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IntroductionThyristor, commonly known as silicon controlled rectifier(SCR), its normative term is reverse blocking three-terminal thyristor. Thyristors are high-power semiconductor devices that have both switching and rectifying functions, and are used in various circuits such as controllable rectification and frequency conversion, inverters, and non-contact switches. As long as it is provided with a weak point trigger signal, it can control the strong electric output. So it is a bridge for semiconductor devices to enter the field of strong electricity from the field of weak electricity. So far, thyristors are the most widely used semiconductor devices in the electronics industry. Despite the continuous emergence of various new semiconductor materials, 98% of semiconductor materials are still silicon materials, which are still the basis of the integrated circuit industry. It is widely used due to its small size, light weight, high power and long life.Intro to Thyristors: the SCRCatalogIntroductionⅠ Thyristor Basics1.1 Brief Introduction of Thyristor1.2 Working Principle of  ThyristorⅡ The Main Characteristics of Thyristors2.1 Basic Structure of Thyristor2.2 Volt-ampere Characteristics of Thyristors2.3 Static Characteristics of Thyristors2.4 Characteristic Equation of ThyristorⅢ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional Thyristors3.2 Main Parameters of TRIACⅣ Main Function of ThyristorⅠ Thyristor Basics1.1 Brief Introduction of ThyristorThyristor, also called silicon controlled rectifier, is an abbreviation of semiconductor thyristor. It is a high-current switching semiconductor device that uses small currents to control. There are two commonly used types: ordinary thyristors (also called unidirectional thyristors) and TRIAC(triode for alternating current). Because of its small size, light weight, high efficiency, long life, vibration resistance and because it is noiseless, easy to use, it has attracted great attention from domestic, foreign, industrial and agricultural production departments in a short period of time and has been widely used in various production equipment and household appliances. According to its working principle, it can be roughly divided into four categories: f— Rectification: change AC power into adjustable DC power. — Inverter: converts DC power to AC power with a certain frequency. — DC switch: used for DC loop switch or DC voltage regulation. — AC switch: used for AC loop switch or AC voltage regulation. According to its service objects, it can be used in industries, agriculture, national defense, transportation, mining, metallurgy, light industry, chemical industry and other departments.In performance, thyristors not only have unidirectional conductivity, but also have more valuable controllability than silicon rectifier elements (commonly known as "dead silicon"). It has only two states: on and off.Thyristors can control high-power electromechanical equipment with milliamp currents. If the frequency exceeds this value, the average switching current allowed to pass will decrease due to the significant increase in the switching losses of the components. At this time, the nominal current should be degraded.Thyristors have many advantages, such as: controlling high power with low power, power amplification multiples up to several hundred thousand times; extremely fast response, turn on and off in microseconds; non-contact operation, no spark, no noise; high efficiency, low cost and so on.Disadvantages of thyristors: poor static and dynamic overload capacity; easy to be misguided due to interference.The two types of thyristors, unidirectional thyristors and three-terminal TRIAC, are briefly introduced below.1.2 Working Principle of  Thyristora. Unidirectional ThyristorThe internal structure of the unidirectional thyristor is shown in figure 1 (a). It can be seen from figure 1 (a) that the unidirectional thyristor is composed of four layers semiconductors P1N1P2N2. There are three PN junctions in the middle: the junction J1, J2, and J3. The anode A is drawn from P1, the cathode K is drawn from N2, and the control electrode (or gate) G is drawn from the middle P2. The circuit symbol of the unidirectional thyristor is shown in figure 1 (b). Figure 1. Schematic Diagram and Circuit Symbol of Unidirectional ThyristorIn order to understand the working principle of the unidirectional thyristor, the unidirectional thyristor can be equivalently regarded as a combination of a PNP transistor T1 and an NPN transistor T2. The middle layer P2 and layer N1 are shared by two transistors. The anode A is equivalent to the emitter of T1, and the cathode K is equivalent to the emitter of T2, as shown in figure 2. Figure 2. Working Principle of Unidirectional ThyristorThe key to understanding how unidirectional thyristors work is to understand the role of the control electrode.(1) No voltage or reverse voltage is applied to the control electrodeWhen the control electrode is left floating or a reverse voltage is applied between the control electrode and the cathode, that is, UGK<0, there must be IG=0. If a reverse voltage is applied between the anode and the cathode, that is, UAK＜0. Due to J, and J2, the transmitting junctions of T1, T2, are both reverse biased and T1 and T2 are in the off state, at this time, the current flowing through the unidirectional thyristor is only the reverse saturation current of the J1 and J3, IA≈0, and the unidirectional thyristor is in the blocking state; if a forward voltage is applied between the anode and the cathode, that is, UAK>0, J2 is in a reverse biased state, because IG=0, T2 must be in the off state. and the current in the unidirectional thyristor is only the reverse of J2. At this time, the current in the unidirectional thyristor is just the reverse saturation current of J2, IA≈0, and the unidirectional thyristor is still in the blocking state. Therefore, when no voltage is applied to the control pole or reverse voltage is applied, IG = 0, the unidirectional thyristor is in a blocking state, and has positive and negative blocking capabilities.(2) Apply forward voltage to the control electrodeWhen a forward voltage is applied between the control electrode and the cathode, that is, UGK> 0, the emitter junction J3 of T2 is in a forward bias, and IG≠0. If a reverse voltage is applied between the anode and the cathode, that is, UAK <0, because the emission junction J1 of T1 is reverse biased and T1 is in the off state, the unidirectional thyristor is in the blocking state, IA≈0; If a forward voltage is applied between the anode and the cathode, that is, UAK> 0, because the emission junctions J1, J3 of T1, T2 are forward biased, and the collector junction J2 is reverse biased, T1, T2 will be in an amplified state. After IG is amplified by T2, the collector current of T2 is IC2 = β2IG. The collector current of T2 is the base current of T1, after being amplified by T1, the collector current of T1 is IC1 = β1β2IG. This current flows into the base of T2 for amplification, and in this cycle, a strong positive feedback is formed, which makes T1, T2 quickly enter the saturation state, and the unidirectional thyristor is in the on state. After the unidirectional thyristor is turned on, UAK, the value of the voltage between the anode and the cathode is very small, and the external power supply voltage is almost completely dropped on the load.(3) Turn-off of the unidirectional thyristorFrom the above analysis, it can be seen that after the unidirectional thyristor is turned on, the base of T2 always has the collector current IC1 of T1 flowing, and the value of IC1 is much larger than the IG applied at the beginning. So even if the control electrode voltage disappears and IG = 0, it can still rely on the positive feedback of the tube itself to maintain conduction. Therefore, once the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. After the unidirectional thyristor is turned on, if you want it to turn off again, the anode current IA must be reduced so that it cannot maintain positive feedback. To this end, the anode can be disconnected or a reverse voltage can be applied between the anode and the cathode.To sum up, under the condition that a forward voltage is applied between the anode and the cathode of the unidirectional thyristor, if a forward voltage is added between the control electrode and the cathode at a certain time, the unidirectional thyristor will change from the blocking state to the conducting state. This is triggered into conduction. After the unidirectional thyristor is turned on, the control electrode will lose the function of controlling. If you want to turn off the unidirectional thyristor again, you must make its anode current less than a certain value IH (called the holding current) or reduce the voltage UAK between anode and cathode to zero. b. TRIACA TRIAC is a three-terminal element with a five-layer structure of N1P1N2P2N3. It has three electrodes: a main electrode A1, a main electrode A2, and a control electrode (or gate) G. It is also a gate control switch. Regardless of its structure or characteristics, it can be regarded as a pair of anti-parallel ordinary thyristors. Its structure, equivalent circuit and symbols are shown in figure 3. Figure 3. Symbol, Structure and Equivalent Circuit of the TRIACThe main electrodes A2 and A1 of the triac are connected in series with the control object (load) RL, which is equivalent to a non-contact switch. The "on" or "off" of this switch is controlled by a signal uG (called a trigger signal) on the control electrode G. When there is a voltage (u ≠ 0) between the main electrodes A2 and A1, the moment the trigger signal uG appears, it will be conductive between A2 and A1 of the TRIAC, which is equivalent to the closed state of the switch. And once it is turned on, even if uG disappears, it can be kept on until u = 0 or the current in the series circuit of the main electrode and the load is reduced to a certain value, then it is turned off. After the cutoff, it is equivalent to the off state of the switch. In this way, the small current signal on the control electrode can be used to control the large current in the main electrode circuit. Figure 4. Volt-ampere Characteristic Curve of TRIACGenerally speaking, regardless of the voltage polarity between the two main electrodes A2 and A1 of TRIAC, as long as a certain amplitude of positive and negative pulses is applied to the control electrode, it can be turned on. So i represents the current in the main electrode and u represents the voltage between A2 and A1. The functional relationship between the two (called the volt-ampere characteristic curve) is shown in figure 4. It can be seen from the curve that the TRIAC has basically the same symmetrical performance in the first quadrant and the third quadrant.According to the voltage u on the main electrode and the polarity of the trigger pulse voltage uG on the control electrode, combined with the volt-ampere characteristic curve, the TRIAC can be divided into four trigger modes, which are defined as follows:(1) I+trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a positive trigger relative to A1.(2) I-trigger: In the first quadrant of the characteristic curve (A2 is positive), the control electrode is a negative trigger relative to A1.(3) Ⅲ+trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a positive trigger relative to A1.(4) Ⅲ-trigger: In the third quadrant of the characteristic curve (A2 is negative), the control electrode is a negative trigger relative to A1.Among these four trigger modes, I+ and III- have higher sensitivity, and are two commonly used trigger modes.In the control circuit of the new type electric heating electric appliance, the trigger signal applied to the control electrode of TRIAC is output by a single chip microcomputer or an integrated circuit. Some output a continuous positive (or negative) voltage signal, and some output a series of zero-crossing trigger pulses synchronized with a 50Hz sinusoidal AC power supply. The former is called a potential trigger, while the latter is called a pulse trigger. Their waveforms are shown in figure 5 and figure 6, respectively. Figure 5. Figure 6. Ⅱ The Main Characteristics of Thyristors2.1 Basic Structure of ThyristorA thyristor (also known as semiconductor controlled rectifier) is a high-power semiconductor device with a four-layer structure (PNPN). It has three lead-out electrodes, namely anode (A), cathode (K) and gate (G). Its symbolic representation and device cross-section are shown in figure 7. Figure 7. Symbol Representation and Device Cross-sectionOrdinary thyristors bidirectionally diffuse P-type impurities (aluminum or boron) in an N-type silicon wafer to form a P1N1P2 structure, and then diffuse N-type impurities (phosphorus or antimony) to form a cathode in most regions of P2, and at the same time lead out a gate electrode on P2 and form an ohmic contact is formed in the P1 as the anode.2.2 Volt-ampere Characteristics of ThyristorsThe on and off states of the thyristor are determined by the anode voltage, anode current and gate current. Volt-ampere characteristic curves are usually used to describe the relationship between them, as shown in figure 8. Figure 8. Volt-ampere Characteristic Curve of ThyristorWhen the thyristor VAK applies a forward voltage, J1 and J3 are forward biased, and J2 is reverse biased. The applied voltage almost falls on J2, and J2 plays a role of blocking the current. With the increase of VAK, as long as VAK <VBO, the passing anode current IA is small, so this region is called a forward blocking state. When VAK increases beyond VBO, the anode current suddenly increases, and it will be in a low voltage and high current state at the moment the characteristic curve passes the negative resistance. The on-state current IT determined by the load flows through the thyristor, the device voltage drop is about 1V, and the state corresponding to the CD section of the characteristic curve is called the on-state. VBO and its corresponding IBO are usually referred to as forward breakover voltage and breakover current. After the thyristor is turned on, it can maintain the on-state by itself. The transition from the on-state to the off-state is usually controlled by an external circuit without using a gate signal, that is, the device can be turned off only when the current is below a certain threshold value called the holding current IH.When the thyristor is in the off-state (VAK <VBO), if the gate electrode is made positive with respect to the cathode and the gate electrode is supplied with current IG, the thyristor will breakover at a lower voltage. The breakover voltage VBO and the breakover current IBO are both functions of IG. The larger the IG, the smaller the VBO. As shown in figure 3, once the thyristor is turned on, the device is turned on even if the gate signal is removed.When the anode of the thyristor is negative with respect to the cathode, as long as VAK <VBO, IA is small and has nothing to do with IG. However, when the reverse voltage is large (VAK≈VBO), the reverse leakage current through the thyristor increases sharply, showing thyristor breakdown. Therefore, VBO is called the reverse breakover voltage and breakover current.2.3 Static Characteristics of ThyristorsThe thyristor has 3 PN junctions, and the characteristic curve can be divided into (0 ~ 1) blocking area, (1 ~ 2) breakover area, (2 ~ 3) negative resistance area and (3 ~ 4) conducting area. a. Forward Working Area— Forward blocking (0 ~ 1) areaWhen a forward voltage is applied between AK, J1 and J3 bear the forward voltage, while J2 bears the reverse voltage, and the applied voltage falls almost entirely on J2. The reverse-biased J2 acts to block the current, and the thyristor is not conducting at this time.— Avalanche area (1 ~ 2 is also called breakover area)When the applied voltage rises close to the avalanche breakdown voltage VBJ2 of J2, the width of the space charge region of the reverse-biased J2 expands, and the internal electric field is greatly enhanced, which causes the multiplication effect to be strengthened. As a result, the current through J2 suddenly increases, and the current flowing through the device also increases. At this time, the current passing through J2 is transformed from the original reverse current to the current which is mainly attenuated by J1 and J3 through the base region and multiplied in the space charge region of J2. This is the avalanche area where the voltage increases and the current increases sharply. Therefore, the characteristic curve turns in the area, so it is called the breakover area.— Load area (2 ~ 3)When the applied voltage is greater than the breakover voltage, a large number of electron-hole pairs generated by the avalanche doubling of the space charge region of J2 are extracted by the reverse electric field. The electrons enter the region N1 and the holes enter the region P2. Due to the inability to recombine quickly, carrier accumulation occurs near both sides of J2: holes in region P2 and electrons in region N1, compensating for the charge of the ionized impurities and narrowing the space charge region. As a result, the potential in region P2 increases and the potential in the region N1 decreases, which acts to offset the external electric field. As the applied voltage at J2 decreases, the avalanche multiplication effect also weakens. On the other hand, the forward voltage of J1 and J3 has been enhanced, and the injection has increased, causing the current through J2 to increase, so a negative resistance phenomenon has occurred in which the current increases and the voltage decreases.— Low resistance on-state region (3 ~ 4)As mentioned above, the multiplication effect causes the accumulation of electrons and holes on both sides of J2, causing the reverse bias voltage of J2 to decrease; at the same time, the injection of J1 and J3 is enhanced, and the circuit is increased, so that charges continue to accumulate on both sides of J2, and the junction voltage continues to decrease. When the voltage drops to the point where the avalanche multiplication stops and all the junction voltages are cancelled, holes and electrons still accumulate on both sides of J2, and J2 becomes forward biased. At this time, J1, J2, and J3 are all forward biased, and large currents can pass through the device  because it is in a low-resistance on-state region. When fully conducting, its volt-ampere characteristic is similar to that of a rectifier element.b. Reverse Working Area (0 ~ 5)When the device is operating in reverse, J1 and J3 are reverse biased. Due to the very low breakdown voltage of the heavily doped J3, J1 withstands almost all of the applied voltage. The volt-ampere characteristic of the device is the volt-ampere characteristic curve of the reverse bias diode. Therefore, the PNPN thyristor has a reverse blocking region, and when the voltage increases above the J1 breakdown voltage, the current increases sharply due to the avalanche multiplication effect, at which time the thyristor is broken down. 2.4 Characteristic Equation of ThyristorA two-terminal device of a PNPN four-layer structure can be regarded as P1N1P2 and N1P2N2 transistors with current amplification coefficients of α1 and α2, respectively, where J2 is a common collector junction. When a forward voltage is applied to the device, the forward-biased J1 injects holes and passes through region N1 to reach the collector junction (J2). The hole current is α1IA; while the forward-biased J3 injects electrons and passes through region P2. The current carried to J2 is α2IK. Because J2 is in the reverse direction, the current through J2 also includes its own reverse saturation current, ICO.The current through J2 is the sum of the above three, that is,（1）Assuming the emission efficiency γ1 = γ2 = 1, according to the principle of current continuity IJ2 = IA = IK, so formula (1) becomes:（2）The formula shows that when the forward voltage is less than the avalanche breakdown voltage VB of J2, the multiplication effect is small and the injection current is also small. So α1 and α2 are also very small, thus（3）The ICO at this time was also small. Therefore, J1 and J3 are forward biased, so increasing VAK can only increase the reverse bias of J2. It cannot increase the ICO and IA a lot, so the device is always in the blocking state, and the current flowing through the device is the same order of magnitude as the ICO. Therefore, formula (3) is called a blocking condition.When the increase in VAK causes the reverse bias of J2 to increase and avalanche multiplication occurs, assuming multiplication factor Mn = Mp = M, then ICO, α1, and α2 will all increase by M times, so (2) becomes（4）At this time, the denominator becomes smaller, and IA will increase rapidly with the growth of VAK, so when（5）The avalanche steady-state limit is reached (VAK = VBO), and the current will tend to infinity, so equation (5) is called the forward breakover condition., , Using this feature, the breakover point conditions are derived from the characteristic curve equation (4). Because α1 and α2 are functions of current, M is a function of VJ2, which can be approximated with M(VJ2)=M(VAK), ICO is a constant and derive  with respect to (4). The outcome is（6）Since the breakover voltage is lower than the breakdown voltage,  must be a constant value. Because , the numerator must also be zero and obtain （7）According to the definition of transistor DC voltage amplification factor,                      （8）We can get the small signal current amplification factor                      （9）Using formula (9), formula (7) can be changed to                        （10）That is, at the breakover point, the product of the multiplication factor and the sum of the small signal is exactly 1. As long as the PNPN structure satisfies the above formula, it has switching characteristics, that is, it can be switched from an off-state to an on-state.Because α changes with the current IE, when IA increases, both α1 and α2 increase. It can be seen that, when the current is large, the value of M satisfying (6) can be reduced instead. This shows that IA increases and VAK decreases accordingly.α is both a function name of the current and a function of the collector junction voltage. When the current increases as α is constant, the corresponding reverse bias of the collector junction decreases. When the current is large,                             （11）According to equation (2), J2 provides an on-state current (ICO <0). Therefore, J2 must be forward biased, so J1, J2, and J3 are all forward biased, and the device is conducting. The off-state of the device changes to the on-state. The key is that J2 junction must be changed from reverse-biased to forward-biased. The condition for J2 to reverse to the forward direction is that holes and electrons should accumulate in regions P2 and N1, respectively. The condition for the accumulation of holes in region P2 is that the amount of holes α1IA injected by the J1 and collected by J2 into region P2 is greater than the amount of holes that disappear by recombination with (1-α2) IK, that is                           （12）Since IA=IK, α1+α2＞1 is obtained. As long as the conditions are true, the hole accumulation in region P2 is the same, and the region electron accumulation condition is（13）Thus （14）It can be seen that when the condition of α1+α2＞1 is satisfied, the potential of region P2 is positive, and the potential of region N1 is negative. J2 becomes forward-biased and the device is in a conducting state, so α1+α2＞1 is called a conducting condition.Figure 9. SCR (Silicon Controlled Rectifier) Symbol Ⅲ The Main Parameters of Thyristor3.1 Main Parameters of Unidirectional ThyristorsIn order to correctly use a unidirectional thyristor, it is necessary not only to understand its working principle, but also to master its main parameters.(1) Forward repetitive peak voltage UFRMUnder the condition that the control electrode is disconnected and the unidirectional thyristor is in the forward blocking state, when the junction temperature of the unidirectional thyristor is the rated value, it is allowed 50 times per second, and the duration should not exceed 10 ms. The forward peak voltage that can be repeatedly applied to the unidirectional thyristor is called the forward repetitive peak voltage, which is expressed by UFRM. Generally, the secondary voltage is specified as 80% of the forward breakover voltage.(2) Reverse repetitive peak voltage URRMUnder the same conditions as the forward repetitive peak voltage, the reverse peak voltage that can be repeatedly applied to the unidirectional thyristor is called the reverse repetitive peak voltage, which is expressed by URRM and is generally 80% of the reverse breakover voltage.(3) Rated voltage UNUsually, the smaller one of UFRM and URRM is used as the rated voltage of the unidirectional thyristor. This is because the voltage added to the tube in practice is generally a positive and negative symmetrical voltage, so the voltage with a smaller value shall prevail. But because the transient over-voltage will also damage the tube, when selecting the tube, for safety reasons, the rated voltage of the tube is required to be greater than 2 to 3 times the actual peak voltage.(4) Rated forward average current IFThe average value of the power frequency sinusoidal half-wave current allowed to pass through the unidirectional thyristor under the ambient temperature of 40°C and specified heat dissipation conditions is called the rated forward average current IF. How many amp of the unidirectional thyristors we generally say refers to this current value. The amount of IF is related to factors such as the ambient temperature, heat dissipation conditions, and the conduction angle of the component. The rated current of the unidirectional thyristor is calibrated by the power frequency sinusoidal half-wave average current under certain conditions. This is because the load connected to the rectifier output often requires the average current to measure its performance. However, from the perspective of the unidirectional thyristor heating, regardless of the current waveform flowing through the unidirectional thyristor and the conduction angle of the unidirectional thyristor, as long as the effective value of the designed current is equal to the effective value of the rated current IF, then the heating of the unidirectional thyristor is equivalent and allowed.(5) Holding current IHAt room temperature, under the condition of the control electrode short circuit, the minimum anode current required to maintain the unidirectional thyristor to continue conducting is called the holding current IH. If the anode current of the unidirectional thyristor is less than this value, the unidirectional thyristor will change from the conducting state to the blocking state.(6) Control electrode trigger voltage UGK and trigger current IGAt room temperature, under the condition that the voltage between the anode and cathode of the unidirectional thyristor is 6V, the minimum DC current value of the control electrode required to change the unidirectional thyristor from the blocking state to the conducting state is called the trigger current IG. The DC voltage UGK between the control electrode and the cathode corresponding to the trigger current IG is called a trigger voltage. Generally, UGK is about 1 to 5V, and IG is tens to hundreds of mA.3.2 Main Parameters of TRIACIn various control circuits, the TRIAC is a relatively easy-to-damage component. Once the TRIAC is found to be damaged, you just need to replace the TRIAC with the same parameters. There are many characteristic parameters of TRIAC, and the following are the main parameters that should be considered during maintenance.— Off-state repetitive peak voltage-rated voltage VDRMWhen the control electrode is disconnected and the component is at the rated junction temperature, the voltage corresponding to the sharp bending point of the forward and reverse volt-ampere characteristics is called the off-state non-repeating peak voltage. 80% of it is called the off-state repetitive peak voltage. It is also called rated voltage, which is expressed by VDRM.When the TRIAC works, the peak value of the applied voltage momentarily exceeds the reverse non-repetitive peak voltage, which can cause permanent damage to the TRIAC. Moreover, due to the increase in ambient temperature or poor heat dissipation, the reverse non-repetitive peak voltage value may decrease. Therefore, when a TRIAC is selected, its rated voltage value should be 2 to 3 times the possible maximum voltage in actual operation. If the power supply voltage is 220V, a TRIAC with a rated voltage above 500V should be selected so that the selected components can withstand the surge voltage.— Rated on-state average current—rated current IT(AV)Under the specified conditions, the maximum average on-state current allowed when the TRIAC is on is called the rated on-state average current. According to the standard series of TRIAC, this current is taken to the corresponding current level, which is often referred to as the rated current for short and represented by IT(AV).Because the current overload capacity of the TRIAC is much smaller than that of ordinary motors and electrical appliances, the rated current of the TRIAC should be 1.5 to 2 times the maximum current in actual operation when selected.— Gate trigger current IGT (voltage UGT)This refers to the minimum trigger signal current (voltage) value that can make the TRIAC conduct reliably and add to the control electrode. If the trigger current (voltage) obtained by the TRIAC control electrode is less than the number of times, the TRIAC may not be turned on.— On-state average voltage UT(AV)Once the TRIAC is turned on, it is equivalent to the closed switch. Because the TRIAC is connected in series with the load, the smaller the voltage between the two main electrodes, the better. After the TRIAC is turned on, the average value of the voltage between the two main electrodes is called the on-state average voltage, which is usually referred to as the tube voltage drop. If the tube pressure drop of the TRIAC is too large, the motors and solenoid valves it controls may not work properly because they cannot get the full voltage.— Holding currentWhen the control electrode is disconnected at room temperature, the TRIAC is reduced from a large on-state current to a minimum main electrode current that is just necessary to maintain conduction, which is called a holding current. The TRIAC is turned off only when the main electrode current decreases below the holding current. Ⅳ Main Function of ThyristorThe functions of thyristors are as follows: first, converter rectification; second, voltage regulation; third, frequency conversion; fourth, switch (contactless switch). The most basic use of ordinary thyristors is controlled rectification. The diode rectifier circuit we are familiar with is an uncontrollable rectifier circuit. If the diode is replaced by a thyristor, it can constitute a controllable rectifier circuit, inverter, non-contact switch, achieve motor speed control, motor excitation, automatic control and so on. In electrical technology, the half cycle of alternating current is often defined as 180°, which is called the electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the beginning of the zero value to the moment when the trigger pulse arrives is called the control angle α; the electrical angle at which the thyristor conducts in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to indicate the on or off range of the thyristor during the half cycle of the forward voltage. Controllable rectification is achieved by changing the control angle α or the conduction angle θ, and changing the average value UL of the pulsed DC voltage on the load. The function of a thyristor is not only rectification, it can also be used as a non-contact switch to quickly turn on or off the circuit, to achieve the inverter that converts DC power to AC power, to change AC power of one frequency to AC power of another frequency, etc. This article mainly introduces the basic principle, characteristics and main parameters of thyristors. Frequently Asked Questions about Thyristors (SCR)1. What are the characteristics of SCR?Characteristics of Thyristor or Characteristics of SCRReverse Blocking Mode of Thyristor. Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S open.Forward Blocking ModeForward Conduction Mode 2. Why SCR is called as thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 3. Is SCR and thyristor are same?Thyristor is a 4 layer device formed by alternate combination of p and n type semiconductor materials. It is a device used for rectification and switching purpose. SCR is the mostly used member of thyristor family and it is the name commonly used when we talk about thyristors. 4. What is a thyristor used for?Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to switch off automatically, referred to as "zero cross" operation. 5. How does a SCR thyristor work?So how does it work? With no current flowing into the gate, the thyristor is switched off and no current flows between the anode and the cathode. When a current flows into the gate, it effectively flows into the base (input) of the lower (n-p-n) transistor, turning it on.

I IntroductionLaser sensor is a kind of sensor which uses laser technology to measure. It is generally composed of laser, optical parts and photoelectric devices. It can convert the measured physical parameters (such as length, flow, speed, etc.) into optical signals, and then use photoelectric converter to convert the optical signals into electrical signals. Through the filtering, amplification and rectification of corresponding circuits, the output signals can be obtained, so as to calculate the measured quantity. Laser technology has the characteristics of strong direction, high brightness and good monochromaticity. It is widely used in industrial and agricultural production, national defense and military, medical and health, scientific research and other aspects, such as distance measurement, precision detection, positioning, etc., as well as length benchmark and optical frequency benchmark.Laser Distance Sensor OverviewCatalogI IntroductionII What is Laser? 2.1 The Concept of Laser 2.2 Important Characteristics of Laser 2.3 Types of Laser 2.4 What can Laser Sensor Detect?III Laser Displacement Sensor 3.1 What is Laser Displacement Sensor 3.2 How Does Laser Displacement Sensor Work? 3.3 Application of Laser Displacement Sensor 3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?IV Laser Distance Sensor 4.1 Classification of Laser Distance Sensors  4.2 Measuring Principle of Different Laser Distance Sensors 4.3 Application of Laser Distance SensorV Laser Sensor Application CaseVI FAQII What is Laser?2.1 The Concept of LaserLaser light is different from ordinary light. (See more about light and photoelectric effect in the article introducing light sensor and photoresistor)We need to use laser to produce laser light. In the normal state, most of the atoms in the laser are in stable low energy level E1. Under the action of appropriate frequency of external light, the atoms in low energy level absorb photon energy to excite and transition to high energy level E2. The photon energy E = e2-e1 = h V, where h is the Planck constant and V is the photon frequency.  On the contrary, when the frequency of light is V, the atom in level E2 will jump to the low energy level to release energy and emit light, which is called stimulated radiation. First of all, the laser makes the atoms of the working materials abnormally in the high-energy level (i.e. inversion distribution of the particle number ), which can make the stimulated radiation process dominant, so that the induced light with the frequency of V can be enhanced, and the large stimulated radiation light can be produced through the avalanche amplification of the parallel reflector, which is called laser light for short.Figure1. Laser2.2 Important Characteristics of Laser（1）High directivity, small divergence angle of light speed, the laser beam extends only a few centimeters from a few kilometers away.（2）High monochromaticity, the frequency width of laser light is more than 10 times smaller than that of ordinary light.（3）High brightness, laser beam convergence can produce temperatures up to several million degrees.2.3 Types of LaserLaser can be divided into four types according to working substance:（1）Solid state laserIts working substance is solid. Ruby laser, neodymium doped yttrium aluminum garnet laser (i.e. YAG laser) and neodymium glass laser are commonly used. Their structures are basically the same, characterized by small and solid, high power. At present, neodymium glass laser is the device with the highest pulse output power, which has reached tens of megawatts.（2）Gas laserIts working substance is gas. Now there are various kinds of gas atoms, ions, metal vapor, gas molecular lasers. Commonly used are carbon dioxide laser, helium neon laser and carbon monoxide laser, whose shape is like a common discharge tube, characterized by stable output, good monochromaticity, long life, but small power, low conversion efficiency.（3）Liquid laserIt can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser. Its main feature is that the wavelength is continuously adjustable.（4）Semiconductor laserIt is a younger laser, and the more mature one is GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. It can be made into range finder and sighting device. However, the output power is small, the directivity is poor, and it is greatly affected by the ambient temperature.2.4 What can Laser Sensor Detect? (1) Laser measurement of lengthPrecise measurement of length is one of the key technologies in precise machinery manufacturing industry and optical processing industry. Modern length measurement is mostly based on the interference phenomenon of light wave, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source. It is 100 thousand times purer than the best monochromatic light source (krypton-86 lamp). Therefore, the laser measurement range of length is large and the accuracy is high.  According to the optical principle, the relationship between the maximum measurable length L of monochromatic light and wavelength λ and spectral line width δ is L = λ 2 / δ. The maximum measurable length of krypton-86 lamp is 38.5cm. For a long object, it is necessary to measure in sections to reduce the accuracy. If He-Ne gas laser is used, it can measure tens of kilometers at most. Generally, the length within several meters can be measured with an accuracy of 0.1 μ M.Figure2. Laser Measure(2) Laser measurement of distanceIts principle is the same as that of the radio radar. After the laser is aimed at the target, the round-trip time is measured, and then the round-trip distance is obtained by multiplying the speed of light. Because of the advantages of laser, such as high directivity, high monochromaticity and high power, these are very important for the measurement of long distance, the determination of target orientation, the improvement of signal-to-noise ratio of the receiving system, and the guarantee of measurement accuracy, so the laser rangefinder is paid more and more attention.  The lidar developed on the basis of the laser rangefinder can not only measure the distance, but also the azimuth, velocity and acceleration of the target. It has been successfully used in the ranging and tracking of the artificial satellite. For example, the lidar using ruby laser has a distance measuring range of 500-2000 km with an error of only a few meters. At present, ruby laser, neodymium glass laser, carbon dioxide laser and Gas laser are often used as the light source of laser rangefinder.Figure3. Measuring Distance with Laser Sensor (3) Laser measurement of thickness Based on the principle of triangle ranging, a precise laser ranging sensor is divided at the upper and lower part of the C-frame. The modulated laser emitted by the laser hits the surface of the measured object. By sampling the signal of the linear CCD, the distance between the measured object and the C-frame is synchronously obtained by the linear CCD camera under the control of the control circuit. The thickness of the middle measured object is calculated by the data fed back by the sensor. Because the detection is continuous, the continuous dynamic thickness of the measured object can be obtained.Figure4. Thickness Measuring with Laser SensorThickness measurement by single laser displacement sensorPut the measured body on the measuring platform, measure the distance from the sensor to the platform surface, then measure the distance from the sensor to the measured body surface, and measure the thickness after calculation. It is required that there is no air gap between the measured body and the measuring platform, and the measured body is not cocked. These strict requirements can only be achieved offline.Thickness measurement by double laser displacement sensorA laser displacement sensor is installed above and below the measured body respectively, and the thickness of the measured body is d = C - (a + b). Among them, C is the distance between two sensors, a is the distance between the upper sensor and the measured body, and B is the distance between the lower sensor and the measured body. The advantage of this method for on-line thickness measurement is that it can eliminate the influence of the vibration of the measured body on the measurement results.  But at the same time, there are requirements for sensor installation and performance. The conditions to ensure the accuracy of measurement are that two sensor beams must be coaxial and that two sensor scans must be synchronous. Coaxiality is realized by installation, and synchronization depends on the selection of laser sensor with synchronization end.Figure5. Thickness MeasurementIII Laser Displacement Sensor3.1 What is Laser Displacement SensorThe laser displacement sensor is called the eyes of the robot and machine, and has an irreplaceable role in welding, blank manufacturing, mechanical processing, heat treatment, loading and unloading, assembly and other operations. So, what is a laser displacement sensor? The laser displacement sensor is a sensor that uses laser technology for measurement, and is composed of a laser, a laser detector, and a measurement circuit. As a new type of measuring equipment, the laser displacement sensor can accurately measure the position, displacement and other changes of the measured object, and can also measure precise geometric measurements such as displacement, thickness, vibration, distance, and diameter.3.2 How Does Laser Displacement Sensor Work?The laser displacement sensor can accurately and non-contactly measure the position, displacement and other changes of the measured object, and is mainly used to measure the displacement, thickness, vibration, distance, diameter and other geometric quantities of the object. According to the measurement principle, the principle of laser displacement sensor is divided into laser triangulation method and laser echo analysis method. Laser triangulation method is generally suitable for high-precision and short-distance measurement, while laser echo analysis method is used for long-distance measurement. The following is the introduction to two measurement methods of laser displacement sensor principle.TriangulationFigure6. Laser Displacement SensorThe laser emitter shoots the visible red laser to the object surface through the lens, and the laser reflected by the object passes through the receiver lens, which is accepted by the internal CCD linear camera. According to different distances, the CCD linear camera can "see" this light point at different angles. According to the distance between the laser and the camera known from this angle, the digital signal processor can calculate the distance between the sensor and the measured object. At the same time, the position of the beam in the receiving element is processed by analog and digital circuits, and the corresponding output value is calculated by microprocessor analysis, and the standard data signal is output in proportion in the analog quantity window set by the user. If switching value output is used, it will be conducted in the settings window and cut off outside the window. In addition, an independent detection window can be set for analog quantity and switch quantity output.Echo analysisThe laser displacement sensor can achieve a certain degree of accuracy by using the echo analysis principle to measure the distance. The sensor is composed of processor unit, echo processing unit, laser transmitter and laser receiver. The laser displacement sensor emits one million pulses per second through the laser transmitter to the detector and returns to the receiver. The processor calculates the time required for the laser pulse to meet the detector and return to the receiver, so as to calculate the distance value.  The output value is the average output of thousands of measurement results. It is the so-called pulse time method. The laser echo analysis method is suitable for long-distance detection, but the measurement accuracy is lower than the laser triangulation method, and the longest detection distance can reach 250m.3.3 Application of Laser Displacement Sensor(1) Dimension measurement: position identification of small parts; monitoring of whether there are parts on the conveyor belt; detection of material overlapping and covering; control of manipulator position (tool center position); device state detection; detection of device position (through the small hole); monitoring of liquid level; thickness measurement; vibration analysis; collision test measurement; automobile-related test, etc. (2) Thickness measurement of sheet metal: laser sensor measures the thickness of sheet metal. Thickness change detection can help to detect wrinkles, small holes or overlaps to avoid machine failure. (3) Cylinder measurement: angle, length, eccentricity of inner and outer diameter, conicity, concentricity and surface profile.Figure7. Application of Laser Displacement Sensor(4) Length measurement: place the measured component on the conveyor belt at the designated position, the laser sensor detects the component and simultaneously measures it with the triggered laser scanner, and finally obtains the length of the component. (5) Uniformity check: place several laser sensors in a row in the tilt direction of the workpiece movement to be measured, and directly output the measurement value through one sensor. In addition, the software can be used to calculate the measurement value and read out the result according to the signal or data. (6) Inspection of electronic components: two laser scanners are used to place the tested components between them. Finally, the data is read out by the sensor, so as to detect the accuracy and integrity of the component size. (7) Inspection of filling level in production line: laser sensor is integrated into the production and manufacturing of filling products. When the filling products pass through the sensor, it can detect whether the filling is full. The sensor can accurately identify whether the filling product is qualified and the quantity of the product by using the extended program of laser beam reflecting surface. 3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?Some parameters that must be understood when selecting a laser displacement sensor are very important.(1) Resolution: generally refers to the minimum range of the sensor, that is, the maximum recognition rate of the sensor. If the parameter is marked as 1mm, then the resolution is equal to 1mm. (2) Repeatability: We must know that even if the measured object is at rest, the measured value will fluctuate slightly. The error margin of repeated measurement of the measured object at the same position in the static state is the repeat accuracy. For example, if the parameter is marked as 1μm, the repeat accuracy of the sensor is 1μm. (3) Full range (effective range): the rated effective range of the sensor. When selecting a sensor, we must select the sensor that contains the effective range according to the required detection distance. (4) Linear accuracy: the error between the measured value and the actual displacement. Linear accuracy is expressed as a percentage, but since the range is a range and the measurement accuracy is more difficult to reach the apex of the range, most sensors will mark the linear accuracy of the apex of the range to intuitively reflect the performance of the sensor. (5) Sampling frequency/sampling period: frequency refers to the number of measurements per second. The higher the frequency, the shorter the time it takes to make a measurement. The shorter the measurement time, the more suitable it is for the detection of high-speed moving objects. (6) Average sampling times: even in the static state, there will be slight measurement fluctuations. At this time, multiple measurements are required to calculate the average number to make the measured value stable and accurate.Figure8. Laser SensorIV Laser Distance SensorLaser ranging is one of the earliest applications of the laser. This is because the laser has many advantages such as strong directivity, high brightness, and good monochromaticity. Before 1965, the Soviet Union used a laser to measure the distance between the earth and the moon (384401km) with an error of only 250m. In 1969, the Americans landed on the moon with a retro-reflector on the lunar surface. They also used a laser to measure the distance between the earth and the moon, with an error of only 15cm. The basic principle of using laser transmission time to measure the distance is to determine the target distance by measuring the time required for the laser to travel to and from the target.Related recommendation: Proximity SensorFigure9. Laser Distance Sensor4.1 Classification of Laser Distance Sensors Laser distance sensor technology is divided into absolute distance measurement method and micro displacement measurement method according to the measurement range. Subdivided according to the measuring method, the absolute distance ranging method mainly includes pulse laser ranging and phase laser ranging, and the micro displacement measuring method mainly includes triangulation laser ranging and interferometric laser ranging.4.2 Measuring Principle of Different Laser Distance Sensors(1) Pulse Laser Distance SensorA pulse laser with a very short duration is emitted by a pulsed laser, and after reaching the target to be measured after the distance to be measured, part of the energy will be reflected back. The reflected pulsed laser is called an echo. The echo returns to the rangefinder and is received by a photoelectric detector. According to the interval between the main wave signal and the echo signal, that is, when the laser pulse travels from the laser to the target to be measured, the distance of the target to be measured can be calculated.  (2) Phase laser Distance SensorThe emitted laser light is emphasized, and the phase change of the modulated signal is used when the laser is propagated in space. According to the wavelength of the modulated wave, the distance represented by the phase delay is calculated. That is, the indirect method of phase delay measurement is used instead of directly measuring the time required for the round trip of the laser to achieve distance measurement. The accuracy of this method can reach the millimeter level.Figure10. Working Principle of Laser Distance Measuring Device (3) Triangulation Laser Distance SensorAs mentioned above, this measurement principle is that the light emitted by the laser is focused on the surface of the measured object after being focused by the condensing lens, and the receiving lens receives the scattered light from the incident light spot and images it on the photoelectric  position detector On the sensitive side. When the object moves, the relative distance of the object movement is calculated by the displacement of the light spot on the imaging surface. The resolution of triangulation laser ranging is very high, which can reach the order of microns.   Figure11. Triangulation Principle (4) Interferometric Laser Distance SensorBy moving the measured target and measuring the coherence, the distance increment measurement is completed by counting, so the sensitivity of the interferometric measurement is very high, which can reach the nanometer level.4.3 Application of Laser Distance SensorThe laser distance sensor is mainly used for: monitoring the position of moving objects; measuring the railway contact network, measuring the boundary of buildings; measuring unsuitable objects; industrial automation and intelligent production management; vehicle speed and flow statistics; industrial monitoring signal trigger control; tower crane XY positioning of crane; automatic target distance control; monitoring of ship's safe docking position; positioning of container; measurement of vehicle's safe distance; measurement of overhead cable and height limitation; measurement of width of boxes on conveyor belt.V Laser Sensor Application Case(1) Over-limit detection of vehicle width and heightThe laser sensor is used for rapid measurement, the network core of the PC industrial control computer and the visual programming software VB are used for real-time data transmission and processing, and the friendly interface control software is also designed. Field test data shows that the system has good real-time performance and high measurement accuracy, and has certain practical value. (2) Expressway toll stationUsed in highway toll stations to count and protect vehicles. Malaysian Teras has applied hundreds of BEA laser sensors to its manual and automatic toll station systems. The laser sensor uses the time-of-flight (TOF) measurement principle, which can form 4 planes in the detection area to detect the vehicle. At the same time, the product also has functions such as anti-collision and vehicle safety protection. Compared with the traditional light curtain, the laser sensor has the advantages of high sensitivity, high accuracy, easy installation, high cost performance and strong stability. (3) Google's second-generation unmanned vehicleIn addition to the laser sensor on the top, Google’s second-generation driverless car prototype is still quite obvious, and the other sensors are set very concealed. The front, rear and sides of the vehicle are clearly marked with the Google unmanned vehicle logo. The driving principle of Google's unmanned vehicle is to continuously collect various accurate data of the vehicle itself and the surroundings through many sensors installed around the car, analyze and calculate it through the processor in the car, and then control the driving of the car according to the calculation results . Unmanned vehicles will use GPS equipment and sensors to accurately locate the vehicle's position and speed, and judge pedestrians, vehicles, bicycles, signal lights and many other objects around it.Figure12. Google's Self-driving CarThe roof of this Lexus is equipped with a 360° rotating laser holographic sensor, which can sense the front, side and rear conditions of the car almost simultaneously. The data collected by the sensor will be input to the processor located on the right rear side of the vehicle through the green data line. This laser sensor can also allow unmanned vehicles to be accurately positioned globally. The original L-shaped Lexus logo on the front of the car was also removed and replaced with a radar sensor; it was used to measure the distance ahead and the speed of the vehicle in order to determine the condition of the vehicle ahead and control the safe acceleration and deceleration of the vehicle. The wheel hub of the tire is also equipped with a position sensor, which is used to detect wheel rotation and help the vehicle to locate. The heart of Google's unmanned vehicles-the processor is located on the right rear side of the vehicle, the data information from each sensor will be transmitted here through the data wire, and analyzed and processed through the software in order to accurately sense and judge the difference between the unmanned vehicles object. In addition to analyzing and judging the current position of objects around the unmanned vehicle, the unmanned vehicle also needs to be calculated by software to accurately predict the possible next position of each object. Finally, the unmanned car will make safe driving decisions based on all the collected data, including controlling the speed of the car and the surrounding distance. VI FAQ1. How does laser sensor work?The basic principle is optical triangulation using a CMOS linear imager. A diffuse triangulating laser distance sensor transmits a laser through a lens and to the target, which reflects the light back to the sensor. A lens focuses this reflected light into a small spot onto the CMOS linear imager. 2. What is the use of laser sensors?The definition of a laser sensor is, it is an electrical device used to sense minute objects and precise positions. This sensor uses a laser to produce light within a straight line. Its visible ray mark of the laser makes the arrangement very simple. Laser light includes light waves with similar wavelengths. 3. What are the types of laser sensors?Laser distance sensors.Displacement sensors.Laser projectors.Laser light curtains.Laser photoelectric sensors.Positioning lasers.Laser edge detection sensors. 4. Are laser sensors dangerous?Improperly used laser devices are potentially dangerous. Effects can range from mild skin burns to irreversible injury to the skin and eye. The biological damage caused by lasers is produced through thermal, acoustical and photochemical processes. 5. Is a laser a sensor?A laser sensor uses a 'laser' to emit light in a straight line. Its visible beam spot makes alignment and positioning very easy. Since the light beam is focused, the sensor can be installed without worries about stray light. The major types of laser sensors include reflective, thru beam, and retro-reflective. 6. What is the range of the laser sensor?Laser distance sensors are designed for non-contact distance measurements: laser gauges for measuring ranges up to 10m, laser distance sensors for up to 3,000m. 7. What is CMOS Laser Sensor?A CMOS image sensor combines with a step-less laser power adjustment algorithm to produce stable detection of all types of workpieces from black rubber with low reflectivity to stainless steel and other highly glossy materials. 8. Which laser sensor is used for measuring very long distances?LDM301 laser distance sensor series – fast measurement of long distances. The laser distance sensors of the LDM301 series use a measured time-of-flight principle to measure distances of 300 m for natural surfaces and 3,000 m for reflective surfaces. 9. How does a laser sensor measure distance?The distance measurement is based on the triangulation principle. The laser beam strikes the object as a small point. The receiver of the sensor (photodiode line) detects the position of this point. The angle of incidence changes according to the distance, and thereby the position of the laser point on the receiver. 10. How accurate are laser distance sensors?Compared to other types of laser sensors, OM70 sensors feature one of the thinnest beam shapes, helping to ensure a more precise measuring focus. For example, most point-type lasers typically only go down to 0.2mm x 0.75mm whereas the OM70 goes down to 0.05mm x 0.05mm. 

IntroductionThyristor is a four semiconductor layers or three PN junctions devicea solid-state semiconductor device with four layers of alternating P- and N-type materials. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is dervid from the words of thyratron (a gas fluid tube which work as SCR) and Transistor. And It acts exclusively as a bistable switch in electronic circuit.What is a Thyristor?CatalogIntroductionⅠ Types of ThyristorsⅡ Thyristor Selection2.1 Specific Requirements of Applying Circuit2.2 Main Parameters of the ThyristorⅢ Replacement of ThyristorⅣ Detection of Thyristor4.1 Detection of Unidirectional Thyristors4.2 Detection of TRIACⅠ Types of ThyristorsCommonly used thyristors include unidirectional thyristors, TRIAC, and turn-off thyristors, etc., which should be selected reasonably according to the needs of the circuit.— Unidirectional ThyristorThe unidirectional thyristor is characterized in that the current can only flow from the anode A to the cathode k, and is mainly used in the control of DC power supply or pulsating direct current, AC power rectification, and DC power inverter.Unidirectional thyristors can be divided into ordinary thyristors and high-frequency thyristors (the working frequency is above 110kHz). Commonly used unidirectional thyristors are 3CT series, 3DT series, KP series and KK series (high frequency thyristors), and imported MCR series, SF series, BST series etc.— TRIACTRIAC was developed on the basis of the unidirectional thyristor and is an AC power control device. TRIAC can not only replace two unidirectional thyristors in anti-parallel, but also requires only one trigger circuit, which is more convenient to use.The characteristic of TRIAC is that alternating current can pass through it, which is mainly used in the control of AC power supply and the adjustment of AC voltage. Commonly used TRIAC include 3CTS series and KS series, as well as imported MAC series, SM series, BCR series, etc.— Gate Turn-off ThyristorThe characteristic of the gate turn-off thyristor is that it can be switched off by the control electrode. It is mainly used in gate turn-off contactless switches, DC inverters, dimmer, speed regulation and other occasions.Gate turn-off thyristors are power-type control devices developed on the basis of ordinary thyristors. After the ordinary thyristor is triggered to be turned on, its control electrode does not work. To turn off the thyristor, the power must be cut off, or the forward current flowing through the thyristor must be less than the holding current. Gate turn-off thyristor overcomes the above drawbacks. When the control electrode G is added with a positive pulse voltage, the thyristor is turned on, and when the control electrode G is added with a negative pulse voltage, the thyristor is turned off.Gate turn-off thyristors are ideal high-voltage, high-current switching devices. For example, the DG series high-power gate turn-off thyristors can reach a maximum voltage of 4500V and a maximum current of 3000A. Ⅱ Thyristor Selection2.1 Specific Requirements of Applying CircuitThere are many types of thyristors, which should be selected reasonably according to the specific requirements of the application circuit.For AC/DC voltage control, controllable rectification, AC voltage regulation, power inverter, switching power supply protection circuit, etc., ordinary thyristors can be selected.For AC switch, AC voltage regulation, AC motor linear speed regulation, lamp linear dimming, solid state relay, solid state contactor, etc., a TRIAC should be selected.For AC motor variable frequency speed regulation, chopper, power inverter and various electronic switch circuits, you can choose gate turn-off thyristor.For sawtooth wave generator, long time delay, over voltage protector and trigger circuit with power transistor, etc., BTG thyristor can be selected.In electromagnetic cookers, electronic ballasts, ultrasonic circuits, superconducting magnetic energy storage systems, switching power supplies and other circuits, reverse conducting thyristors can be selected.In the photocoupler, light detector, light alarm, light counter, photoelectric logic circuit and operation monitoring circuit of automatic production line, the light-control thyristor can be selected.2.2 Main Parameters of the ThyristorThe main parameters of the thyristor should be determined according to the specific requirements of the application circuit.The selected thyristor should have a certain power margin, and its rated peak voltage and rated current (on-state average current) should be higher than the maximum operating voltage and maximum working current of the controlled circuit by 1.5 to 2 times.The parameters of the thyristor's forward voltage drop, gate trigger current, and trigger voltage should meet the requirements of the application circuit (this refers to the control circuit of the gate), and should not be high or low, otherwise it will affect the normal operation of the thyristor. Ⅲ Replacement of ThyristorAfter the thyristor is damaged, if no thyristor of the same type is replaced, another type of thyristor with similar performance parameters can be used instead.When designing an application circuit, a large margin is generally left. When replacing the thyristor, just pay attention to its rated peak voltage (repeated peak voltage), rated current (on-state average current), gate trigger voltage and gate trigger current, especially the two indicators of rated peak voltage and rated current.The switching speed of the thyristor used for replacement should be consistent with the switching speed of the damaged thyristor. For example: After the high-speed thyristor used in the pulse circuit and high-speed inverter circuit is damaged, only the same type of fast thyristor can be used instead of the ordinary thyristor.When selecting a thyristor to be used for replacement, it is not necessary to leave too much margin for any parameter, and the parameter of it should be as close as possible to the parameter of the replaced thyristor, because an excessively large margin is not only a waste, but also sometimes has side effects, such as non-triggering or insensitive triggering.In addition, the appearance of the two thyristors should be the same, otherwise it will cause inconvenience to the installation. Ⅳ Detection of ThyristorThyristors are usually represented by the letters "SCR" in circuit schematic diagrams. For example, SCR2 refers to the thyristor numbered 2. The symbol of the thyristor in the schematic diagram is shown in figure 1. Figure 1. Symbols of Thyristor4.1 Detection of Unidirectional Thyristors(1) Discrimination of each electrode: According to the structure of an ordinary thyristor, it can be seen that there is a PN junction between the gate G and the cathode K, which has unidirectional conductive characteristics, while there are two PN junctions of opposite polarities connected in series between the anode A and the gate. Therefore, by measuring the resistance between the pins of an ordinary thyristor with the R × 100 or R × 1 k Q level of the multimeter, three electrodes can be determined.The specific method is: use the black probe of the multimeter to connect one electrode of the thyristor, and use the red probe to touch the other two electrodes in turn. If the measurement result has a resistance value of several thousand ohms (kΩ) and another resistance value of several hundred ohms(Ω), it can be determined that the black probe is connected to gate G. In the measurement with a resistance value of several hundred ohms, the red probe was connected to the cathode K, and in the measurement with a resistance value of several thousand ohms, the red probe was connected to the anode A. If the measured resistance values are both very large, it means that the black probe is not connected to gate G. Apply the same method to test other electrodes until three electrodes are found.You can also measure the forward and reverse resistance between any two pins. If the forward and reverse resistance are close to infinity, the two electrodes are anode A and cathode K, and the other pin is gate G.Each electrode of the ordinary thyristors can also be judged according to its packaging form.For example, the bolt end of the bolt-type ordinary thyristor is anode A, the thinner lead end is gate G, and the thicker lead end is cathode K.The lead end of the flat thyristor is gate G, the flat end is anode A, and the other end is cathode K.A thyristor of metal package (T0-3) is a common thyristor and its shell is anode A.The middle pin of the plastic thyristor (T0-220) is anode A, and it is mostly connected with its own heat sink. Figure 2. Pin Arrangement of Several Common Thyristors(2) Judging whether it is good or bad: Use the R×1 kΩ level of a multimeter to measure the forward and reverse resistance values between anode A and cathode K of ordinary thyristor, which should normally be infinite (∞) ; If the forward and reverse resistance values are zero or the resistance values are both small, it indicates that a breakdown short circuit or leakage occurs inside the thyristor.Measure the forward and reverse resistance values between gate G and cathode K. Normally, there should be forward and reverse resistance values similar to diodes (the actual measurement results are smaller than those of ordinary diodes), that is, the forward resistance value is small (less than 2 kΩ) and the reverse resistance value is large (greater than 80 kΩ). If the resistance values of the two measurements are both large or small, it means that the thyristor is open or short-circuited between electrode G and K. If the forward and reverse resistance values are equal or close, it indicates that the thyristor has failed, and the PN junction between its electrodes G and K has lost its unidirectional conduction effect.Measure the forward and reverse resistance value between anode A and gate G. In normal conditions, both resistances should be several hundred kiloohms (kΩ) or infinite. If the forward and reverse resistance values are not the same (there is unidirectional conduction like a diode). One of the two PN junctions connected in reverse series between gate G and electrode A has been short-circuited.(3) Detection of triggering capability: For ordinary thyristors with low power (working current is below 5A), it can be measured with R×1 level of the multimeter . During the measurement, the black probe is connected to anode A and the red probe is connected to cathode K. At this time, the watch hand does not move, and the resistance value is displayed as infinite (∞). Use tweezers or wires to make anode A and gate G of the thyristor be short-circuited(see figure 3), which is equivalent to applying a forward trigger voltage to gate G. At this time, if the resistance value is several ohms to tens of ohms (the specific resistance value will vary according to the part number of the thyristor), it indicates that the thyristor is conducting due to the forward trigger. Then disconnect electrode A and gate G(the probes on electrode A and K do not move, only the trigger voltage of gate G is cut off). If the value indicated by the watch hand is still in the position of several ohms to tens of ohms, it indicates that the triggering performance of the thyristor is good. Figure 3. Detection of Triggering CapabilityFor medium and high power ordinary thyristors with a working current above 5 A, the on-state voltage drop VT, holding current IH and the gate trigger voltage Vo are relatively large. The current provided by the R × 1 kΩ level of the multimeter is low, and the thyristor cannot be completely turned on, so a 200Ω adjustable resistor and one to three 1.5 V dry batteries can be connected in series at the end of the black probe (depending on the capacity of the thyristor being tested, if its working current is greater than 100 A, three 1.5 V dry batteries are applied), as shown in figure 4. Figure 4. Detection of Trigger VoltageYou can also use the test circuit in figure 5 to test the triggering capability of an ordinary thyristor. In the circuit, vT is the thyristor under test, HL is a 6.3 V indicator (small electric beads in a flashlight), GB is a 6 V power supply (four 1.5 V dry batteries or 6 V regulated power supply can be used), and S is the button, R is the current limiting resistor. Figure 5. Test Circuit to Test the Triggering CapabilityWhen the button S is not connected, the thyristor VT is in a blocking state, and the indicator light HL is not on (if HL is on at this time, there may be breakdown of vT or leakage damage). After pressing the button S once (turn S on for a moment to provide the trigger voltage for gate G of the thyristor VT), if the indicator HL is always on, it means that the thyristor has a good triggering capability. If the brightness of the indicator is low, it indicates that the thyristor has poor performance and a large conduction voltage drop (the conduction voltage drop should be about 1 V under normal conditions). If button S is on, the indicator light is on, and when button S is off, the indicator light is off, indicating that the thyristor is damaged and the triggering performance is poor.4.2 Detection of TRIAC(1) Discrimination of each electrode: Use the R×1 or R×10 level of the multimeter  to measure the forward and reverse resistance values between three pins of the TRIAC. If it is measured that one pin is not connected with the other two pins, then this pin is the main electrode T2.After finding the electrode T2, the remaining two pins are the main electrode T1 and the gate G3. Measuring the forward and reverse resistance values between these two pins will gain two smaller resistance values. In a measurement with a small resistance value (about tens of ohms), the black probe is connected to the main electrode T1, and the red probe is connected to gate G.One end of the bolt of the bolt-shaped TRIAC is the main electrode T2, the thinner lead end is gate G, and the thicker lead end is the main electrode T1.    The shell of the metal-encapsulated (TO-3) TRIAC is the main electrode T2.The middle pin of the plastic-encapsulated (TO-220) TRIAC is the main electrode T2, which is usually connected to its own small heat sink. Figure 6. Pin Arrangement of Several TRIAC(2) Judging whether it is good or bad: Use the R×1 or R×10 level of a multimeter to measure the forward and reverse resistance values between the main electrode T1 and the main electrode T2 and between the main electrode T2 and gate G of the TRIAC. Normally it should be close to infinity. If the measured resistance values are all very small, it means that the electrodes of the TRIAC have been broken down or are short-circuited.Measure the forward and reverse resistance of the main electrode T1 and gate G. Normally, it should be between tens of ohms (Ω) and one hundred ohms (Ω) (when the black probe is connected to electrode T1 and the red probe is connected to gate G, the measured forward resistance value is slightly smaller than the reverse resistance value). If the forward and reverse resistance values between electrode T1 and gate G are measured to be infinite, it indicates that the thyristor has been damaged by an open circuit.(3) Detection of triggering capability: For small power TRIAC with working current below 8A, it can be measured directly with R×1 level of the multimeter. When measuring, first connect the black probe to the main electrode T2 and the red probe to the main electrode T1, then use tweezers to make electrode T2 and gate G be short-circuited, and add a positive polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T2 → T1.Then connect the black probe to the main electrode T1, and the red probe to the main electrode T2. Use tweezers to make electrode T2 and gate G be short-circuited, and add a negative polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T1 → T2.If gate G is disconnected after the thyristor is triggered to be turned on, the low-resistance conduction state cannot be maintained between electrode T2 and T1 and the resistance value becomes infinite, it indicates that the TRIAC has poor performance or is damaged. If a positive (or negative) polarity trigger signal is added to gate G, the thyristor still does not conduct (the forward and reverse resistance values between T1 and T2 are still infinite), then the thyristor is damaged and has no trigger continuity.For medium and high power TRIAC with a working current of 8A or more, when measuring their triggering capability, one to three 1.5V dry batteries can be connected in series to a probe of a multimeter, and then measure by using R×1 level as described above.For a TRIAC with a withstand voltage of 400V or more, its trigger capability and performance can also be tested by using 220V AC voltage.Figure 7 is a test circuit of a TRIAC. In the circuit, FL is a 60W /220V incandescent bulb, VT is the TRIAC under test, R is a 100Ω current limiting resistor, and S is a button. Figure 7. TRIAC CircuitAfter the power plug is connected to the working frequency AC, the TRIAC is in the off-state and the light bulb is off. (If the bulb is glowing normally at this time, it means that electrode T1 and T2 of the thyristor under test have been broken down and short-circuited; if the light bulb is slightly light, it means that the thyristor under test is damaged by leakage). Press the button S once to provide the trigger voltage signal for gate G of the thyristor. In normal conditions, the thyristor should be immediately triggered to turn on, and the light bulb will glow normally. If the bulb fails to emit light, the internal circuit of the tested thyristor is damaged. If the light bulb is turned on when the button S is pressed, and the light bulb is turned off when the button is released, it indicates that the triggering performance of the tested thyristor is poor.When using a multimeter to detect low-power light-controlled thyristors, put the multimeter in R × 1 level, connect one to three 1.5V dry batteries in series to a black probe, and measure the forward and reverse resistance values between the two pins. Normally it should be infinite. Then use a small flashlight or laser pen to illuminate the light receiving window of the light controlled thyristor. At this time, a small forward resistance value can be measured, but the reverse resistance value is still infinite. In a measurement with a small resistance value, the black probe is connected to the anode A, and the red probe is connected to the cathode K.The following method can also be used to measure light-controlled thyristors. Turn on the power switch S and illuminate the light receiving window of the thyristor VT with a flashlight. After adding a trigger light source (high-power light-controlled thyristor has its own light source, as long as the light-emitting diode or semiconductor laser in its optical cable is added with the working voltage, no external light source is required), the indicator EL should be on. After the light source is evacuated, the indicator light EL should remain illuminated. There is only one PN junction. Therefore, you just need to measure electrode A and G with a multimeter.Put the multimeter in the R × 1 kΩ level, and the two probes can be connected to one of the two pins of the thyristor under test (measure their forward and reverse resistance values). If a pair of pins is measured with a low resistance value, the black probe is connected to the anode A, while the red probe is connected to gate G, and the other pin is the cathode K. (2) Judging whether it is good or bad: Use the R×1 level of a multimeter to measure the forward and reverse resistance values between the electrodes of the BTG thyristor. Under normal conditions, the forward and reverse resistances between the anode A and the cathode K are infinite; the forward resistance between the anode A and gate G (when the black probe is connected to electrode A) is several hundred ohms to several thousand ohms and the reverse resistance value is infinite. If the forward and reverse resistance values between two electrodes are measured to be very small, it indicates that the thyristor has been short-circuited and damaged.(3) Detection of triggering capability: Put the multimeter in the R × 1 Ω level, connect the black probe to anode A, and the red probe to cathode K. The measured resistance should be infinite. Then touch gate G with your finger and add a human body induction signal to it. If the resistance between electrodes A and K changes from infinity to low resistance (a few ohms) at this time, it indicates that the thyristor has a good triggering ability. Otherwise, the performance of the thyristor is poor. Frequently Asked Questions about Thyristors1. What is thyristor and its types?A thyristor is a four-layer device with alternating P-type and N-type semiconductors (P-N-P-N). In its most basic form, a thyristor has three terminals: anode (positive terminal), cathode (negative terminal), and gate (control terminal). The gate controls the flow of current between the anode and cathode. 2. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction. 3. Where is thyristor used?Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. 4. Why SCR is called Thyristor?Silicon Controlled Rectifier (SCR) is a unidirectional semiconductor device made of silicon. This device is the solid state equivalent of thyratron and hence it is also referred to as thyristor or thyroid transistor. 5. What is thyristor diagram?In general, Thyristors are also switching devices similar to the transistors. ... SCR or Thyristor is a four-layered, three-junction semiconductor switching device. It has three terminals anode, cathode, and gate. Thyristor is also a unidirectional device like a diode, which means it flows current only in one direction.

IntroductionA printed circuit board (PCB) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. A PCB allows signals and power to be routed between physical devices.What are PCBs? How Do PCBs Work?CatalogIntroductionⅠ PCB Basics1.1 PCB MaterialsⅡ PCB Product CharacteristicsⅢ Common Sense of PCB ProcessⅠ PCB Basics1.1 PCB Materialsa. Copper Clad Laminate（referred to as CCL, or sheet material）Tg: Glass Transition Temperature, which is the temperature at which glassy substances are transformed between glassy and highly elastic (usually softened). In the PCB industry, this glassy substance is generally referred to as resin or dielectric layer composed of resin and glass fiber cloth. Tg is an important technical index reflecting the heat resistance of printed circuit board substrates. Generally, the higher the Tg value of printed circuit boards, the better the heat resistance. The Tg of general FR-4 copper-clad epoxy glass laminates for SMT printed circuit boards is 130 ～ 140℃, which can meet the requirements when using Sn-Pn solder. For lead-free solders with higher melting points, the Tg of the substrate can not withstand the high temperature of welding if the Tg ≤ 150 ° C. In special cases (high temperature use), the Tg can be greater than 170 ° C. However, excessive Tg will cause the hardness of the substrate to increase and the material to become brittle. Therefore, we cannot simply pursue high Tg. We should comprehensively consider the performance of the board and choose a suitable printed board substrate with a higher Tg, which is one of the requirements for lead-free welding.CTI: Comparative Tracking lndex (or comparative leakage index, tracking index). The highest voltage value that the surface of the material can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without the formation of leakage traces. The unit is V.CTE: Coefficient of thermal expansion. Generally, the sheet material performance of a PCB is measured by the linear expansion coefficient, which is defined as the ratio of the increase in length to the original length under a unit temperature change, such as Z-CTE. The lower the CTE value, the better the dimensional stability and vice versa.TD: Thermal decomposition temperature refers to the temperature at which the base material resin loses weight by 5%. It is a sign of delamination and performance degradation caused by the heat of the base material of printed circuit boards.CAF: Conductive Anodic Filament of printed boards is the phenomenon of electrochemical insulation damage on insulating substrates. It refers to the state where dendritic metals are precipitated between wires under the action of an electric field after a voltage is applied to parallel circuits on printed circuit boards. Or conductive anodic filament(CAF) occurs along the glass fiber surface of the substrate, thereby reducing the insulation between the wires.T288: It is a technical index that reflects the welding resistance of the printed circuit board substrate. It refers to the maximum time that the printed circuit board substrate can withstand the high temperature of welding at 288°C without blistering and delamination. The longer this time, the better it is for welding. For traditional Sn-Pb alloys with low welding temperature (220 ～ 230℃), when the thermal decomposition time of printed circuit board substrate is 260 ℃, T260≥30s can meet the requirements of SMT printed circuit board. For lead-free welding, the temperature is generally 250 ~ 260℃, the thermal decomposition temperature of the substrate of the printed circuit board will increase, and only T288≥300s at 288 ℃ can ensure that the substrate does not decompose and the performance is not damaged during welding.DK: dielectric constant.DF: Dissipation factor refers to the ratio of the energy that has been lost in the insulation sheet material of the signal line to the energy that still exists in the line.OZ: oz is the abbreviation of the symbol ounce, which is British measurement unit and also unit of weight. 1 OZ means the thickness of copper with a weight of 1 OZ evenly spread over an area of 1 square foot (FT2). It is the average thickness of copper foil expressed by the weight per unit area. It is expressed by a formula, that is, 1OZ = 28.35g / FT2. b. Copper FoilED Copper Foil: Electrodeposited copper, copper foil commonly used in PCB, cheap.RA Copper Foil: Rolled annealed copper, copper foil commonly used in FPC.Drum Side: smooth side of electrodeposited copper foilMatt Side: rough side of electrodeposited copper foilCopper: Elemental symbol Cu, atomic weight 63.5, density 8.89 g / cm3, and the electrochemical equivalent of Cu2 + is 1.186 g / Ah. c. Prepreg: referred to as PPEpoxy Resin: an organic polymer compound containing two or more epoxy groups in the resin molecule, which is a resin component used in prepregs that are currently commonly used.DICY: Dicyandiamide, a common hardenerR.C: resin contentR.F: resin flowG.T: gel timeV.C: volatile contentHarden: Under certain conditions (high temperature, high pressure or light), the epoxy resin and the hardener undergo cross-linking polymerization to form a polymer with a three-dimensional mesh structure. d. InkViscosity: Viscosity refers to the relative movement between adjacent fluid layers when fluid is flowing, and frictional resistance will be generated between the two fluid layers. Unit: Pascal. Seconds (pa.s).Hardness: The hardness of the ink after pre-baking is 2B, the hardness of the ink after exposure is 2H, and the hardness of the ink after finishing is 6H. Thixotropic: a property that the ink is gelatinous when it is left to stand, but its viscosity changes when it is touched. It is a physical property of a liquid, that is, its viscosity decreases under agitation, and it will return to its original viscosity soon after standing. By stirring, the thixotropic effect lasts for a long time and it is enough to reconstitute its internal structure. To achieve high-quality screen printing, the thixotropic of the ink is very important. In particular, during the squeegee process, the ink is agitated to make it liquid. This action speeds up the speed of ink passing through the mesh, and promotes the original ink with separate wires to be evenly integrated. Once the squeegee stops moving, the ink returns to a stationary state, and its viscosity quickly returns to the original required data. e. Dry Film· Structure of dry film Figure 1. Structure of Dry Film· Dry film consists of three parts and ingredients:   — Supporting film(Polyester)   — Photo-resist dry film   — Covering film(Polyethylene)· Main ingredients① Binder(film-forming resin) ②Monomer ③Photo-initiator ④Plasticiser ⑤Adhesion promoter ⑥Thermal polymerization inhibitor ⑦Dye ⑧Solvent· The types of dry film are divided into three types according to the different methods of developing and removing the dry film: solvent-based dry film, water-soluble dry film, and peel-off dry film; according to the purpose of dry film, it is divided into: dry resist film, masking dry film and solder mask dry film.· Photosensitivity: It refers to the amount of light energy required for the photoresist to react to form a polymer with a certain resistance under the irradiation of ultraviolet light. In the case of constant light source intensity and light distance, the sensing speed is expressed as the length of exposure time. Short exposure time means that the sensing speed is fast.· Resolution: refers to the number of lines (or intervals) that can be formed by the dry film resist within a distance of 1mm. The resolution can also be expressed by the absolute size of the lines (or intervals). f. Net YarnNet density:— T number \ mesh number: refers to the number of meshes within 1 cm. g. Drill bit· geometry structure name of drill bitFigure 2. Geometry Structure Name of Drill Bit· Point AngleThe point angle is composed of two narrow and long first point angle surfaces and two triangular hook-shaped second point angle surfaces. These four sides meet at the point angle, forming two short edges called chisel edges at the center of the joint. This is the place that the sheet material first touches. This chisel edge is first positioned under pressure and rotation to drill into the stack. A protruding square strip on each of the two outer sides of the first point angle surface is called a margin. This margin tends to spiral upward along with the drill body part, which is the contact part between the drill pin and the hole wall. The right angle at the intersection of the margin and the edge lip is very important to the quality of the hole wall. The point angle has a long edge between the first and second point angle surfaces. The point where the two long edges and the two chisel edges meet in the middle is the point angle. The angle formed by the two long edges is called the point angle. When the drilling paper is made of phenolic resin substrate, the drill point angle is about 90° ~ 110° due to less resistance. When drilling paper is FR4 glass fiber board, the point angle should be slightly blunt at 115° ~ 135°. The most common one is 130°. The angle between the first point angle surface and the horizontal plane of the long edge is about 15°, which is called primary face angle. The second point angle is about 30°, and the angle formed by the chisel edge and the edge lip is called a cheisel edge angle. · Types of drill bitFigure 3. Types of Drill Bit Ⅱ PCB Product Characteristics1) Impedance· The sum of resistance and reactance (capacitance, inductive reactance) on a vector. Common impedance types are characteristic impedance and differential impedance.2) Warpage· Maximum bow (Figure a) and twist (Figure b) of printed circuit boards using surface mount components should be less than or equal to 0.75%3) RoHSRoHS, the abbreviation of restriction of hazardous substances, is the restriction of the use of certain hazardous substances in electrical and electronic equipment. RoHS lists a total of six harmful substances, including: lead(Pb), cadmium(Cd), mercury(Hg), hexavalent chromium( Cr6+), polybrominated diphenyl ether(PBDE), polybrominated biphenyl(PBB).4) BacklightIt is an enlarged visual inspection method to check the integrity of the copper wall of the through hole. The method is to carefully thin the substrate outside the hole wall from a certain direction, and then use the principle of resin translucency to shoot light from the back. If the quality of the chemical copper hole wall is intact and there is no any holes or pinholes, the copper layer must be able to block light and be dark in the microscope. Once there are holes in the copper wall, light spots must appear and be observed , and can be enlarged as photographic evidence. The ground sample is about 4-6mm wide.Figure 5. Backlight Standard Diageam5) Anode Phosphor Copper Ball· Purity requirementsElementContent(%)ElementContent(%)Cu≥99.91Ni≤0.002P0.040-0.06Sb≤0.002Pb≤0.002As≤0.002Fe≤0.0025S≤0.002Sn≤0.002O≤0.002 · Features— A black (or brown-black) film forms on the surface of phosphor copper after power is applied.— Black (or brown-black) film is Cu3P, also called phosphor copper anode film.· Role of phosphor copper anode film— The anode film itself can catalyze and accelerate the (Cu+-E → Cu2+) reaction, thereby reducing the accumulation of CU+.— After the anode film is formed, it can inhibit the continuous generation of Cu+.— The electrical conductivity of the anode film is 1.5X104-1 cm-1, which has metal conductivity.— Phosphor copper is less anodized than pure copper (1A / DM2 P0.04-0.065%, phosphor copper is less anodized than oxygen-free copper, 50MV-80MV) and will not cause anode passivation.— Anode film will greatly reduce the phenomenon of tiny grains falling off the anode.— Anode film prevents the copper anode from dissolving too quickly.6) Method for Estimating Surface Area of Electroplated Copper Anode· Method for estimating surface area of round titanium basket copper anode: pDLF/2p = 3.14 D = diameter of titanium basket L = length of titanium basket F = factor· Method for estimating surface area of square titanium basket copper anode: 1.33LWFL = length of titanium basket W = width of titanium basket F = factor· F is related to the diameter of the copper ball:Diameter = 12 mm    F = 2.2Diameter = 15 mm    F = 2.0 Diameter = 25 mm    F = 1.7Diameter = 28 mm    F = 1.6 Diameter = 38 mm    F = 1.27) ICD IssuesInternal Connection Defects Ⅲ Common Sense of PCB Processa. Etching FactorThe index used to consider the amount of etching lateral erosion, because the amount of lateral erosion will be different for different copper thickness, so the etching factor is different from the total copper thickness.Calculation method:b. Lateral ErosionThe etching of the side wall of the wire under the resist pattern is called lateral erosion, and the degree of lateral erosion is expressed by the width of the side etching:· lateral erosion is related to the type, composition and etching process and equipment of the etching solution. c. Pool EffectDuring the etching process, the circuit board passes through the etching machine horizontally. Due to the gravity acting on it, the fresh medicine is blocked by the old one and cannot effectively react with the copper surface. d. Different Stages of ResinsA-stage resin: Some thermosetting resins are liquid in the early stages of manufacture or liquid when heated, and can still dissolve in some liquids at this time.B-stage resin: Some thermosetting resins can soften when heated in the middle stage of the reaction, but they cannot be completely dissolved or melted. At this time, they can swell or partially dissolve when contacted with some solvents.C-stage resin: The later stage of the reaction of some thermosetting resins, when it is practically insoluble or infusible. e. Font Color of SubstrateRed font: flame retardant grade, other fonts: non flame retardant grade. f. OthersMSDS: Material Safety Data Sheet provides a variety of information on safety, health and environmental protection for chemical substances and products, and can provide information on basic knowledge, protective measures and emergency actions of chemicals. In some countries, MSDS is also called SDS, and SDS terminology is used in ISO 11014.SGS: Societe Generale de Surveillance S.A. Founded in 1887, it is currently the world's largest and oldest non-governmental third party engaged in product quality control and technical certification of multinational companies. Headquartered in Geneva, it has 251 branches around the world, 256 professional laboratories and 27,000 professional and technical personnel, and carries out product quality inspection, monitoring and assurance activities in 142 countries.UL: UNDER WRITERS LABORATORIES INCIPC: The Institute for International and Packaging Electronic CircuitsISO: International Standards OrganizationMIL: Military StandardJPC: Japan Printed Circuit AssociationCOV: Coefficient of variationFR4: Abbreviation for Flame Retardant Type 4. It is the name of a flame-resistant printed circuit board material composed of a composite material of glass fiber and epoxy resin. It is the most widely used printed circuit board. g. pH ValueAlso known as the hydrogen ion concentration index and pH value, it is a scale of the hydrogen ion activity in a solution, which is a measure of the acidity and alkalinity of a solution in the usual sense. The concept was proposed by Danish biochemist Søren Peter Lauritz Sørensen in 1909. P stands for German Potenz, which means strength or concentration, and H stands for hydrogen ion (H +). Sometimes pH is also written in Latin as pondus hydrogenii.Under normal conditions (25°C, about 298K), when pH <7, the solution is acidic, when pH> 7, the solution is alkaline, and when pH = 7, the solution is neutral. h. Hull Cell TestHull designed Hull cell in 1939. The Hull cell test only requires a small amount of plating solution. After a short time test, the plating effect of the plating solution can be obtained in a wide range of current density. Because this test is sensitive to the composition and operating conditions of the plating solution, it is commonly used to determine the concentration and pH value of each component of the plating solution, and to determine the current density range for obtaining a good coating. The Hull cell has become an indispensable tool for electroplating research and electroplating process control.Hull cells are usually made of insulating materials such as plexiglass or rigid polyvinyl chloride. The bottom surface is trapezoidal, and the cathode and anode are placed on two sides that are not parallel. There are five types of capacity: 250ml, 267ml, 320ml, 534ml, and 1000ml. Generally, 250ml plating solution is often added to a 267ml test cell, which is convenient for converting the additives into how many grams per liter.Figure 8. Dimensions of Hull Celli. Current Density A / dm2A/dm2 — how many amperes per square decimeter area, 1A / dm2 — This is the current density of electroplating, which means that the current passes through the plating area of the workpiece per square decimeter is 1A and dm means decimeter. Generally, we use a 267ml Hull cell, place the test piece at the cathode and immerse it in the test solution. The area of the test piece is approximately 1dm2. j. TP: THROUGH POWERCalculation method:k. Replacement ReactionA replacement reaction is a chemical reaction in which a simple substance reacts with a compound to form another simple substance and a compound. Any replacement reaction is a metathesis reaction, including a reaction of metal and metal salt, the reaction of metal and acid, etc.; the replacement reaction must be a redox reaction, and the redox reaction is not necessarily a replacement reaction; the replacement reaction occurs according to the active list of metals. l. EDS：Energy Dispersive X-ray SpectroscopyThe surface of the sample being tested is irradiated with a condensed electron beam. Due to the interaction between the electron beam and the sample, various electrons or X-rays, photons, and other information are generated. Then, this information is collected and processed in different ways to display various characteristics of the sample (morphology, microstructure, composition, crystal plane, etc.) m. SEM：Scanning Electron MicroscopeAn electron beam with a diameter of 20 mm to 30 mm emitted from the cathode of the electron gun is accelerated by the voltage between the cathode and the anode, and is directed toward the lens barrel, and is condensed by the condenser lens and the objective lens, and is reduced into an electron probe with a diameter of about several nanometers. Under the action of the scanning coil on the upper part of the objective lens, the electron probe scans the surface of the sample in a raster pattern and excites a variety of electronic signals. These electronic signals are detected by corresponding detectors, amplified, converted into voltage signals, and finally sent to the grid of the picture tube and modulate the brightness of the picture tube. The electron beam in the picture tube is also raster scanned on the phosphor screen, and this scanning movement is strictly synchronized with the scanning movement of the electron beam on the sample surface, so that a scanning electron image corresponding to the contrast and the intensity of the received signal is obtained. The image reflects the topographical features of the sample surface. n. BONDINGBonding is a wiring method in the chip production process. It is generally used to connect the chip's internal circuit with gold or aluminum wires to the package pins or gold-plated copper foil on the circuit board before packaging. Ultrasonic waves from an ultrasonic generator (generally 40-140KHz), which generates high-frequency vibration through the transducer, are transmitted to the splitter through the horn. When the splitter is in contact with the lead wire and the welded part, under the action of pressure and vibration, the surfaces of the metal to be welded rub against each other, the oxide film is destroyed, and plastic deformation occurs, causing the two pure metal surfaces to closely contact each other to achieve the combination of atomic distance, and finally form a strong mechanical connection. After bonding (ie, after the circuit is connected to the pins), the chip is packaged with black glue. o. The Giovanni EffectIt means that due to the potential difference between two metals, an electric current is generated through the medium, and then an electrochemical reaction occurs, and the anode with a high potential is oxidized. p. Vacuum Degree; Degree of VacuumThe degree of thinness of the gas under vacuum is usually expressed by "high vacuum" and "low vacuum", high vacuum indicates "good" vacuum, and low vacuum indicates "poor" vacuum. If the pressure in the device under test is lower than atmospheric pressure, a vacuum gauge is required for pressure measurement. The value read from the vacuum gauge is called the degree of vacuum. The degree of vacuum is a value indicating that the actual value of the system pressure is lower than the atmospheric pressure, that is: the degree of vacuum = atmospheric pressure-absolute pressureThere are usually two ways to identify the degree of vacuum:   — First, it is marked with "absolute pressure" and "absolute vacuum degree" (that is, how much pressure is higher than "theoretical vacuum"); in actual cases, the absolute pressure value of the vacuum pump is between 0 and 101.325KPa. The absolute pressure value needs to be measured with an absolute pressure meter. The initial value of the gauge (absolute vacuum gauge) for measuring the vacuum degree at 20°C and the place where the altitude = 0 is 101.325KPa (ie, a standard atmospheric pressure).   — The second is to use "relative pressure" and "relative vacuum degree" (that is, how much pressure is lower than "atmospheric pressure") to identify. "Relative vacuum degree" refers to the difference between the pressure of the measured object and the atmospheric pressure at the measurement site and is measured with an ordinary vacuum gauge. In the absence of vacuum (that is, at atmospheric pressure), the initial value of the gauge is 0. When measuring vacuum, its value is between 0 and -101.325KPa (usually expressed as a negative number).Commonly used vacuum units are Pa, Kpa, Mpa, atmospheric pressure, kilogram (Kgf / cm2), mmHg, mbar, bar, PSI, etc. The approximate conversion relationship is as follows:   — 1MPa = 1000KPa   — 1KPa = 1000Pa   — 1 atmospheric pressure = 100KPa = 0.1MPa   — 1 atmospheric pressure = 1 kg (Kgf / cm2) = 760mmHg   — 1 atmospheric pressure = 14.5 PSI   — 1KPa = 10mbar   — 1bar = 1000mbar Frequently Asked Questions about PCB Basic1. What is PCB and types of PCB?A printed circuit board (PCB) is a thin board made from fiberglass, composite epoxy, or other laminate materials. PCBs are found in various electrical and electronic components such as beepers, radios, radars, computer systems, etc. Different types of PCBs are used based on the applications. 2. What is the basis of PCB?PCB is the acronym of Printed Circuit Board, a mechanical base that contains tracks and footprints reflecting the schematic of the design. Modern PCBs are typically made of a non-conductive substrate that is overlayed by copper layers. 3. What are the basic steps of PCB design?Here's the full list of PCB layout and design steps:Create the Schematic.Create a Blank PCB Layout.Schematic Capture: Linking to Your PCB.Designing Your PCB Stackup.Defining Design Rules and DFM Requirements.Place Components.Insert Drill Holes.Route Traces. 4. What are the common types of PCB?Common Types of Printed Circuit BoardsSingle Layer PCB. Single layer printed circuit boards are among some of the simplest to design and manufacture. ...Double Layer PCB.Multi-Layer PCB.High Density Interconnect (HDI) PCB.High Frequency PCB. 5. Which material is used in PCB?copper circuitryPrinted circuit boards (PCBs) are usually a flat laminated composite made from non-conductive substrate materials with layers of copper circuitry buried internally or on the external surfaces. They can be as simple as one or two layers of copper, or in high density applications they can have fifty layers or more.

Ⅰ IntroductionAn operational amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. Learn more about the most common opamp basics, essential knowledge when selecting and using an op amp in electronics. We can conclude our section and look at Op Amp basics with the following properties and questions. Opamp Basics: Op-Amp CircuitsCatalogⅠ IntroductionⅡ Amplifier Figures of MeritⅢ Q & AⅣ Application: LM358 Classic CircuitsⅡ Amplifier Figures of MeritNegative FeedbackIt is a important technique to improve bandwidth and distortion and control gain.Open-loop GainIt refers to the ratio of the voltage change at the output of the amplifier to the voltage change at the input when the amplifier input and output are open. Common-mode Rejection Ratio (dB)It is the ratio of the amplifier's amplification factor of the differential voltage signal to the amplification factor of the common mode voltage signal.Input Current NoiseIt is the equivalent current noise applied in parallel with the input of the noiseless amplifier.Output CurrentIt refers to the current driven by the load at the output of the op amp. It is usually a function: input overdrive, correlation between output voltage and power supply, temperature, source, and drain characteristics will differ.Phase MarginIt is the phase shift between an output of the same frequency and an inverting input in an open-loop circuit.Voltage GainIt is the ratio of the change in output voltage to the change in input voltage.Programmable Gain BufferIt can set the gain resistance of the op amp (integrated on the template), and the gain can be set to +1, +2, or -1 through simple external connections.Saturation VoltageIt is the voltage between the collector and emitter of the transistor under saturation conditions. In the saturated state, the emitter-base and collector-base are forward biased, so that the voltage between the collector-emitter is very low.Rise TimeThis refers to the time required for the output voltage to change from 10% of its final value to 90% of its final value.Unity-gain BandwidthIt refers to the frequency at which the amplifier's open-loop gain is equal to one. If the op amp frequency response has a single-pole roll-off, the unity-gain bandwidth is equal to 1UGBW.Strobe “OFF” VoltageThe strobe “OFF” voltage is the minimum voltage at the strobe pulse and is guaranteed not to interfere with the comparator operation.Input Current IndexIt refers to the average of the current drawn from the two input pins. In addition, the input current is also commonly called bias current.Gain Bandwidth ProductIt refers to the product of the amplifier's bandwidth and the gain at which the bandwidth is measured.Large Signal Voltage GainIt refers to the ratio of the change in output voltage to the change in input voltage. This parameter is usually specified at a large output voltage, smaller than the maximum output voltage, which is the typical value under direct current conditions.Offset Voltage Temperature CoefficientIt refers to the average rate of change in offset voltage due to changes in junction temperature within a specified temperature range.Output High VoltageIt refers to the high DC output voltage of the comparator, which produces the high output current. And it is usually related to the totem pole or push-pull output of the comparator.Input Source CurrentIt refers to the maximum output positive current produced under the comparator's push-pull output state.Total Harmonic Distortion (THD)When a pure sinusoidal signal is input to the op amp as Vin (w) = Vpsin (wt):Input harmonic distortion: Vout(w)a1Vpsin(wt)+a2Vpsin(wt)+...+anVpsin(nwt)The expression of THD is: THD(%)=[sqrt(a2xa2+a3xa3+...+anxan)/a1]x100Common-Mode Input Impedance (RINCM)It refers to the ratio of the change in the common-mode input voltage to the change in the input current at the inverting or non-inverting terminal.Output Low VoltageIt refers to a low DC output voltage. The output drive is a low voltage sink current. This specification is usually related to the totem pole or push-pull output of the comparator.Using a CMOS op amp as the output driver, although the circuit works well, but requiring a 1m shielded cable, and the oscillation of the operational amplifier is about 1MHz when there is no input signal. If shorten the cable to 10cm, the oscillation is stable.Some op amps are not suitable for driving capacitive loads directly, such as long shielded cables, which is a capacitive load. In addition, coaxial cables have about 60-100pF capacitance per meter.Harmonic DistortionIt refers to the unwanted spurious signals generated at the amplifier output due to the non-linearity of the signal line. When the input is a sinusoidal signal, these spurious signals will appear as integer times of the input frequency (for example, second harmonic, third harmonic).Output Leakage Current (ILEAKAGE)It means that the current enters the comparator output (the output is driven high). It often appears at the output of open collector and open drain.Power Supply Rejection Ratio (PSRR)It refers to the ratio of the change in the input offset voltage to the change in the power supply voltage, PSRR (dB) = 20log10 (DVOS / DVS)Linear Phase DeviationIt refers to how a closed-loop phase response of an operational amplifier approaches and follows the linear relationship between phase change and frequency in a specific frequency band.-3dbIt refers to the frequency when the value of the small signal output amplitude of the closed-loop amplifier decreases to 3dB.Common-mode Voltage RangeIt refers to the typical value of the voltage range at the input, which determines the performance of the amplifier.Specified Power Supply RangeIt describes the power supply voltage required for the operational amplifier to operate.Output Absorption CurrentIt refers to the highest output negative current of the comparator.Output Voltage SwingIt refers to the maximum peak-to-peak swing of the output voltage under a specific load and power supply voltage.Current FeedbackIt refers to a technology used in current feedback amplifiers whose output signal reflects the value of the current input to the inverting input (transimpedance gain function). In some aspects, this topology has operational advantages over traditional voltage feedback.Closed Loop BufferIt refers to an amplifier with high input impedance and low output impedance and a fixed gain of +1. Its typical applications are used for isolation, increased output drive, capacitive load, etc., in addition, there is no need to set the gain resistance.Closed-loop Gain It is the ratio of the change in the output voltage to the change in the input voltage after the feedback and input network added. Generally, this value is set using an external resistance.Common-mode RangeThe common-mode range, also known as the input voltage range, is a measure of the range of input voltages that the input pins of an op amp can accept. This specification is usually relative to the power supply amplitude.Output ImpedanceIt refers to the ideal series output impedance of the ideal operational amplifier when there is no impedance, which is the approximate output impedance of the op amp measured under AC  conditions.Transient ResponseIt refers to the step function response of the closed-loop system of the amplifier under the condition of small signal (usually less than 100mV).Slew RateWhen given a transition or square wave input, the amount of change in the amplifier's output from one level to another. Typical values are averages of values measured based on a change in total output voltage from 10% to 90%.Response TimeIt is the time interval when the input step function makes the output from the initial value to the logic threshold voltage.Unity Gain FrequencyIt refers to the frequency at which the gain of the voltage feedback op amp is 1 (0 dB). For an ideal operational amplifier, its gain-bandwidth product is equal.Intercept PointIt refers to the output power of the fundamental frequency, which is equal to the power value of the fundamental frequency in the specified distortion term (2nd, 3rd, or 3rd intermodulation).Input Offset CurrentIt refers to the current difference between the two inputs.Voltage OverdriveIt means that a certain amount of input step voltage exceeds the minimum drive input voltage required by the comparator to change from one logic level to the opposite logic level.Differential Gain & Differential PhaseDifferential gain refers to the change in the input and output of the gain, and differential phase refers to the phase change in the input stage. They are video measurements, and are a standard measurement in the broadcasting field to measure relative changes in the interpretation of video signal consistency.Voltage FeedbackA technique used in traditional operational amplifiers, where part of the output voltage is fed back to the input, and the voltage difference between the two inputs is amplified by the operational amplifier. Avol Open-loop Voltage Gain“A” is a sign of gain. The letter “V” written below indicates the gain of voltage, and the letter “ol” also written below is an abbreviation for open loop. Open-loop voltage gain refers to the gain (Vout / Vin) of the amplifier without feedback. Due to the existence of the bias voltage, these errors must be compensated.Logic Threshold VoltageIt refers to the voltage that causes the comparator output state to change when the input offset voltage is exceeded.Output ResistanceIt refers to the value of the series resistance at the output of an ideal op amp with zero output resistance, which measured under DC conditions.Gain FlatnessIt refers to the volume of gains “violently increasing” and “rapidly decreasing” in a given bandwidth range measured in decibels (dB), which affects the most important parameter specifications such as phase margin, gain margin, and closed-loop gain.Offset Current Temperature CoefficientIt refers to the average rate of change in deviation current due to changes in junction temperature within a specified temperature range.Input ImpedanceIt is the ratio of input AC voltage to input AC current.Input Voltage NoiseIt refers to the equivalent voltage noise in series with a noiseless amplifier.Input Offset VoltageIt is the product of the DC error voltage between the inputs and the closed-loop gain, because of the non-ideal balance between the input stage and the output is caused by the DC error voltage of the input terminals.Gain MarginOpen loop gain when the phase between the inverting input and output crosses zero at a certain frequency.Supply CurrentIt refers to the current required from the power supply to the unloaded amplifier and to the power supply at the output midpoint.Settling TimeIt refers to the time between the input step function initial value and the output voltage reaching the specified error band. The error band refers to the percentage of the total voltage change.Differential Input ResistanceIt is the ratio of the change in the input voltage to the change in the input current.Ⅲ Q & AQ1: What is the difference between a voltage feedback amplifier and a current feedback amplifier?A: The internal circuits of these two op amps are different. The voltage feedback op amp is restricted by the internal design, and it only has a very low input bias current, but there is no internal limit on the differential input voltage, because it is limited only when external feedback is required. In contrast, for a current feedback amplifier, its differential input voltage is subject to internal design, but it does not limit its input bias current, so it is limited only when external feedback is required.Q2: What is the difference between open and closed loops?A: The open loop gain is actually the internal gain of the op amp without feedback, and usually takes any value between 1,000 and 10,000. Closed loop gain is the gain of the entire circuit, which is equal to the open loop gain divided by 1 plus the loop gain (the improvement coefficient). In fact, the gain of the op amp when there is no feedback is the open loop gain, and the gain when feedback is considered is closed-loop gain.Q3: If the op amp has ideal AC characteristics, the Bode plot (gain-frequency response) is a unipolar system. What is the gain slip rate in dB / decade?A: In a unipolar system, the gain drops (or decreases) at 20dB / decade, which is 6dB / octave. This is responsive to any single pole, and it is also suitable for a simple RC filter or an ideal operational amplifier. However, because op amps have more high-frequency poles, the phase shift will begin to increase as the frequency approaches the unity gain frequency of the op amp.Q4: What is the difference between unity gain bandwidth, gain bandwidth product (GBP), and -3dB frequency?A: Many op amps have an open-loop gain reduction rate of -20db / decade when the frequency is stable. At any point during this descent phase, the GBW is a constant. If the unity-gain operation of the op amp is stable, then the unity-gain bandwidth, or the frequency at which the open-loop gain is 1, is usually equal to GBP. In addition, GBP is not equal to (usually higher than) the unity gain bandwidth. The -3dB frequency is a measure of the bandwidth of an operational amplifier when it is operating in a closed loop. The -3dB point is the frequency at which the gain of the overall closed-loop system drops by 3dB. The unity gain frequency for closed loop applications can be calculated using BW=GBP/Av. The -3dB frequency and unity-gain bandwidth applied depend on the feedback gain setting, output swing, load, and circuit layout.Q5: Why do some amplifiers oscillate with a capacitive load?A: The output impedance of the op amp and the capacitance of the capacitive load may form a resistance-capacitance oscillation. Also they form an R-C oscillation at the output stage, which causes additional phase lag in the feedback signal. CMOS amplifiers have a high output impedance which will cause the electrodes to be approached or lower the unity gain frequency of the op amp. The additional phase lag of the electrodes will weaken the phase margin of the op amp The total phase lag of the amplifier causes the phase angle of the unity gain frequency to increase by more than 180 degrees to cause the total feedback phase shift in unity gain to exceed 180. degree. In addition, the output impedance of a CMOS amplifier is between 100 and 500, causing a relatively low pole frequency. And meanwhile, the output impedance of the high-speed bipolar operational amplifier is in the range of 1 to 100, which causes the pole frequency to be much higher than that of the CMOS operational amplifier, so that the pole is far from the unity gain frequency of the device. The drive of a CMOS amplifier to a capacitive load can be improved by placing an output resistor at the output and an external positive feedback capacitor.Q6: If the output of the op amp stays close to the voltage rail, that is, the output rail, what is the reason?A: There are many ways for operational amplifiers to “rail”. The difficulty is keeping it away from the "rail". If the input exceeds the input voltage range, the output is usually near to a supply voltage rail. In theory, if the output exceeds the actual supply voltage, and a higher supply voltage is given, the op amp will go to rail output again. If there is no feedback or the polarity of the feedback is wrong, the op amp goes to rail output again. At the same time, if the non-inverting input is higher than the negative inverting input, the op amp also goes to rail output. The application of the operational amplifier should be analyzed to ensure that the power supply voltage used has a proper input and gain, so that in normal operation, its input voltage is within the rated value and the output voltage is within the normal range.Q7: What is the difference between the common-mode voltage and input voltage range of an op amp?A: Common mode voltage means that one voltage is applied to both inputs at the same time. Input voltage range is the range of voltages that can be accepted by the input pins. It is necessary to remember that the op amp should suppress the common-mode voltage, and amplify the difference between the two input pins only.Q8: The SPICE model of the bipolar operational amplifier works well, but the SPICE model of the CMOS operational amplifier does not work. Is there a need to set SPICE?A: To input the appropriate bias current to the model, the SPICE model applied on CMOS operational amplifier needs to set the default GMIN option to the largest SPICE package value.Q9: What is the difference between the amplifier's output current and short-circuit current?A: Short circuit current refers to the current generated by the device if the output is connected directly to the power line. This indicates that the output current is limited depending on the design of the device. However, the short-circuit current does not represent the true output of the drive capability of the output. Due to the impedance characteristics of the output stage, the maximum output current is determined by the swing of the output voltage under load. In facet, the smaller the load, the larger the output swing; the larger the load, the smaller the output swing.Q10: How to check the stability of an op amp circuit?A: Check the stability of the control loop, such as the pulse load and related changes in output voltage. The pulse load may be a load current with a pulse or step change, so that the output of the op amp circuit should be connected to a series R-C circuit. The greater the circuit swing or vibration, the worse the stability of the circuit.Q11: Are there any good ways to minimize noise when amplifying a low-level DC signal?A: To obtain a high signal-to-noise ratio, the circuit must be well designed. This includes choosing the best amplifier bandwidth and knowing the impedance of the input signal. If the input signal source has a fairly high impedance, it makes no sense to choose a low voltage noise amplifier, which has high current noise.Q12: How should design a low frequency (<1Hz) differentiator to minimize the output noise?A: The only reason that the output of the differentiator contains noise is because there is a lot of gain and the input is noisy. The traditional differentiator uses Rs-Cs in series at the input and the Rf-Cf in parallel near the operational amplifier. It is not necessary to try more Rs or Cf to minimize noise. The noise of the output come from the differentiator does not mean that it is harmful, because it also amplifies useful signals. In addition, if disconnect a loop, the differential output noise may be beneficial and will stabilize the loop. If the output of the differentiator is quite noisy or has too much input noise, analyze which are the real sources of them.Q13: How to protect the amplifier input from being higher or lower than the supply voltage?A: What must be done is either to clamp the input of the device, or to limit the input current of the device, or ideally, do both. The easiest way is to choose a current limiting resistor to limit this current. The selection is based on the fact that the current generated by the circuit input at the maximum input voltage is less than the maximum current rating of the input pin. Usually, a 1K to 100K resistor in series with this input pin is effective. However, since the signal is usually connected directly to a non-inverting input pin, a non-inverting amplifier may need a protective resistor connected to this pin. For high impedance circuits, a large resistor and or low leakage current diode can be used.Q14: What is the difference between a single-supply amplifier and a dual-supply amplifier?A: There is no difference in the actual circuit, layout, and characteristics of the amplifier. When an operational amplifier is designated as dual power supply, the output load is usually referenced to ground (GND), while a single power supply operational amplifier is usually referenced to the midpoint voltage of a single supply, and it is usually specified to operate on lower voltages, but this is not a necessary requirement. Therefore, whether the op amp is powered by a single 5V power supply and ground (GND), or powered by +2.5 and -2.5V, these is no different. Ⅳ Application: LM358 Classic CircuitsThis Video is Going to Show Top 5 Electronics Project Using OP-AMP LM358The LM358 includes two independent, high-gain, internal frequency-compensated dual operational amplifiers. It is suitable for single-supply operation with a wide range of power supply voltages. It is also suitable for dual-supply operation. LM358 applications include sensor amplifiers, DC gain modules and all other operational amplifiers that can be powered by a single power supply. The classic circuits of LM358 are as shown as following:Figure 1. Active DC-coupled Low Pass RC Filter Figure 2. LED Driver Figure 3. Transistor-Transistor-Logic (TTL) Drive Circuit Figure 4. Active RC Band Pass Filter Figure 5. Squareware Oscillator Figure 6. Hysteresis ComparatorFigure 7. Active Band Pass filter Figure 8. Lamp Driver Figure 9. Current Monitor Figure 10. Low Drift Peak Detector Figure 11. Voltage Follower Figure 12. Power Amplifier Peripheral CircuitFigure 13. Voltage Controlled Oscillator VCOFigure 14. Fixed Current Source Figure 15. Pulse Generator Figure 16. AC Coupled Non-inverting Amplifier Figure 17. AC Coupled Inverting Amplifier Figure 18. Adjustable Gain Instrumentation Amplifier Figure 19. DC Amplifier Figure 20. Pulse Generator Figure 21. Bridge Current Amplifier Figure 22. Introducing Differential Input Signal Figure 23. DC Differential Amplifier Frequently Asked Questions about Op Amps Basics1. What is an op amp basics for dummies?An op amp is a super-sensitive electronic amplifier circuit that's designed to amplify the difference of two input voltages. Thus, an op amp has two inputs and one output. ... Most op amps require both a positive and a negative voltage power supply, with voltages usually ranging from 6 V to 18 V. 2. What is the basic use of op amp?An operational amplifier is an integrated circuit that can amplify weak electric signals. An operational amplifier has two input pins and one output pin. Its basic role is to amplify and output the voltage difference between the two input pins. 3. What is operational amplifier and its types?An operational amplifier (op amp) is an analog circuit block that takes a differential voltage input and produces a single-ended voltage output. Op amps usually have three terminals: two high-impedance inputs and a low-impedance output port. 4. Which purpose the op amp is used?As the name suggests, the purpose of an amplifier or an op amp is to amplify or increase the input signal to produce an output signal which is much larger than that of the input, with a similar waveform as that of the input. The main change in the output signal will be the increase in the power level. 5. What does it mean when an op amp saturates?Originally Answered: What happens when an op-amp is saturated? that means the amplification or gain is so high as to make the output signal with a given input signal, so large as to exceed the compliance range of the power supply of the ope amp. More simply put, if you have an op amp supplied with +/-15V supply rails.

Ⅰ IntroductionAs we all know, capacitors have always played a very important role in electronic circuits. They are responsible for the coupling of signals in electronic circuits, the differentiation of volt-ampere characteristics in RC circuits, such as integration, the "channel" in oscillating circuits, bypass and power filter, etc. Aluminum electrolytic capacitor is made of anodized aluminum foil, corroded cathode aluminum foil and electrolytic paper in the middle, then impregnated with working electrolyte and sealed in aluminum shell. CatalogⅠ IntroductionⅡ Common problems of electrolytic capacitor  2.1 Why can't an aluminum electrolyte capacitor withstand reverse voltage?  2.2 What are the similarities and differences between nonpolar capacitance and polar capacitance?  2.3 What will happen when a polar capacitor is reversed?  2.4 The reverse connection of the polar capacitor will explode. Does it mean that it can't be directly connected to the AC power supply?  2.5 If the polarity capacitor is reversed, why is it short-circuited?  2.6 Why does the resistivity of electrolytic capacitor become smaller when the positive and negative poles are reversed?  2.7 Why can we only use a nonpolar capacitor in a pure AC circuit?  2.8 What is electrolytic capacitance?  2.9 The characteristics of electrolytic capacitors are as follows  2.10 What are the similarities and differences that cannot be ignored between polar and nonpolar capacitors in performance, principle and structure?Ⅲ SummaryⅣ FAQ Ⅱ Common problems of electrolytic capacitor2.1 Why can't an aluminum electrolyte capacitor withstand reverse voltage?Due to the polarity of electrolytic capacitors, it is necessary to pay attention to the correct connection of positive and negative electrodes in use, otherwise, not only the capacitors can not play a role, but also the leakage current is very large. In a short time, the inside of the capacitors will heat up, damage the oxide film, and then damage.  As shown in the figure, the basic structure of the aluminum electrolytic capacitor is composed of an anode, aluminum layer attached to the insulating medium, cathode aluminum layer of the receiving electrode and the real cathode composed of electrolyte. The electrolyte is soaked in the paper between the two aluminum layers. Aluminum oxide layer is plated on the aluminum layer, which is very thin compared with the voltage applied on it, and it is easy to be broken down, leading to capacitor failure. The alumina layer can withstand the forward DC voltage. If it bears the reverse DC voltage, it is easy to fail in a few seconds. This phenomenon is called the "valve effect", which is why the aluminum electrolytic capacitor has polarity. If both electrodes of the electrolytic capacitor have an oxide layer, the non-polar capacitor will be formed.Many articles report the mechanism of the threshold phenomenon of the reverse voltage of the aluminum electrolytic capacitor, which is called the hydrogen ion theory. When the electrolytic capacitor bears the reverse DC voltage, that is, the cathode of the electrolyte bears the positive voltage while the oxide bears the negative voltage, The hydrogen ions gathered in the oxide layer will pass through the medium and reach the boundary between the medium and the metal layer, and then they will be converted into hydrogen. And the expansion force of the gas causes the oxide layer to fall off. Therefore, the current flows directly through the capacitor after breaking through the electrolyte, and the capacitor fails. This DC voltage is very small. Under the reverse DC voltage of 1 ~ 2V, the aluminum electrolytic capacitor will immediately fail due to the hydrogen ion effect in a few seconds. On the contrary, when the positive voltage is applied to the electrolytic capacitor, the negative ions are concentrated between the oxide layers. Because the diameter of the negative ions is very large, they can not break through the oxide layer, so they can withstand higher voltage.2.2 What are the similarities and differences between nonpolar capacitance and polar capacitance?Are nonpolar capacitors the same as nonpolar electrolytic capacitors? Most kinds of capacitors are nonpolar, only the electrolytic capacitors have polarity. Among them, there are very special nonpolar electrolytic capacitors. Compared with ordinary capacitors, electrolytic capacitors have a larger capacity, lower price and smaller volume than other capacitors, but electrolytic capacitors generally have polarity, and their working reliability, withstand voltage, temperature resistance, dielectric loss and other indicators are not as good as other capacitors.The so-called non-polar electrolytic capacitor is actually the back-to-back packaging of two identical electrolytic capacitors. This kind of capacitor has large loss, low reliability and low voltage withstand, which can only be used in a few occasions with low requirements.2.3 What will happen when a polar capacitor is reversed?If the capacitance capacity is very small, the withstand voltage is very high, and the working voltage is low, there will be nothing wrong with the reverse connection. If the capacity is slightly large (above 100uF) and the withstand voltage is close to the working voltage, the capacitance can just work for nearly 10 minutes, then it will bulge and burst.2.4 The reverse connection of the polar capacitor will explode. Does it mean that it can't be directly connected to the AC power supply?It can't be connected to the AC power supply, because the polar capacitor is designed to be used in the DC power supply for filtering, and there is special material inside the polar capacitor, which can not bear the backpressure. If it is connected to the AC power supply, it will breakdown reversely or explode.2.5 If the polarity capacitor is reversed, why is it short-circuited?The internal structure of the polar capacitor is divided into the positive electrode, a dielectric layer and the negative electrode. The dielectric layer has the property of unidirectional conduction. Of course, the dielectric layer of the product will not play the role of insulation after being connected reversely, and the capacitor will be short-circuited. 2.6 Why does the resistivity of electrolytic capacitor become smaller when the positive and negative poles are reversed?It involves the principle of electrolytic capacitor. When the positive electrode of the capacitor is connected positively, a very thin oxide film (alumina) will be formed as the dielectric. When the negative electrode of the capacitor is connected reversely, H2 will be produced without forming the oxide film, and the other electrode will not form the oxide film which can be used as the dielectric due to different materials. 2.7 Why can we only use a nonpolar capacitor in a pure AC circuit?In the circuit of DC voltage superposing AC signal, if we can ensure that the lowest voltage after superposing will not become negative, we can use a capacitor with polarity. In the case of the same capacity, the volume and cost of the polar capacitor are far less than that of the nonpolar capacitor, so when we need a larger capacity, the volume of the capacitor is a big contradiction. We usually replace non-polar capacitors with polar ones, which not only solves the volume problem but also reduces the cost. Large capacitance can filter the AC signal with a lower frequency and above, while small capacitance can only filter the signal with higher frequency and above. 2.8 What is electrolytic capacitance?Electrolytic capacitor is a kind of capacitor. Its medium is coated with electrolytes. It can be divided into positive and negative electrodes and cannot be connected wrongly. The capacitance is composed of two metal poles and the insulating material (medium) sandwiched between them.2.9 The characteristics of electrolytic capacitors are as follows①The capacitance per unit volume is dozens to hundreds of times larger than other kinds of capacitance.②Rated capacity can easily reach tens of thousands of μ for even several F, but it is not as good as double electric layer capacitance.③The price is much lower than other kinds because the components of electrolytic capacitors are ordinary industrial materials, such as aluminum. The equipment for manufacturing electrolytic capacitors is also common industrial equipment, which can be mass-produced at a relatively low cost. Electrolytic capacitors are usually made up of metal foil (aluminum/tantalum) as the positive electrode, and the insulating oxide layer (alumina/tantalum pentaoxide) of metal foil as the dielectric. The negative electrode of aluminum electrolytic capacitor is composed of thin paper/film or electrolyte polymer soaked in electrolyte, the negative electrode of the tantalum electrolytic capacitor is usually manganese dioxide. As both of them use electrolytes as the negative electrodes, the electrolytic capacitor gets its name. The polar electrolytic capacitor usually plays the role of power filter, decoupling, signal coupling, the time constant setting, DC isolation and so on in the power circuit or IF and LF circuits. It can't be used in an AC power circuit. When it is used as a filter capacitor in the DC power circuit, its anode (positive) should be connected with the positive end of the power voltage, and the cathode (negative) should be connected with the negative end of the power voltage. It can't be reversed, or it will be damaged. 2.10 What are the similarities and differences that cannot be ignored between polar and nonpolar capacitors in performance, principle and structure? Polar capacitance is a kind of electrolytic capacitance. It consists of two electrodes formed by the anode aluminum foil and the cathode electrolyte. A layer of aluminum oxide film produced on the anode aluminum foil is used as the dielectric of capacitance. As a result of this structure, it has polarity. When the capacitance is directly connected, the aluminum oxide film will remain stable due to the electrochemical reaction. When the reverse connection is made, the aluminum oxide layer will become thinner, which makes the capacitor easy to be broken down and damaged.  Therefore, we must pay attention to the polarity of the electrolytic capacitor in the circuit. Ordinary capacitors are nonpolar. We can also connect two anodes or cathodes of electrolytic capacitors in series to form nonpolar electrolytic capacitors. ①The same principleThey both store and release charges.The voltage on the plate shall not change suddenly. (voltage here refers to the electromotive force of charge accumulation) ②Different mediaWhat's the medium? It's the material between the two plates of the capacitor. Most of the polar capacitors use electrolytes as a dielectric material, and the capacity of the polar capacitor is larger than that of the same volume. In addition, the capacity of the same volume of polar capacitance produced by different electrolyte materials and processes will be different. The withstand voltage of the capacitor is closely related to the dielectric materials used. There are also many dielectric materials for non-polar capacitance, most of which are metal oxide film, polyester and so on. The reversibility or irreversibility of dielectric determines the use environment of polar and nonpolar capacitors. ③Different performance.Performance and maximization of requirements are the requirements for use. If a metal oxide film capacitor is used for filtering in the power supply part of the TV set, and the capacity and withstand voltage of the capacitor should meet the requirements of filtering, a power supply must be installed in the shell. For a filter, only the polar capacitance can be used, which is irreversible. The positive terminal must be connected to the high potential terminal and the negative terminal to the low potential terminal. Generally, when the electrolytic capacitance is more than 1 microfarad, it is used for coupling, decoupling, power filtering, etc. Most of the nonpolar capacitors are below 1 microfarad, which participates in resonance, coupling, frequency selection, current limiting, etc. Large capacity and high withstand voltage capacitors are usually used for reactive power compensation, motor phase-shifting and frequency conversion power supply phase-shifting. There are many kinds of nonpolar capacitors. ④Different capacityFor capacitors of the same volume, when the medium is different, the capacity is also different. ⑤Different structureWe can use capacitors of any shape without considering the tip discharge. The polar capacitance is usually round, and there are few polar capacitances of square type. There are many shapes of nonpolar capacitors, including tube type, deformed rectangle, sheet type, square type, circular type, combined square type and circular type, and of course there are intangible ones. Here intangible refers to distributed capacitance.The distributed capacitance in HF and IF devices should not be ignored. The function is the same. The main difference is their capacity. Due to the influence of material structure, the capacity of non-polar capacitance is relatively small, generally below 10uF, while the capacity of polar capacitance is usually large. When filtering the power supply, you have to use a polar capacitor of large capacity. Ⅲ SummaryOne of the basic principles of circuit design is to require the designer to fully understand and master the real components. The components used should be standard parts, general parts, and the most common models on the market (the better the versatility of components, the easier the procurement, the larger the supplier's output, and the lower the procurement cost). For the components used in the drawings, if the materials can only be obtained by customization, the cost is certainly not low. If you can't get the customized material, this design is wastepaper. In addition, large capacitance is suitable for filtering low-frequency signals and small capacitance for filtering high-frequency signals. However, decoupling is only one function of capacitance. Different kinds of capacitance have different characteristics and usages. This aspect has a lot to do with experience. It is impossible to achieve it quickly. It can only be accumulated through practice. Ⅳ FAQ1. Which capacitor gives a long-term service: ceramic capacitors or aluminum and tantalum electrolytic capacitors?Electrolytics have a limited lifetime, 10,000 hours at high temps.Tantalums are really good capacitors, until they short out, whenever they feel like it.Ceramics tend to live the longest. 2. Why is aluminum used in electrolytic capacitors?Aluminum has been found to be among ideal materials for electrolytic capacitors due to the following reasons—1) It easily forms a thin oxide layer with a high dielectric constant.2) This layer can be formed in a wide range of thicknesses to suit different applications.3) The aluminum oxide layer can be formed and can withstand high voltages exceeding 400 V. Other materials Tantalum/ Niobium can only take small voltages of below 25 V. 4) Aluminum can be made into foil/ plate / formed into shape. In yesteryears, it was common to use shapes of this metal mechanically formed into different shapes.5) These properties allow high capacitor values for low and high voltages in small size.6) Most amenable to convenient manufacturing processes like winding, punching, forming (oxidation).7) Most abundant material on earth, hence very cheap.There are hardly any other materials that have these properties. 3. What is the role of aluminum electrolytic capacitors?Aluminum electrolytic capacitors are polarized capacitors because of their anodization principle. They can only be operated with DC voltage applied with the correct polarity. Operating the capacitor with the wrong polarity or with AC voltage leads to a short circuit and can destroy the component. 4. What happens if the electrolytic capacitor backward?Electrolytic capacitors are polar by nature and have positive and negative terminals clearly marked. If the polarity is reversed while connecting, the dielectric in the form of an oxide layer is damaged. A heavy current flows, a large amount of heat is generated, and the capacitor is damaged. 5. How do you determine the polarity of the Aluminium electrolytic capacitor?If the case is insulated, you can try applying a small bias voltage (3-5V) to the capacitor in each direction (through a current-limiting resistor of 100K or so) and see which direction allows the least current; this will be the correct polarity of the capacitor. 6. What are aluminum electrolytic capacitors used for?Especially aluminum electrolytic capacitors are used in many applications as decoupling capacitors to filter or bypass undesired biased AC frequencies to the ground or for capacitive coupling of audio AC signals. Then the dielectric is used only for blocking DC. 7. How long do aluminum electrolytic capacitors last?Today's aluminum electrolytic capacitors have a longer shelf life, usually around 2 years, as compared to their predecessors. For aluminum electrolytic capacitors, the changes in ESR, capacitance, and leakage current are caused by the chemical reactions between the aluminum oxide film and the electrolyte. 8. How do you read an electrolytic aluminum capacitor?The value of the capacitor is denoted in picofarads for ceramic, film, and tantalum capacitors, but for aluminum electrolytic capacitors the value is denoted in microfarads. For small values the letter R is used to denote a decimal point, e.g. 0R5 is 0.5, 1R0 is 1.0 and 2R2 is 2.2, etc. 9. How are aluminum electrolytic capacitors made?Aluminum electrolytic capacitors are made by layering the electrolytic paper between the anode and cathode foils and then coiling the result. The process of preparing an electrode facing the etched anode foil surface is extremely difficult. Due to this process, the electrolyte essentially functions as the cathode. 10. What are aluminum capacitors used for?Aluminum electrolytic capacitors (electrolytic) are widely used in power supply applications requiring high capacitance in energy-dense, small-volume packages having very low equivalent series resistance (ESR).