω) and whether the delay contact is disconnected (resistance is infinite).  Ⅲ Relay for Life: Automotive Relay Test3.1 Automotive RelayRelays are widely used in automotive circuits, such as starting system circuits, wiper circuits, and rear window heating circuits. When the vehicle starts, a larger starting current is required. If the ignition switch is used for direct control, the starting contacts will ignite and burn, which will affect the service life of the ignition switch and even cause serious consequences such as line ablation and fire. Using a relay to control a large current with a small current will not cause the above problems. When a certain voltage or current is applied to both ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the core, yoke, armature, and the working air gap of the magnetic circuit. Under the action of the magnetic field, the armature attracts the pole face of the iron core, making the normally closed contact opens and the normally open contact close. When the voltage or current at both ends of the coil is less than a certain value, the mechanical reaction force is greater than the electromagnetic attraction force, and the armature returns to the initial state: the normally closed contact is on and the normally open contact is off. One of the automobile relays functions is a switch; the other is load overload protection; the third is fault protection. 3.2 Common Faults of Automotive RelaysIncluding coil burnt, short circuit, insulation part aging, contact ablation, etc.1) Relay MalfunctionWhen the controlled circuit is required to be closed, the relay will not act, on the contrary, when the controlled circuit is not required to be closed, the relay will act. This kind of problem occurs mainly because the interference voltage in the circuit exceeds the allowable range of the drive circuit of the relay. When designing the circuit, pay attention to the factors that can cause interference (such as chip command errors, short circuits, grid fluctuations, etc.). 2) Relay BurnedThere are many reasons for burnout. For example, the actual switching current exceeds the rated switching current of the relay, and the actual inrush current exceeds the rated switching current of the relay. According to design experience, in order to avoid these problems, the rated current should be selected to be 2-3 times the actual switching current, and the impact current of the relay is 2-3 times the actual current. 3) Contact WeldingGenerally speaking, the temperature rise of the AC conversion relay coil is higher than that of the DC conversion relay. This is because of the eddy current loss and hysteresis loss in the magnetic circuit. In addition, when the AC conversion relay is operating at a voltage lower than the rated voltage, a bounce phenomenon may occur. This will cause burnout, welding of contacts and damage to the relay, or disconnection of the self-protection circuit. Therefore, measures must be taken to prevent fluctuations in the power supply voltage.In addition, regardless of the length of the fluctuation time, it will cause the failure of the relay. So ensure that there is a power supply with sufficient capacity. 4) Coil Temperature Rise is Too HighThe loss of magnetic materials such as copper wires and iron cores or the heat transfer of the contacts will cause the temperature rise. Therefore, the heat resistance of the insulating material and the distance between the relay and the heat-generating device should be paid special attention in the circuit design. 3.3 Detection MethodStatic detection: check the resistance of the coil and the resistance of the normally closed contact.Dynamic detection: energize the coil and detect the resistance of the normally open contact. 3.4 Specific OperationTurn on the ignition switch and hear whether there is a pull-in sound in the control relay or feel the relay with your hands for vibration. If so, it means that the relay is basically in routine. The failure of the circuit may be caused by other reasons. On the contrary, it means that the relay is faulty.Replace the relay to be tested with an identical working relay. Turn on the switch, and if the electrical equipment is working normally, it can be determined there is a problem with the relay to be tested.Use the multimeter Rx100Ω gear and combine the resistance of each pin of the circuit to analyze. If the conduction and disconnection are normal, it means that there is no problem with the relay, otherwise, it means the relay is faulty.Open the relay shell to check whether the contacts are ablated or oxidized. If there are bumps and rust on the contact, it means that the contact is ablated or oxidized and does not work properly.Check whether the coil is ablated or discolored. If the coil is ablated with jelly, the coil is black or has a gluey smell, which means the coil is short-circuited by ablation.Ⅳ One Question Related to Relay Test and Going Further4.1 QuestionWhat are the symptoms of a bad car relay?4.2 AnswerThe car suddenly stalls while operating. One of the most common symptoms of a failed ignition relay is a car that suddenly stalls while operating. Car not starting. Another symptom of a faulty ignition relay is a no-power condition.Dead battery. A dead battery is another symptom of a faulty ignition relay.Burned relay. Ⅴ Frequently Asked Questions about Relay Test1. How do you check if a relay is bad?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 2. How do you test a 12-volt relay? 3. How do you check an overload relay with a multimeter?CEP7 Overload Relay test proceduresMeasure the normal motor running current (i motor).Turn off the motor and let it cool for about 10 minutes.Calculate the following ratio: i (motor) / i (overload min FLA).Set the overload to its minimum FLA and turn on the motor.Wait for the overload to trip. 4. How do I test a solid-state relay?The SSR can be tested as described below if a load is connected. Connect a load and power supply, and check the voltage of the load terminals with the input ON and OFF. The output voltage will be close to the load power supply voltage with the SSR turned OFF. 5. Can a bad relay drain your battery?Battery drain or dead batteryA failed ECM power relay can also cause a battery drain or a dead battery. If the relay shorts, it can leave power on to the computer, even when the vehicle is turned off. This will place a parasitic drain on the battery, which will eventually cause it to go dead. 6. What happens when the main relay goes bad?The engine will not startIf the main relay is not supplying the engine computer with the power it needs, then the engine will not be able to crank and run the right way. Failing to get the main relay replaced will usually lead to the car being unusable. 7. How do you test a battery relay? 8. How do you test a protection relay?Protection relay self-test procedureThis will normally involve checking the relay watchdog circuit, exercising all digital inputs and outputs and checking that the relay analog inputs are within calibration by applying a test current or voltage. 9. How do you check if a relay is working?The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 positions in the fuse box where the relay plugs in (or wherever the relay is located). 10. How do you test an electromagnetic relay?Grab a multimeter and set it to Ohms. Touch the leads across the electromagnet coil pins and measure resistance. Anywhere from 50-120 ohms is OK. Out of range or open means a bad electromagnet coil winding and time for a new relay.

IntroductionWhat is the difference between a three-phase AC motor and a single-phase AC motor? If you pay attention to it, you will find that single-phase AC motors have more equipment than three-phase AC motors, which is the start capacitor (starting capacitor). The most common one is in various household appliances. Almost all household appliances with motors are equipped with start capacitors. This article will start with the explanation of the principle of the motor start capacitor, and then describe in detail the failure phenomena, causes and test methods of the start capacitors in the two common home appliances, air conditioners and fans. In addition, the article will also explain some basic questions about start/run capacitors. If you want to learn the related knowledge of motor start capacitors, this article must be worthy of your reading.How to test a RUN or START CAPACITOR the CORRECT wayCatalogIntroductionCatalogI The Principle and Causes of Damage of Start Capacitor 1.1 How Does the Motor Work? 1.2 How Does the Start Capacitor Work? 1.3 Reasons for the Burning of the Start CapacitorII Troubleshooting of Start Capacitor in Air Conditioner 2.1 Functional Characteristics of Start Capacitor 2.2 How to Quickly Figure out Whether the Start Capacitor is Damaged 2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage? 2.4 Symptoms and Test MethodsIII How Test the Motor Start Capacitor of a Fan?IV Relevant Knowledge in the Step of Start Capacitor Test 4.1 How to Choose Start Capacitor? 4.2 Precautions for Replacing the Start CapacitorV How to Test the Motor Start Capacitor without a MultimeterVI Frequently Asked Questions about Start Capacitors 6.1 Which Motors are Served by the Start Capacitor? 6.2 Why Does a Three-phase Motor do not Need a Start Capacitor? 6.3 The Relationship Between Start Capacitors and Motor 6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?VII QuizⅧ FAQI The Principle and Causes of Damage of Start Capacitor1.1 How Does the Motor Work?The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a capacitor is needed to separate the phases. The purpose is to make the current in the two windings produce a phase difference of nearly 90 ゜ to generate a rotating magnetic field. The capacitive induction motor has two windings, namely the starting winding and the running winding. The two windings are 90 degrees apart in space. A large-capacity capacitor is connected in series to the starting winding. When the running winding and the starting winding pass single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding due to the action of the capacitor, thus reaching the maximum value first. Two identical pulsed magnetic fields are formed in time and space so that a rotating magnetic field is generated in the air gap between the stator and the rotor. Under the action of the rotating magnetic field, an induced current is generated in the motor rotor, and the current interacts with the rotating magnetic field. The electromagnetic field torque causes the motor to rotate.Figure1. Electric-Motor1.2 How Does the Start Capacitor Work?A single-phase power supply is different from a three-phase power supply in that a three-phase rotating magnetic field is generated in the motor. The principle of capacitor starting of single-phase motor is: using the principle that the current of the capacitor in the circuit is advanced by 90 degrees so that a magnetic field of 90 degrees ahead of the main winding is generated in the starting winding so that there will be an alternating 90-degree angle in the motor. The magnetic field, to put it plainly, uses the phase-shifting principle of the capacitor to transform a single-phase power supply into a two-phase power supply of 90 degrees to each other, and a rotating magnetic field of 90 degrees to each other is generated in the motor. Maybe it is easier to understand to explain from this aspect. The start capacitor is to give the motor a thrust when the motor is started so that the motor can turn from moving to rotating. Without it, when a single-phase AC motor starts, it will shake at the origin instead of rotating; the start capacitor is a two-phase AC motor, so the magnetic field cannot exert force on the rotor without it, and of course, it is impossible to rotate.Figure2. Capacitor Start Run Induction Motor1.3 Reasons for the Burning of the Start CapacitorGenerally, the start capacitor is not easy to burn, because its working time is very short, and it is thrown off by the centrifugal switch at the moment of starting, with no current flowing through the start capacitor. However, not easy to burn does not mean that it will never burn. If the start capacitor burns out, the possible reasons are as follows:① Capacitors have low voltage resistance or poor quality, so it is best to use capacitors with a voltage resistance of 500V. ② The centrifugal switch will often produce an arc when it is turned off. It is possible to burn the switch to the motor. After the switch is started, the switch cannot be turned off. There is always current through the capacitor. It is easy to burn the secondary winding of the motor and the start capacitor within a certain period of time. ③ The capacity of the selected capacitor is too small, and the starting current exceeds the allowable value of the capacitor. ④ The motor is bored or the bearing is damaged. It is difficult for the motor to start the centrifugal switch within a certain period of time and it is difficult to reach the disconnected speed, and the start capacitor is easy to burn.Figure3. Deteriorated CapacitorII Troubleshooting of Start Capacitor in Air Conditioner2.1 Functional Characteristics of Start CapacitorThe start capacitor is an important part of the auxiliary compressor to start. The capacitor is a large-capacity capacitor (1~6uF), which is used to provide starting current for the auxiliary winding of the motor to assist the compressor to start. The start capacitor is generally fixed on the bracket or support plate above the compressor, and the pin is connected to the starting end of the compressor.2.2 How to Quickly Figure out Whether the Start Capacitor is DamagedFirst of all, it depends on what kind of capacitors are used in electrical parts.● If the compressor start/run capacitor is damaged, the compressor cannot start or run intermittently.● If the capacitor of the fan motor is damaged, some malfunctions such as excessive exhaust temperature, excessive exhaust pressure, compressor overload, and small air output will occur during the operation of the air conditioner.2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage?The capacitor of the air conditioner compressor is installed in the outdoor unit. Due to the high temperature of the outdoor unit (ambient temperature + temperature emitted by the condenser), capacitors that have been used for a long time will easily dry up and fail the electrolyte.When replacing capacitors, be sure to use high-quality capacitors. The capacity must be the same, and the withstand voltage must not be lower than the original standard.Figure4. Air Conditioner Outdoor Unit2.4 Symptoms and Test Methods① Smoke.② Cannot operate normally.③ The display cannot be displayed normally.④ The power supply cannot be charged and discharged normally.⑤ Can not heat normally.⑥ The power factor compensates the capacitor damage, resulting in a waste of electricity. Different symptoms of damage to compressor start capacitor and external motor start capacitor:There are two capacitors in the outdoor electromechanical packaging, the larger one is the compressor capacitor, and the smaller one is the external motor capacitor. Different capacitors have different failure phenomena. ① Compressor capacitor damagethe performance phenomenon is that normal compressor startup will be accompanied by loud noise and jitter. If the capacitor is damaged, you will feel a slight jitter in the compressor by pressing the casing above the compressor, and the sound is like the sound of current passing. The compressor will stop running after a period of time. ② The damage of the external motor capacitorAfter the compressor is working normally, the external motor stops working for a period of time. If there is a fault code, there will be high-pressure protection and compressor exhaust pipe temperature protection.Figure5. Basic Electrical Controls of Air-Conditioning UnitsTest methods of air conditioner start capacitor● Method 1:The start capacitor of the air-conditioning compressor is a large-capacity electrolytic capacitor. When testing, using the capacitance setting of the digital multimeter to determine whether there is any abnormality.Under normal circumstances, the capacitance of the capacitor used to detect the capacitance of a multimeter should be the same or very close to the nominal capacitance, otherwise, the start capacitor is mostly deteriorating, such as dry electrolyte, leakage, etc., which should be replaced. ● Method 2:In addition to using a multimeter to test its capacitance, the ohmic setting of a pointer multimeter can also be used to test the charge and discharge performance of the start capacitor.Steps:① Connect the red and black test leads to the two poles of the compressor start capacitor.② The multimeter gear is set in ohm gear.③ Under normal circumstances, the pointer of the multimeter first swings to the right to a position, then slowly swings to the left, and finally stops at a fixed position.④If the pointer does not swing or the swing range is small, it indicates that the performance of the compressor start capacitor is poor.Suggested Reading: 5 Ways to Test Capacitors How to replace the start capacitor?If it is ensured that the air conditioner failure is caused by the damage of the compressor start capacitor itself, the damaged compressor start capacitor needs to be replaced. Replacing the start capacitor can generally be divided into three steps: removing the start capacitor, finding a replaceable start capacitor, and replacing the start capacitor. ①Remove the start capacitorThe compressor start capacitor is located on the circuit support board above the compressor. When disassembling, unplug the connecting wire and use a screwdriver to remove the fixing screw of the snap ring. ② Looking for replaceable capacitorsAfter removing the damaged compressor start capacitor, then select a suitable new start capacitor to replace it according to the specifications and volume of the damaged start capacitor.The specific content of how to select the start capacitor will be explained in detail below. ③Replace the start capacitorAfter selecting the compressor start capacitor, install the new compressor start capacitor in the outdoor unit, fix the metal fixing ring, reconnect the connecting cable, and then power on and test the machine to complete the replacement.Figure6. Motor Start CapacitorIII How Test the Motor Start Capacitor of a Fan?①Connect all parts on the circuit board of the air conditioner completely.②Turn on the power supply.③Use the remote control to adjust the temperature to make the fan motor rotate.④Connect the ground terminal of the oscilloscope probe to the ground terminal of the circuit board.⑤Use an oscilloscope probe to detect the white lead on the Hall element plug.⑥The oscilloscope shows signal waveform. When testing the start capacitor of the fan motor, a multimeter should be used to measure the resistance of the capacitor. Due to the large size of the capacitor, it is impossible to use the capacitor input jack for testing. At this time, you can use the multimeter pen to test and judge the quality of the capacitor by the change of the value displayed by the multimeter. If the resistance value displayed by the multimeter changes from small to large and then changes to infinity, it means that this is a good capacitor with charging and discharging functions. Check again after changing the test leads, the displayed value still changes from small to large, and changes to infinity. After measuring the capacitor, if the test leads are not replaced when the test is performed again, and the resistance value is displayed as infinite, it means that charging and discharging are not performed, but it does not mean that the capacitor is damaged. Therefore, when testing the capacitance, the test leads must be replaced no matter which multimeter is used.Figure7. Fan MotorRelated recommendation: How to Test Ceramic Disc Capacitor IV Relevant Knowledge in the Step of Start Capacitor Test4.1 How to Choose Start Capacitor?● How to calculate the starting and running capacitance of a single motorrun capacitance C=120000*I/2.4*f*U*cosφWhere: I is current; f is the frequency; U is the voltage; cosφ is the power factor, taking 0.5 to 0.7. The run voltage of the run capacitor is greater than or equal to (2～2.3) U.Start capacitor capacity = (1.5 ~ 2.5) run capacitor capacity.The run voltage of the start capacitor is greater than or equal to 1.42 U.(It is best when the voltage across the capacitor is 311V during operation) The working capacitor is 1-4UF per 100W, and the start capacitor is 4-10 times the working capacitor (the motor requires a larger starting torque). Empirical data, if your motor does not exceed 200W, the start capacitor will not exceed 100uF. If you run the capacitor, you can choose several values for the power-on test, and see which capacitor has the smallest current in the whole machine, then the capacity of the capacitor is the most Good value.) The capacity of the single-phase split-phase motor capacitor can be calculated by the empirical formula C=35000I/2PUfcos&Such as I=250W/220V=1.2AC=35000x1.2/2x1x50x220X0.8=24ufCan choose 350V30uf capacitance.Figure8. Starting Capacitor Table ● How to calculate the voltage across the run capacitor of a single-phase motor?① First of all, you must know the impedance value of the secondary winding. You can measure the resistance value by measuring the DC resistance with a multimeter. Then, the secondary winding is connected to a 12V AC voltage and the current value is measured. According to the winding impedance equal to the resistance and reactance in series, it can be calculated by phasor Out the winding inductance value. ② In normal operation, the capacitor is connected in series on the secondary winding, that is, the three equivalent parameters of winding resistance, winding reactance, and capacitance are connected in series and then connected to 220V voltage. It is easy to calculate the phasor according to the formula of the series circuit. Calculate the voltage on the capacitor. ③ When a single-phase motor is running, the voltage at both ends of the capacitor is generally above 300VAC, so the capacitor voltage is generally selected for a capacitor with a withstand voltage of 400V or more, and a capacitor with a voltage of more than 450V is better. ④ For the calculation of the capacitance withstand voltage, please refer to Article 2. First, measure the resistance R and reactance XL of the secondary winding, and then select the capacitance C according to the power of the motor to calculate the capacitive reactance Xc.The actual voltage across the capacitor during operation: Uc= Xc*220/(R+jXL-jXc); the withstand voltage value of the capacitor: Uce=1.3~1.5Uc.Figure9. Single Phase Motor Starting ● Detailed selection guide of start capacitor and run capacitorSingle-phase motor capacitor selection.Withstand voltage formula: U (capacitance) is greater than or equal to 1.5*USingle-phase run capacitor formula: C=1950×I/U×cosφ (using a capacitor, which is both a start capacitor and a run capacitor, is commonly used for small-capacity motors such as electric fans and washing machines)Start capacitor capacity formula: C=3500*I/U*cosφ (a capacitor is only used when starting, disconnected during normal operation, and switched with a transfer switch or a centrifugal switch.Dual-value capacitor run capacitor capacity formula: C=1200*I/U*cosφ (use 2 capacitors, one for operation and one for startup)Dual-value capacitor start capacitor capacity formula: C=(2～3)*C (run capacitor) C: Capacitor capacity: I: motor rated current, U: motor rated voltage, cosφ: power factor 0.7.Generally, there is no need to calculate. The run capacitor is 2~3μF per 100W, and the start capacitor is 2~3 times the run capacitor. The capacitor selection of the motor has strict requirements on the voltage, and it must be equal to or greater than 1.5 times the rated voltage of the motor. For a power supply with a rated voltage of 220V, the rated voltage of the capacitor cannot be lower than 400V. The capacitance value has a certain broadness, it doesn't matter if it is larger or smaller, especially the start capacitor, which can be selected at 2-6 times the working capacitor. ● How to choose the capacitor of single-phase asynchronous capacitor start the motorWe can calculate according to the following formulaPhase start capacitor capacity:C=350000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-the larger power factor is 2, and the smaller power factor is 4; cosφ---power factor (0.4～0.8).Split-phase start capacitor withstand voltage:The capacitor withstand voltage is greater than or equal to 1.42*U. Run capacitor capacity:C=120000*I/2p*f*U*cosφIn the formula: I---current;f-frequency;U---voltage;2p-take 2.4;cosφ---power factor (0.4～0.8). Run capacitor withstand voltage:The withstand voltage of the capacitor is greater than or equal to (2～2.3)*U.start capacitor capacity of double-value capacitor motor:C=(1.5～2.5)*operating capacitor capacity.Withstand voltage of start capacitor:The capacitor withstand voltage is greater than or equal to 1.42*U.4.2 Precautions for Replacing the Start CapacitorThe start capacitor is an important part of the electronic circuit. Once the start capacitor is broken, the motor cannot be started. The damaged start capacitor will only make a buzzing sound when it is energized for a short time, causing the current to surge, and long-time energization will cause severe overheating and even burn the motor, so it should be replaced immediately. And it is not difficult to judge that the start capacitor is broken. Most of the damaged start capacitors are bulging, and the surface will be burnt due to excessive current, and the rotor speed will be slow and weak. Of course, the most intuitive and accurate way is to use the capacitance setting of a multimeter to measure the quality. Once we have confirmed that the start capacitor has failed, the things that should be noted when replacing the start capacitor:① After the start capacitor is discharged, there will still be part of the residual charge that cannot be discharged for a while, and an artificial discharge should be performed again. ② Since the failed start capacitor may have poor lead contact, internal disconnection or fuse blown, etc., part of the charge may not be discharged. Therefore, the maintenance personnel should wear insulating gloves before touching the failed start capacitor. Using the short-circuit wire to short the two poles of the faulty capacitor first, and then it can be removed and replaced by hand. ③ If multiple start capacitors are used in series, they should be discharged separately.Figure10. Replace the Start Capacitor④ When handling or replacing a malfunctioning start capacitor, disconnect the power supply of the start capacitor, disconnect the switch or unplug the plug, and discharge the start capacitor. ⑤ When discharging, first connect the grounding terminal of the grounding wire, and then use the grounding rod to discharge the start capacitor several times until there is no discharge spark or discharge sound, and then fix the grounding terminal. ⑥ It should also be noted that general users often ignore the instruction manual, and the precautions for use must be carefully understood and followed during installation. As we all know, the impedance of a capacitor is inversely proportional to frequency. As the frequency increases, the loss also increases. Measures should be taken to limit the harmonics and inrush current in the circuit. Capacitors always generate heat, so pay special attention to ventilation and cooling. After the reactive power compensation device is installed, during the trial operation, the system should be tested, and measures should be taken in time if over-voltage, over-current, oscillation, and harmonics are found, which is very necessary for the normal operation of the capacitor. V How to Test the Motor Start Capacitor without a MultimeterA DC voltmeter can be connected to the capacitor in parallel, and an insulating shaker can be used to charge the capacitor (note the + and-poles)(1) See if the voltage can rise to the rated voltage of the capacitor:① 0V, the capacitor is short-circuited.② Slowly rise to the rated voltage of the capacitor, it proves that the capacitor is good.③ Raised quickly to the rated voltage of the capacitor, and the insulation resistance is about the internal resistance of the DC voltmeter, then the capacitor is open.(2) When it is stable at the rated voltage value of the capacitor, look at the insulation resistance of the capacitor:①The insulation resistance is close to the internal resistance of the DC voltmeter, so the capacitance is good.②If the insulation resistance is less than the internal resistance of the DC voltmeter, it means that the leakage of the capacitor is large, and it is easy to generate heat and cannot be used.VI Frequently Asked Questions about Start Capacitors6.1 Which Motors are Served by the Start Capacitor?Although some electrical appliances seem to have similar principles, they are different in the selection of motors, such as electric fans and air conditioners. Most electric fans use single-phase motors. Single-phase motors have only one 220v live wire and one neutral wire, while air conditioners often uses the three-phase motor, which has three wires, 220v live wire, neutral wire, and 380v live wire. The most obvious difference between a single-phase motor and a three-phase motor is that the number of start capacitors is different. A single-phase motor is equipped with a start capacitor, while a three-phase motor has no start capacitor.6.2 Why Does a Three-phase Motor do not Need a Start Capacitor?Because the three-phase motor itself has three running windings and can generate a magnetic field by itself, the appearance of the magnetic field can effectively replace the start capacitor, so the three-phase motor is generally not equipped with a start capacitor. However, the start capacitor still plays an irreplaceable role in a single-phase motor, because there is only one running winding in a single-phase motor, which cannot form a rotating magnetic field, and the operation of electrical appliances can only rely on the start capacitor. In addition to the start capacitor in a single-phase motor, there is also a run capacitor. Although these two capacitors work together, the function of the start capacitor is much greater than that of the run capacitor, so once they start capacitor is damaged, the fan will make a lot of noise, The blade speed is reduced. If this happens to your electric fan, you might as well try to replace a start capacitor, the problem should be solved.6.3 The Relationship Between Start Capacitors and MotorAt present, in single-phase motors with low-power motors, the start capacitor is connected in series with the starting coil and then connected in parallel with the running coil to work at the same time. In order to speed up the start-up time of the high-power motor, a large capacitor to help start is added. After the motor is started, the additional large start capacitor is disconnected by the centrifugal switch. The smaller capacitor connected in series with the starting coil is responsible for the phase shift required during normal operation. Electric current, the power supply machine is operating normally. Is there a single-phase motor that is connected to the starting coil and connected in parallel with the running coil in the circuit from start to run and does not require other large capacitors to help start? Low-power motors are always used in the circuit. High-power motors have to add additional capacitors due to their large power and large starting distance.Figure11. Torque-speed characteristic6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?The start capacitor is used for phase separation, and the purpose is to make the current in the two windings produce a phase difference close to 90 ゜ to generate a rotating magnetic field, allowing the motor to run quickly in a static state. There is an automatic clutch switch in the motor. When the motor is started, the motor will continue to run due to inertia. When the speed reaches a certain speed, the start capacitor will be separated by centrifugal action and automatically connected to the run capacitor, and the motor will enter the normal working state. The function of the run capacitor is to keep the current in the two windings with a phase difference of 90° to generate a continuous rotating magnetic field. For motors with start capacitors, the rotational torque generated by the start capacitors is larger than that of the run capacitors, which is more suitable for starting with a load. Motors without start capacitors are not suitable for starting with a larger load.VII QuizThe starting capacitor of a single phase motor is(A) Electrolytic capacitor(B) Ceramic capacitor(C) Paper capacitor(D) None of the above.Answer: AⅧ FAQ1. What happens when a start capacitor goes bad?A motor connected to a run and start capacitor may still attempt to start if one or both of the capacitors has failed, and this will result in a motor that hums and will not remain running for long. ... In most cases of capacitor problems, such as damage or a loss of charge, the capacitor will need to be replaced. 2. What's the difference between a run capacitor and a start capacitor?The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current. A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. 3. How do you test a start capacitor with an ohmmeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 4. How to test a motor start capacitor?Motor run capacitor failure symptoms include warm air flowing from the vents inside the home, the air conditioner taking more time than usual to kick on or it turns off before it is programmed to, or there is a constant low hum emitting from the machine that isn't typical. 5. How do I test a capacitor with a multimeter?To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. 6. How do you check if a capacitor is bad with a multimeter?If the capacitance value is within the measurement range, the multimeter will display the capacitor's value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty. 7. How do I test a capacitor without a multimeter?Just connect those two ends of the capacitor to a single-phase supply and switch it ON for a few seconds. Then take that two-terminal and short it, you will get a spark. 8. How can I check a capacitor?Put the Analog Multimeter in the Ohmmeter position and if there are multiple ranges, choose a higher range. Connect the leads of the capacitor to the multimeter probes and observe the readings on the multimeter. For a good capacitor, the resistance will be low in the beginning and will gradually increase. 9. Can I replace a start capacitor with a run capacitor?Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). A start capacitor can never be used as a run capacitor because it cannot handle current continuously. 10. Which is bigger, start or run capacitor?A lot of torque is necessary to start up an AC system, so a start capacitor will have greater capacitance than a run capacitor. 

Ⅰ IntroductionThe vacuum tube was an essential part of early radios, used to create and amplify the electrical signals needed for the radio to work. The vacuum tube radio was a new stuff of the early 20th century, and immediately became the "new favorite" of that era with the launch of the radio show. Due to the continuous development of technology and the emergence of transistors, vacuum tubes were swept away by the powerful transistors in the 1960s and 1970s. With the improvement of people's lives, they have a new understanding of sound quality, and people are once again interested in the beautiful sound of vacuum tube radios.Vacuum Tube Radio RepairCatalogⅠ IntroductionⅡ Vacuum Tube Radio Circuit2.1 Input Circuit2.2 Converter Stage2.3 Detection Stage2.4 Intermediate Amplifier Stage2.5 Audio Voltage Amplifier2.6 Audio Power Amplifier Stage2.7 RectifierⅢ Classification3.1 AM Radio3.2 Heterodyne Radio3.3 FM Radio3.4 Service LifeⅣ Audio Maintenance PrincipleⅤ Specific Troubleshooting5.1 Tube Inspection5.2 Resistor Inspection5.3 Capacitor Inspection5.4 Transformer InspectionⅥ One Question Related to Vacuum Tube Radio and Going FurtherCompared with the semiconductor radio, the biggest advantage of the tube radio is that the sound quality is obviously better than that of the semiconductor radio. The second is that it is stable and elegant, with classical temperament. In addition, the history of tube radios is far greater than that of semiconductor radios, which are more valuable for collection. Ⅱ Vacuum Tube Radio CircuitFigure 1. 12AV6 Vacuum Tube Radio Circuit2.1 Input CircuitThe circuit from the antenna of the radio to the input of the first-stage tube is called the input circuit. It has two tasks to complete: One is to transmit the high-frequency signal voltage induced on the antenna to the grid of the first electron tube (usually the frequency conversion stage). To accomplish this task, there should be a certain coupling between the radio antenna and the input circuit to facilitate signal transmission. Second, because the antenna induces a lot of signals, it is necessary to select the radio broadcast signal you want to listen to  suppress unnecessary signals. To accomplish this task, the input circuit should be composed of a selective resonant circuit, so the input circuit should compose of coupling elements and the resonant circuit.Depending on the coupling form of the antenna and the resonance circuit. The input circuit can be divided into three types: inductive coupling (transformer coupling) circuit, capacitive coupling circuit, and inductive capacitive coupling circuit. Since the voltage transmission coefficient of the inductive coupling circuit is relatively high and uniform, tube radio usually adopts this type. In order to improve the suppressing ability of mid-frequency interference (because the intermediate-frequency detuning is smaller for medium wave, and it is larger for short wave.). An intermediate-frequency trap circuit is also applied in the antenna loop.2.2 Converter StageThe converter stage is located between the input circuit and the intermediate amplifier stage. Its function is to change the high-frequency amplitude modulation signal into a fixed intermediate-frequency amplitude modulation signal. In order to complete the task, the frequency converter should be composed of four parts: oscillator (generating a high frequency constant-amplitude oscillating voltage with an intermediate-frequency different from the frequency of the external radio signal), a high frequency circuit (used to select the radio signal, usually it is the input circuit), non-linear components (usually pentagrid converter, used to change the frequency), the intermediate-frequency loop.The converter stage of a tube radio is generally composed of a pentagrid converter tube. The high-frequency circuit is connected to the third grid (signal grid). The oscillator is composed of the screen grid (oscillation anode), the first grid (oscillation grid) and the cathode. The anode load is an intermediate frequency resonance circuit. Therefore, the main mark that distinguishes the converter stage from the tube amplifier is the pentagrid tube and three resonant circuits. The frequency converter can be divided into single-grid converter and double-grid converter according to the different ways of inputting signal voltage and local oscillation voltage. The single-grid inverter applies the signal voltage and the local oscillation voltage to the same electrode of the inverter tube, so the traction effect is large, and the operation is unstable. The double-gate inverter adds the signal voltage and the local oscillation voltage to the different electrodes of the inverter tube separately, the traction effect is greatly reduced, thus double-bar frequency converters are commonly used in tube radios.The structural feature of pentagrid tubes is that there are two metal sheets on the second grid, called the collection screen. It blocks the electrons moving to the anode. There are two metal rods on the third grid in the gap of the collecting screen, which carry negative potential. When the electrons passing through the second grid hit the negatively charged metal rod while advancing, the electrons flying to the anode is divided into two streams, one flying to the anode, and the other being collected by the collecting screen without returning to the cathode. In this way, when the signal grid voltage changes, it will only affect the distribution of the two currents, and has little effect on the total current. Because the local oscillator adopts the total current feedback, the oscillator is basically not affected, so the oscillation frequency is stable. There are many types of frequency converter circuits with different performances, but their principle difference lies in the way of local oscillation feedback. 2.3 Detection StageThe detection stage is located between the intermediate amplifier stage and the audio voltage amplifier stage. Its task is to transform the intermediate-frequency signal into the original modulated audio signal. To complete the task of frequency conversion, the detector must consist of an intermediate-frequency signal circuit (it is the output circuit of an intermediate-frequency amplifier), a non-linear elements (usually a diode), and a load (resistor). 2.4 Intermediate Amplifier StageThe intermediate amplifier stage is located between the frequency converter stage and the detection stage, and specifically amplifies the 465kHz intermediate-frequency signal. The main feature of the intermediate amplifier stage is: in order to achieve automatic gain control, the intermediate amplifier tube generally uses a remote cut-off tube. The intermediate-frequency transformer is the main component of the intermediate-frequency amplifier, and its quality directly affects the quality of the radio. In addition, a shielding cover is usually used to avoid the parasitic coupling between the intermediate-frequency loop and the eternal to improve the working stability. The shielding cover is made of good conductors such as aluminum and copper, and is attached to the case (ground potential), to isolate the entry of high frequency electromagnetic fields. 2.5 Audio Voltage AmplifierTo make the power amplifier stage output a certain amount of power, its anode current must have a certain amount of AC component, which requires that the grid audio signal voltage used to control the anode current must have a sufficiently large amplitude (voltage). The output voltage of the detector stage is usually very low. Therefore, between the power amplifier stage and the detector stage, there is always one or several audio voltage amplifier stages. Its function is to amplify the audio signal voltage to meet the input signal requirements.The audio voltage amplifier stage can be divided into a resistance-capacitance coupling amplifier and a transformer-coupled amplifier according to the different anode load forms. In the vacuum tube radio, the resistance-capacitance coupling amplifier is used mostly. The audio voltage amplifier stage and the detection stage usually share an electron tube. The input signal is generally controlled by a volume control potentiometer to adapt to different broadcasts and listening situations. Some radios have a tone control circuit connected to the output end of the audio voltage amplifier stage to flexibly change the frequency characteristics of the amplifier to make the sound pleasant. Due to the different forms of volume and tone control circuits. There are many types of audio voltage amplifier stages. 2.6 Audio Power Amplifier StageThe audio power amplifier stage is the last stage of the tube radio, and its function is to output a certain amount of audio power to make the speaker work normally. Someone may wonder, can a voltage amplifier be used instead of a power amplifier? This will not work. Although there is no difference between the basic circuits of a power amplifier and a voltage amplifier, due to the different tasks of the two, there are significant differences in the selection of electron tubes, the circuit components, and the nature of the load. It should be pointed out that the so-called power amplifier does not amplify the input power itself, but uses the control function of the electron tube grid voltage to convert the power supplied by the anode circuit DC power supply into the required audio power. From the viewpoint of energy conversion, the power amplifier is an energy converter. 2.7 RectifierThe rectifier stage of the tube radio is composed of three parts: power transformer, rectifier tube and filter. There are many forms of rectifier circuits, and the most common is a full-wave rectifier circuit.Ⅲ Classification3.1 AM RadioIn the era when tube radios were popular. AM radios were the mainstream products. Amplitude modulation is to modulate the high frequency carrier with audio signal. Its waveform is symmetrical up and down, and the amplitude is the same as the modulated signal. After detection, the highfrequency component is filtered out to obtain the audio signal. The frequency of the carrier signal (the frequency of the broadcasting station) is the carrier frequency.AM radios can receive medium wave and short wave broadcasts, and some can receive long wave broadcasts. Since the mid-band frequency interval has been unified to 9KHz, its highest audio frequency is only 4KHz. The sound quality is affected, and electromagnetic interference is relatively large.There are two main types of AM radios: direct-amplifier type and heterodyne type.1)Direct-amplified radio, also known as high-amplification radio, its typical circuit structure is such as: High-amplification→Detection→Low-amplification→Power amplifierA circuit that uses a grid detector circuit and a high-frequency positive feedback, called a regenerative radio, can achieve better sensitivity and selectivity. It can receive AM telegraph signals with high-amplifier and short-wave. Old Japanese-made radios mostly have this circuit. High-frequency signals of direct-amp radios are prone to self-excitation, and the high-end and low-end gains are uneven, and there is no high-amplifier regeneration. Widespread use of reed speakers with poor sound quality made it obsolete. Out of nostalgia, some enthusiasts are still keen on getting recycled radios. The regenerative circuit can not only be applied to direct-amplified radios, but also in simple heterodyne radios, because proper positive feedback can also be used to improve sensitivity.Simple regenerative radios often use reed speakers, which have high impedance (about 10K) and high sensitivity, and can be directly used as the load of the power amplifier tube. Its frequency range is only 350~3000Hz, so the sound quality is poor. Later regenerative radios used moving coil speakers, and the sound quality was better. However, due to the low impedance, an output transformer is needed, and its primary impedance must match the load impedance of the power amplifier tube. Moving coil speakers are divided into permanent magnet, constant magnet and excitation type. In addition, excitation horns are used in AC tube radios, and their excitation coils can also be used as filter chokes. 3.2 Heterodyne RadioThe heterodyne radio uses a frequency conversion circuit. The signal generated by the high-frequency oscillation circuit is different from the input signal at a certain frequency. After the two are combined, a fixed intermediate frequency signal (455~465KHz) is generated. The oscillation circuit needs independent vacuum tube as a part before the emergence of the dedicated frequency conversion tube. Some people call the oscillation frequency higher than the signal frequency by a heterodyne type.Heterodyne plus intermediate-frequency amplifier circuit is called superheterodyne. This type of circuit requires a single electron tube to oscillate. Later a multi-pole or composite tube dedicated to frequency conversion appears, such as 1A2, 6A2, 6SA7GT, 6U1, 6K8, etc. The superheterodyne type is the most common circuit of commercial radios. It has an automatic volume control circuit and can add tuning instructions. The superheterodyne radio can obtain more stable and higher gain due to the fixed frequency amplified. The disadvantage is that there is image frequency interference.The circuit structure of a typical superheterodyne radio is as follows:Frequency conversion→Middle amplifier→Detection→Low amplifier→Power amplifier 3.3 FM RadioFM is to use audio signal to modulate the frequency of high-frequency carrier. Its advantages are strong anti-interference ability, high signal-to-noise ratio, good frequency bandwidth and sound quality, and the audio frequency can reach 20~15000Hz. Because the FM audio work in the ultra-high frequency band, it can accommodate many radio stations. Because of its linear propagation characteristics, the same frequency can be reused at a distance of hundreds of kilometers, which can effectively solve the problem of congestion of medium and short-wave radio stations.Modern FM broadcasting is compatible with stereo and mono (in the early days of stereo broadcasting, two frequencies were used and received by two radios). Amateurs like to use a simple super-regenerative circuit to receive FM broadcasts. Because it works in a self-oscillation state, it is unstable and has strong super noise. 3.4 Service LifeThe general design life of the valves is 2000 hours (special tube has 5000~10000 hours), but in reality, many electron tubes break after using 2000 hours, for example, there is no sound. Its performance is not up to the design standard. When there is not any sound from the actual tube, it may have reached 3000 hours or more. The actual service life of the amplifier tube of the front stage is longer. To cite the simplest example: the old picture tube (picture tube is also a kind of vacuum tube) of a TV set, the actual life span usually excesses 10 years.If use it for 5 hours a day, you need to change the tube once a year to meet the actual design performance, and the replaced tube can also be used for other purposes.Ⅳ Audio Maintenance PrincipleHow to repair the failure of the vacuum tube radio depends on the difficulty of the failure. Under normal circumstances, the following steps are used.First point is to seek clues. What is the difference in the use of the radio before and after the failure, thinking about whether it has been repaired, or what components have been replaced, etc.Second point is the fault display. For faults such as flashover, smoking, burning, etc. Turn off the power immediately and find the factor to avoid damage to the audio. According to the displayed failure phenomenon, rotate audio button to compress the failure to clarify where the failure may occur, and to provide necessary information for analysis and judgment.Third point is analysis and judgment. Make comprehensive analysis and judgment based on the information obtained by self-examination inference and fault display. List all possible direct causes of the failure, and develop a scientific inspection procedure. This not only prevents blind movement, but also accumulates maintenance experience and improves efficiency.Fourth point is troubleshooting. Gradually narrow the scope of the fault, and accurately detect the fault point. Sometimes it refers to (such as electron tubes, capacitors, resistors, coils, transformers, etc.) connecting wires, welding points of a certain component. Determine where the failure occurred, which is important for fault repair. Tube radios are usually divided into high-frequency part, intermediate frequency part, audio part, power supply part and auxiliary circuit according to their working frequency and circuit function. The fault should be compressed to a specific circuit, such as DC circuit, AC circuit, or the anode circuit, the screen grid circuit, grid circuit, cathode circuit, etc. Since each loop in the electronic tube circuit affects each other, a failure of a component may affect several loops at the same time. Therefore, in the inspection process, comprehensive analysis should be performed, that is, each circuit should not be isolated.Fifth point is repairing practice. The repair test can be performed after determining the cause of the failure.Sixth point is repair inspection. In the actual maintenance process, if a failure point is detected, it must be immediately eliminated and repaired. After all the faults are investigated, the radio should be inspected as necessary to ensure the quality of the repair. The specific content of the inspection depends on the specific fault and the type and quantity of the instrument. The simplest one should also be auditioned and take general technical observations.Ⅴ Specific TroubleshootingA radio is composed of many components, and component damage is the main cause of radio failure. When the fault location is gradually reduced, the quality of the components must be checked to determine the fault. Therefore, inspecting components is one of the main methods to find the cause of failure.5.1 Tube InspectionThe common faults of electronic tubes are mostly filament broken, poor contact of the electric board, broken pole, pole touching, leakage, breakdown between the cathode and the filament, aging, air leakage, and micro-sound effects. When the radio is turned on, if the filament of a certain tube does not light up, the filament may be broken or a serious leak. The tube shell is cold when the filament is broken, and the tube shell is often warm when the gas leaks. Unplug the tube for further inspection. Use a multimeter to measure the filament resistance at the "R×1"gear. If the meter indicates infinite, the filament is disconnected. If the indicator of the needle is zero, it means that the tube is short-circuit. When the indicator of the needle indicates a constant value, it means that there is leakage between the electrodes. The smaller the resistance, the more serious the leakage. At this time, the top of the tube is usually milky white.The electron tube electrode is in poor contact, intermittently electrode touching (some are hot electrode touching), and the radio will sometimes have no sound or noise. This type of failure is characterized by being greatly affected by vibration. Therefore, you can tap the tube shell lightly. If there is no sound or noisy sound (sometimes there may be no sound or normal sound after tapping), it means that the tube electrode is in poor contact or intermittently electrode touching. When the fault is serious, the electrode is completely disconnected, which means a broken pole. The tube does not work, and the radio is silent.When the electron tube ages, the positive current and the amplification will decrease. If it was frequency conversion tube, the local oscillator will oscillate abnormally, resulting in the radio cannot receive the radio signal, low sound, or the high frequency end of the band can receive one or two radio stations, and the low-frequency end cannot receive the radio signal. The micro-phonic effect of the vacuum tube is often caused by improperly fixing the electrode, and it can be judged by tapping the tube case lightly. Sometimes it is difficult to determine whether the valve is normal or not. The easiest way is to replace it with the same type of electron tube. If the failure disappears, it means that there is a problem with the vacuum tube. 5.2 Resistor InspectionThere are two types of resistors used in radios: fixed resistors and variable resistors (potentiometers). They are not easy to damage. Possible damages include: resistor burnout, resistor body broken or lead broken, internal open circuit, or poor lead contact, etc. Some of failure can be observed. For example, the increase of fault current in a certain part of the circuit, which means the resistor is burned out. At this time, the paint layer on the outside of the resistor is burnt and hot. If the resistor is internally disconnected or the resistance value changes, it can be checked by measuring the ohm gear of a multimeter. If the meter indicates infinity, it means that the resistor is internally open; if the measurement result is too far from the nominal value (the error exceeds 20%), it means that the resistance has deteriorated.The lead of the resistor body is in poor contact and will be broken unbroken, which will cause noise or sound from the radio. This kind of failure is very susceptible to vibration and produces vibration noise. Therefore, you can gently shake the resistor body to see if the sound changes. It can also be measured with a multimeter. When measuring, shake the resistor body. If the pointer is unstable, it means that the contact is bad.The potentiometer that controls the volume of the radio usually has a power switch. The common faults of the potentiometer are: dirty carbon film, wear of the carbon film, poor contact of the sliding contact, loose lead-out piece, burnt of the carbon film, broken bakelite frame of the sliding contact, leakage, and damage to the power switch. When the potentiometer is normal, the radio can only hear a very light sound after power on. If the potentiometer is closed, the broadcast voice should not be heard. After turning off the power, there is no sound. If you cannot control the power on and off, it indicates that the power switch is partially damaged; if the noise is very loud, especially when the volume is adjusted, the loudspeaker has a large sound or interrupted sound, it means that the potentiometer is in poor contact. 5.3 Capacitor InspectionThere are two types of capacitors used in radios: fixed capacitors and variable (semi-variable) capacitors. The faults of fixed capacitors are usually leakage, breakdown, internal open circuit, poor internal contact, broken lead, or capacity failure, etc. The common faults of variable capacitors are bump, short circuit, leakage and so on. 5.4 Transformer InspectionCommon faults in transformers are coil mold breakage, a partial short circuit, leakage and other faults. A multimeter can also check out the coil mold. When high-frequency coils and intermediate-frequency transformers are locally short-circuited, it is not easy to measure. The substitution method can be employed to test or adjust the loop frequency to figure problems out.Ⅵ One Question Related to Vacuum Tube Radio and Going Further6.1 QuestionWhy do vacuum tubes sound better?6.2 AnswerTubes sound better because their distortion products are more musical. Those are the fundamental reasons why tubes simply sound better. Vacuum tubes are the more linear and require less feedback. Tubes are voltage amplifiers as opposed to transistors which are current amplification devices. Frequently Asked Questions about Vacuum Tube Radio1. What is a vacuum tube radio?VACUUM TUBE RADIO. Vacuum Tube Radio. By the late 1920s, vacuum tube radio equipment replaced the primitive spark-gap systems on most merchant ships. This new equipment could send and receive signals virtually worldwide, by using high frequency or "short-wave" bands. 2. How do radio vacuum tubes work?The basic working principle of a vacuum tube is a phenomenon called thermionic emission. It works like this: you heat up a metal, and the thermal energy knocks some electrons loose. ... When the cathode is heated, and a positive voltage is applied to the anode, electrons can flow from the cathode to the anode. 3. Why are vacuum tubes better for audio?Tube amplifiers sound better because of the euphonic distortions they add to the music, as well as plenty of other reasons I'll cover below. ... We use tubes simply because they make the music we create sound better: smoother, warmer and cleaner. Ditto for guitar amplifiers used in creating music. 4. How do you know if a vacuum tube is bad?When a vacuum tube develops an air leak (a small crack or bad seal by a pin for example) this getter color will change to pure white. If you see this you know with 100% certainty that the tube is bad. Third, look for a purple glow that is very focused around specific elements inside the tube.