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Introduction As a beginner, what the functions of capacitors in a circuit? A Capacitor is a passive electronic component that stores and releases the energy. Its unique characteristic is blocking direct current while allowing alternating current to pass. The main functions of capacitors are based on these characteristics. The use of capacitors is also based on this. Here are collecting 20 questions about capacitor use in electronics enclosed with details. Let you have a more comprehensive understanding of the use of capacitors. How Capacitors Work? 20 Questions about the Role of Capacitors in Circuits Help you learn about capacitors functions in 30 minutes. These questions forcus on how capacitors work, where capacitors are used, why capacitors are used, the different types. 1) What is the function of a capacitor connected in parallel with the positive and negative terminals of the voltage source?When cap used in a rectifier circuit, it has a good filtering effect. When the voltage is alternating, the voltage at both ends cannot be changed suddenly due to the charging effect of the capacitor, which ensures the stability of the voltage. When cap used as a battery power supply, it is equivalent to short-circuiting the battery's AC signal, avoiding the increase in battery internal resistance and parasitic oscillation of the circuit due to the battery voltage drop. 2) A capacitor in series or in parallel can achieve the effect of coupling in the circuit. What is the difference between whether there is a capacitor in the circuit?In the AC multi-stage amplifying circuit, because of the different gains and powers of each stage, the DC working offset values of each stage are different. If the levels are directly coupled, it will cause the bias values of all levels to be mixed and unable to work normally. The “Pass AC, Block DC” characteristic not only solves the coupling of inter-stage exchanges, but also isolates the inter-stage biased value intermixing. 3) The two coupling capacitors in the basic amplifying circuit, the positive pole of the capacitor and the DC positive pole are connected to pass the AC and block the DC. Can the reverse connection also have this function?If the connection is reversed, the electrolytic capacitor will leak, which will change the DC operating point of the circuit and make the amplifying circuit abnormal or unable to work. 4) What is the role of the capacitor in the resistance-capacitance coupling amplifier circuit?Block the DC signal so that the static operating points of adjacent amplifying circuits are independent of each other and do not affect each other. 5) Can the analog circuit amplifier do not have a coupling capacitor? The theory in amplifier circuit adds a coupling capacitor between the transformer secondary coil and the transistor. Turn the output of the former stage into the input of the latter stage, so that two stages do not affect. The former stage is alternating current, so does the latter stage, so there no mutual influence.The former stage is indeed alternating current, but the latter stage is alternating current superimposed direct current. The transistor needs a DC bias. If there is no capacitor to block the DC, the coil of the transformer will bypass the DC bias of the transistor (because the inductor passes DC). 6) In the basic amplifier circuit, can the coupling capacitor be non-polar?In the basic amplifying circuit, the coupling capacitor depends on the frequency. When the frequency is high, a non-polar capacitor is needed. It is characterized by relatively stable, high withstand voltage, small size and capacity. Its biggest use is to block direct current and pass alternating current. Coupling capacitor is widely used in high-frequency alternating current paths, bypass, resonance and other circuits. (high-pass)When the frequency is low, since the capacitance of the non-polar capacitor is relatively low, the capacitive reactance is relatively increased, so it is necessary to use a polar electrolytic capacitor. Because of the electrolyte inside, the capacity can be made large, allowing low-frequency alternating current to pass. However, because of the organic medium between the internal two poles, the withstand voltage is limited. Non-polar capacitor is mostly used in circuits such as low-frequency AC paths, filtering, decoupling, and bypassing. (low-pass) 7) In a battery-powered circuit, why does the capacitor charging and discharging have the delay effect?Capacitors accumulate electric charge. During the charging process, the voltage rises slowly, while discharging vice versa. During charging, at the beginning, the voltage across the capacitor is zero, as the time goes by, the voltage gradually rises to the voltage you set to control the switching of the circuit. Of course, the discharging process can also be used to achieve this. The delay time is related to capacitor capacity, capacitor leakage, charging resistance, voltage, and sometimes the load resistance is also taken into consideration. 8) The resistance-capacitance coupling amplifier circuit can only amplify AC signals, but cannot amplify DC signals?Capacitor is an electronic component that blocks DC and AC. Therefore, the resistance-capacitance coupling amplifier circuit can only amplify AC signals. A direct coupling amplifier circuit is used to amplify DC signals. 9) How to tell the coupling capacitor and the bypass capacitor in the amplifying circuit?The negative pole of the coupling capacitor is not grounded, but is connected to the input of the next stage, and the negative pole of the bypass capacitor is grounded. 10) How to choose coupling capacitor for the multi-stage AC amplifier circuit?Generally ceramic capacitors can be done, and tantalum capacitors can be used if the performance is good. According to the frequency range of your input signal, capacitance of 103,104 can be used for high frequencies, and also electrolytic capacitors of about 22uF can be used for lower frequency AC signals. 11) The amplifying circuit adopts direct coupling, and the feedback network is a pure resistor network. Why is the circuit only possible to produce high-frequency oscillation?The oscillation comes from the phase shift of the closed loop reaching 180 degrees and the loop gain at this time is greater than zero. Using a pure resistor network as a feedback network will definitely not have phase shift, which comes from the open loop circuit of the amplifier only. Using a direct-coupled open-loop amplifier, there will be no capacitive elements between stages that will cause phase shifts, but the capacitor inside the transistor or MOS tube will cause it. These capacitances are all fF, the maximum is pF. The resonant frequency of the circuit composed of these capacitors and the equivalent resistance of the circuit is quite high. Therefore, the amplifier adopts direct coupling, with a pure resistor feedback network, which can only produce high-frequency oscillations. 12) How to estimate the output resistance of the first-stage amplifier and the input resistance of the second-stage amplifier? When the amplitude of the signal source is too large, what will happen at the output of the two-stage amplifier? Shake the input end of the amplifier and observe the output end to see what appears? why?A. The input resistance of the second stage amplifier is the output resistance of the first stage amplifier. B. Have distortion. C. Cause clutter due to human body induction. 13) How to use the charge and discharge of capacitors to understand filtering, decoupling and bypassing?Capacitors block DC and pass AC. Blocking DC is easy to understand, but passing AC is not easy to understand. As long as you understand it, you can understand filtering, decoupling and bypass.Capacitors are charging and discharging, but the direction of alternating current changes alternately. The magnitude of the amplitude also changes periodically. The entire changing image is a sine curve.The capacitor is connected to the AC circuit, and due to the periodic change of the AC voltage, it is also periodically changes. There is a charging and discharging current in the line. This charging and discharging current has the same shape as the voltage except that the phase is 90 degrees ahead of the voltage, which is equivalent to the AC passing through the capacitor.The alternating current passing through the resistance consumes electric energy (heating) on the resistor. However, the capacitor only exchanges energy with the power supply. The power supply sends energy to the capacitor when charging, and the capacitor returns the electrical energy to the power supply when discharging. Therefore, the power generated by multiplying the voltage by the current here is called reactive power.What needs to be clear is that when the capacitor is connected to an AC circuit, the flowing electrons (current) do not really rush through the insulating layer, but generate current in the circuit. This is because in the circuit, reverse discharge and forward charge are in the same direction, the forward discharge and reverse charge either. Understand that the capacitor is connected to AC, then the AC component is bypassed to the ground, and also the filtering is completed. 14) How to use bypass capacitor, filter capacitor and decoupling capacitor respectively?These three types of capacitors are actually used for filtering, but they are used in different circuits, so their names and usages are different.Filter capacitor, this is the capacitor we usually use after power rectification. It is a capacitor that rectifies the AC of the rectifier circuit into a pulsating DC and smoothes it by charging and discharging. This type of capacitor is generally an electrolytic capacitor with a large capacity.Bypass capacitors are used to filter out the high frequency components in the input signal. They are mainly used to filter high frequency clutter. Usually, ceramic capacitors and polyester capacitors are used. The capacity is small and is at the picofarad level.The decoupling capacitor takes the interference of the output signal as the filtering object. It is equivalent to the battery and uses its charge and discharge so that the amplified signal will not be interfered by the sudden change of the current. Its capacity depends on the frequency of the signal and the degree of ripple suppression. 15) Regarding the function of the capacitor, under what circumstances use the coupling capacitor and under what circumstances use the filtering capacitor?After the alternating voltage is applied to the two ends of the capacitor, it will continue to charge and discharge with the alternating frequency of the current. At this time, there is an alternating current of the same frequency in the circuit, which is the passing characteristic of the capacitor.When the frequency is appropriate, the capacitor can be regarded as a path to the circuit, and the AC output of the previous stage can be transmitted to the subsequent circuit through the capacitor.For direct current, it is isolated, because when the voltage at both ends is charged to be equal to the circuit voltage, there will be no more charging current.When acting on the transmission of front and rear AC signals, it is coupling, and when acting on filtering out fluctuation components and useless AC components, it is filtering. 16) The capacitor filter of the rectifier circuit uses its charge and discharge, but sometimes the filter uses the capacitor to have a different capacitive reactance to the non-pass frequency signal, such as a bypass capacitor. So which point is used when analyzing capacitor filtering?The theoretical explanation of using capacitor characteristic is more general, and the theory using capacitive reactance is more in-depth. The role of capacitor is to use its charge and discharge characteristics, depending on what components you want to filter out. Use large capacitors to filter low frequencies and a small capacitor for high frequency. In theory, the filtering in the low-frequency rectifier circuit and the bypassing in the high-frequency circuit are the same, and the difference is the capacitive reactance. 17) After the filter capacitor is fully charged, it will discharge the back circuit and then in cycle?Such a working process in the circuit, capacitor is related to the frequency of the signal. First of all, it depends on what you want to put the capacitor in the circuit. When used as a filter, it filters out a certain frequency signal to the ground. For example, the capacitors at the front end of the chip power supply are decoupling. The phenomenon you mentioned is like the filter capacitor before the voltage regulator is turned off and the filter capacitor of the switching power supply. 18) What is the specific coupling of capacitors? Is there any difference compared with filtering?Coupling refers to the process of signal transmission from the first stage to the second stage, and usually refers to AC coupling when it is not specified. Decoupling refers to taking further filtering measures on the power supply to remove the influence of mutual interference between the two levels of signals through the power supply. The coupling constant refers to the time constant corresponding to the product of the coupling capacitance value and the second-stage input impedance value.Decoupling has three purposes: D.Remove the high-frequency ripple in the power supply, and cut off the high-frequency signal of the multi-stage amplifier through the crosstalk path of the power supply.E.When the large signal is working, the circuit's demand for the power supply increases, causing the power supply fluctuations, here decoupling reduces the impact of power fluctuations on the input stage/high voltage gain stage during large signals.F. Form a floating ground or floating power supply, and complete the coordination of various parts of the ground or power supply in a complex system.The high-frequency switching noise generated by the active device during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the noise on the board and to guide it to the ground. 19) How to distinguish whether the capacitor in the circuit is a filter capacitor or a bypass capacitor?The filter capacitor is in the power circuit; the bypass capacitor is in the signal circuit.In fact, their function is basically the same. The filter capacitor: Bypasses or filters out the pulsating current components and plays the role of charging and discharging. Bypass capacitor: Filter or bypass high frequency or low frequency components in the circuit. 20) Is the coupling capacitor a decoupling capacitor?It is completely different. The coupling capacitor is for signal transmission, and the decoupling capacitor is for reducing interference.

Introduction The audio power amplifier, also called power amp, is a sound-producing device that reconstructs the input audio signal on the output element. The reconstructed signal volume and power level must be idea, effective and low distortion. The audio frequency range is about 20Hz to 20000Hz, so the amplifier must have a good frequency response in this range. Depending on the application, the power level varies greatly, from the milliwatt level of earphones to the several watts of TV or PC audio, to the tens of watts of home stereo and car audio, to the more powerful home and commercial audio, even the system’s hundreds of watts are large enough to meet the sound requirements of the movie theater or auditorium. Catalog Introduction Ⅰ What is Audio Power Amplifier? Ⅱ Types of Audio Power Amplifiers 2.1 Class A Power Amplifier 2.2 Class B Power Amplifier 2.3 Class AB Power Amplifier 2.4 Class D Power Amplifier Ⅲ Audio Power Amplifiers Comparisons Ⅳ Audio Power Amplifier Circuit Architecture Ⅴ Typical Audio Amplifier Circuits Examples Ⅵ FAQ Ⅰ What is Audio Power Amplifier? Audio power amplifier is one of the important components of multimedia products and is widely used in the field of consumer electronics. Linear audio power amplifiers have always dominated the traditional audio amplifier market due to their low distortion and good sound quality. With the popularization of portable multimedia devices such as mobile phones, tablet PC, and notebook computers, the efficiency and volume of linear power amplifiers can meet the requirements of the market, for example, class D power amplifiers have become more and more popular for their advantages such as high efficiency and small size.The development of audio amplifier has experienced three generations of electron tube (vacuum tube), bipolar transistor and field effect tube. The tube audio amplifier has a mellow tone, but it is bulky, with the disadvantages of high power consumption, extremely unstable, and poor high-frequency response. Bipolar transistor audio amplifiers have bandwidth, large dynamic range, high reliability, long life, and high-frequency response, but its static power consumption and on-resistance are very large, so it is difficult to improve its efficiency. The FET audio amplifier has the same mellow tone as the electronic tube, and its dynamic range is wide, and more importantly, its on-resistance is small, which can achieve very high efficiency. audio power amplifier" width="410" height="250" /> Figure 1. Hi-Fi Audio Power Amplifier Ⅱ Types of Audio Power Amplifiers There are many types of audio power amplifiers, and more than ten modes of it commonly used, such as Class A, Class B, Class AB, Class C, Class D, Class E, Class F, Class G, Class H, and Class S, but only four types suitable for audio applications: Class A, Class B, Class AB and Class D. 2.1 Class A Power Amplifier The main feature of the Class A power amp is: The operating point Q of the amplifier is set near the midpoint of the load line, and the transistor is turned on during the entire cycle of the input signal. The amplifier can work in a single tube or push-pull state. Since the amplifier works in the linear range of the characteristic curve, transient and alternating distortion are small. The circuit is simple and the debugging is convenient. Class A power amplifiers only need one transistor to provide current to the load, and the conduction angle is 360°C during a complete signal cycle. This kind of power amplifier has low distortion, but often requires a large static current and is low in efficiency. Theoretically, the maximum working efficiency of a class A power amplifier is 25%, so a heat sink is required when working. 2.2 Class B Power Amplifier Class B power amplifier is composed of two complementary transistors. In a complete signal cycle, each amplifier tube will be turned on in one half cycle and turned off in the other half cycle, that is, the conduction angle is only 180°C. Because it has no static current, it has a higher efficiency. In theory, the maximum efficiency of a Class B power amplifier can reach 78%. However, when the input signal is close to zero, the amplifier tube has a critical conduction state, resulting in crossover distortion. 2.3 Class AB Power Amplifier Class AB power amplifier inserts two diodes into the input end of the Class B power amplifier. When the input is close to zero, the amplifier tube has been slightly turned on, so that the conduction angle of each amplifier tube is greater than 180°C and less than 360°C. It overcomes the crossover distortion of Class B power amplifiers, and the efficiency is between Class A and Class B power amplifiers, based on a common structure of traditional linear power amplifiers. However, the medium output voltage is usually far away from the power supply voltage, and a lot of power consumption is consumed in the transistor. Therefore, even a well-designed amplifier, its efficiency is still low. 2.4 Class D Power Amplifier Class D (digital audio power) power amplifier is a kind of input analog audio signal or PCM digital information into PWM (pulse brightness modulation) or PDM (pulse density modulation) pulse signals, which is used to control the amplifier power switching devices turn on/off audio power amplifiers, also known as switching amplifiers. It has the outstanding advantage of high efficiency.The output stage of the Class D power amplifier consists of two complementary power tubes. Driven by the high-frequency control pulse signal, the power tubes work in the on-off state. One is turned on and the other is turned off. Therefore, the amplifier does not require static power consumption, that is, it has a very high efficiency. In theory, the efficiency of Class D power amplifiers can reach 100%, but in fact it is over 80%, which is 2 to 3 times that of traditional linear power amplifiers.The digital audio power amplifier also looks like a one-bit power digital-to-analog converter. The amplifier is composed of four parts: input signal processing circuit, switching signal forming circuit, high-power switching circuit (half-bridge and full-bridge) and low-pass filter (LC). In electronics, Class D amplifiers use a very high frequency switch circuit to amplify the audio signal. DIY Class D Audio Amplifier Class D amplifiers have the following advantages:1) It has a high efficiency, usually above 85%.2) Small size, which can save a lot of space than analog amplifier circuits.3) Connection without crack noise.4) Have low distortion and good frequency response curve. Few peripheral components, easy to design and debug.   Ⅲ Audio Power Amplifiers Comparisons 1) Class A, B, and AB amplifiers are analog amplifiers, and class D amplifiers are digital amplifiers. 2) Class B and Class AB push-pull amplifiers have higher efficiency and less distortion than Class A amplifiers, and their transistors consume less power and have better heat dissipation. However, Class B amplifiers will have poor switching characteristics during the transition between transistor on and off states or cause alternate distortion due to improper selection of circuit parameters. 3) The Class D amplifier has high efficiency and low distortion, a good frequency response curve, and fewer peripheral components. 4) Class AB amplifier and Class D amplifier are the basic circuit forms of audio power amplifiers at present. Figure 2. Amplifier Circuit Example Ⅳ Audio Power Amplifier Circuit Architecture The purpose of audio amplifier is to reproduce the audio input signal with high efficiency and low distortion on the sound output element at the required volume and power level. The frequency range of the audio signal is 20 Hz to 20000 Hz, so the audio amplifier must have a good frequency response. Audio amplifiers usually consist of preamplifiers and power amplifiers.PreamplifierThe amplitude of the audio signal source signal is generally very small and cannot directly drive the power amplifier. Therefore, they must be amplified to a certain condition first, which requires the use of a preamplifier. In addition to the signal amplification function, it can also have functions such as volume adjustment, tone control, loudness control, and channel equalization.Power amplifierThe power amplifier is referred to as the power amplifier for short, and its purpose is to provide the load with a large enough current drive capability to achieve power amplification. Class D power amplifier works in the on-off state. In theory, it does not require static current and has high efficiency.The topological structure of a typical Class D power amplifier circuit is shown in Figure 2: It consists of a triangle wave generator, a comparator, a power output stage and an LC low-pass filter. Figure 3. Class D Power Amplifier Circuit Figure 2 is a schematic diagram of the working principle of a Class D power amplifier. The sine wave audio input signal and the triangular wave signal with a much higher frequency are modulated by the comparator to obtain a PWM modulation signal whose duty cycle is proportional to the amplitude of the input signal. It pushes the output power tube to work in the on-off state. The output end of the tube obtains an output signal with a constant duty cycle. The amplitude of the output signal is the power supply voltage and has a strong current drive capability. After signal modulation, the output signal contains both the input signal and the fundamental component of the modulated triangle wave, as well as their higher harmonics and combinations. After LC low-pass filtering, the high-frequency components in the output signal are filtered out, and a low-frequency signal with the same frequency and amplified amplitude as the original audio signal is obtained on the load.   Ⅴ Typical Audio Amplifier Circuits Examples The following is a brief introduction of AN7115 audio power amplifier circuit. Figure 4. AN7115 Audio Amplifier Circuit AN7115 has an output power of 2.1W and a noise output of 3mV under the conditions of V=9.0V, THD=10%, and RL=8Ω.Limit parameters: Vcc=13V, power dissipation (without radiator) is 1.2W, and 2.25W with a radiator. Working temperature: -20℃ to 70℃, suitable for small portable radio recorders and audio equipment as power amplifiers. TDA2030 adopts V-shaped 5-pin single in-line plastic package structure. According to the shape of the pin, it can be divided into H-type and V-type. This integrated circuit is widely used in car stereo radio and tape recorders and mid-power audio equipment. It has the characteristics of small size, high output power, low distortion, etc., and has an internal protection circuit. Circuit characteristics are as followed: Figure 5. TDA2030 Audio Power Amplifier Circuit 1) Very few external components.2) The output power is large, Po=18W (RL=4Ω).3) The use of ultra-small package (TO-220) can increase the assembly density.4) The boot impact is minimal.5) It contains various protection circuits, so it is safe and reliable to work. The main protection circuits include: short circuit protection, thermal protection, ground wire coupling open circuit, power supply polarity reverse connection (Vsmax=12V), and load discharge voltage kickback, etc. Figure 6. Mini Audio Power Amplifier Ⅵ FAQ 1. What is audio power amplifier?An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from radio receiver or electric guitar pickup to a level that is high enough for driving loudspeakers or headphones. Audio power amplifiers are found in all manner of sound systems including sound reinforcement, public address and home audio systems and musical instrument amplifiers like guitar amplifiers.   2. How do audio amplifiers work?An amplifier takes an input signal from a source, such as a laptop, turntable or CD player, and creates a larger copy of the original signal before it's sent to the speakers. It gets the power to do this from your mains electricity, which is sent directly to the power supply within the amplifier.   3. What does an audio power amplifier do?An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from radio receiver or electric guitar pickup to a level that is high enough for driving loudspeakers or headphones.   4. What is the most powerful audio amplifier?Classic audio brand McIntosh has announced its most powerful integrated amplifier. The fully loaded, hybrid MA12000 Integrated Amplifier is McIntosh's most powerful integrated amplifier ever offering 350W per channel.   5. Do you need amplifier for speakers?Powered speakers do not need an amplifier. They have an amplifier already installed in them which is why they are called 'powered speakers' to begin with. Depending on the speakers' input options, you can hook them up to different audio sources without the need for a separate amplifier.   6. Does an amplifier improve sound quality?Amplifiers ideally amplify audio signals linearly and, therefore, do not technically improve or worsen sound quality. However, less-than-ideal amplifiers, amp settings and amplifier-speaker combinations may worsen sound quality. Amps are nevertheless needed to drive speakers and headphones properly.   7. Why do I need a power amplifier?Some users prefer to separate input switching and AV processing from the task of providing power for, and connection of, loudspeakers through separate AV preamp/processors and power amplifiers. ... A separate preamp and power amp results in more equipment and cable clutter.   8. Which is better power amplifier or integrated amplifier?A preamplifier / power amp does generally offer better quality; while flat output has become common place, keeping your switching circuitry separate from your amplification and having two separate power supplies can reduce the internal interference that can be introduced into your signal.   9. What is a home audio amplifier?A home theater amplifier (also known as an audio-visual receiver or simply an AVR) is a powerful piece of kit that combines amplifiers and digital signal processors to convert output from a range of source devices into high quality sound and video. In short, it is the hub of a home theater set up.   10. What is the most powerful audio amplifier?Classic audio brand McIntosh has announced its most powerful integrated amplifier. The fully loaded, hybrid MA12000 Integrated Amplifier is McIntosh's most powerful integrated amplifier ever offering 350W per channel.   11. What is the purpose of an audio amplifier?The goal of audio amplifiers is to reproduce input audio signals at sound-producing output elements, with desired volume and power levels—faithfully, efficiently, and at low distortion.   12. What do you mean by audio amplifier?Any electronic device that increases the power of an electrical signal whose vibrations are confined to the audio frequency range—the range that can be perceived by the human ear—is an audio amplifier.

Introduction A resistor is a passive two-terminal electrical component. After it is connected to the circuit, the resistance is fixed, which can limit the current through the branch connected to it. On one hand, the resistance that cannot be changed is called a fixed resistor, on the other hand, the resistances of potentiometers or variable resistors are changeable. The main physical characteristic of a resistor is to transform electrical energy into thermal energy. It can also be said that it is an energy-consuming element, because internal energy is generated when current passes through it. Figure 1. Use Resistor in Circuit Catalog Introduction Ⅰ Functions of Resistor Ⅱ Three Basic Principles for Resistor Selection Ⅲ The Role of Resistors in Transistor Circuits 3.1 Why Should a Resistor Be Added to the Base of the Transistor? 3.2 Pull-down Resistor in Transistor Circuits Ⅳ FAQ Ⅰ Functions of Resistor In short, the function of resistance is to limit current, divide current, divide voltage, and convert electric energy into internal energy (heating) in the circuit. According to Ohm's law, through calculations, resistors in parallel and series connections can be used to achieve the desired current and voltage. Also there are different resistors and switches combined to produce voice-activated switches, photosensitive switches, infrared switches and so on. How to Use Resistors in circuits? 1) Limit CurrentIn order to prevent the components connected in series from being burnt out by the excessive current and to ensure the normal operation of the electrical appliances, a variable resistor can usually be connected in series in the circuit.2) Current DiversionThe resistor is connected in parallel to the component or circuit that needs to be shunted, and the voltage does not change. The function of this resistor is to divide current.3) Voltage DiversionGenerally, electrical appliances are marked with a rated voltage value. If the power supply is higher than it, the electrical appliance cannot be directly connected to the power supply for a normal operation. In this case, a resistor with suitable resistance can be connected in series in the circuit to share a part of the voltage, therefore the electrical appliance can work at the rated voltage. At this time, the role of the resistor is to divide the voltage.4) Provide Bias VoltageIn the transistor circuit, the resistor is connected between the base of it and the working voltage. At this time, the power supply provides a bias voltage to the base through the resistor, and the resistance can determine the bias voltage. The role of the resistor in the circuit at this time is to provide a bias voltage.5) Negative FeedbackUsed in the resistance between the base and collector of the transistor, then the feedback branch of the negative feedback circuit is formed in the circuit. At this time, the resistor plays a negative feedback role in the circuit.6) OscillationResistor and capacitor form an RC circuit, which can be combined in parallel and in series.7) Damping EffectConnecting a resistor in parallel in the LC resonant circuit can reduce the Q value, at this time, resistor plays a damping effect.8) DecouplingThe use of resistors in multi-stage amplifier circuits can prevent harmful low-frequency interference, which play a decoupling effect.9) Convert Electrical Energy into Internal Energy (Heating)When the current passes through the resistor, it will convert all (or part) of the electrical energy into internal energy, which will generate heat. This principle is often used in electric stoves and heaters in our lives.10) Convert Current into VoltageWhen current flows through the resistor, a voltage will be generated across the resistor. As shown in the figure below, the collector load resistor R2 plays this role, converting the current flowing through the resistor R2 into a voltage and outputting it from U0. Figure 2. Resistor Circuit   Ⅱ Three Basic Principles for Resistor Selection 1) Choose resistors that are manufactured by a certification body that implement high-level standards.2) Choose resistors produced by manufacturers with functional advantages, quality advantages, efficiency advantages, price advantages, and service advantages.3) Choose a manufacturer that can meet the above-mentioned requirements in the model catalog.   Ⅲ The Role of Resistors in Transistor Circuits 3.1 Why Should a Resistor Be Added to the Base of the Transistor? First of all, we must understand the basic principle of the transistor. It is a current-controlled element, which is different from the MOFET, a voltage-controlled element. The transistor has three working areas: cut-off area, amplification area and saturation area. Take NPN transistor as an example, the voltage difference（UBE）of BE is about 0.6V (the actual size depends on the model of the component). When UBE＜0.6V , the transistor is off; when UBE=0.6V, the transistor is in the amplification or saturation region. Figure 3. Schematic Diagram of the NPN Transistor Current When the transistor is in the amplification area, the added resistance between the base and VCC is a bias resistance. The following explains why the base should be added when the transistor is used as a switch. What is the difference between transistor and MOSFET circuits when adding a resistor.The following figure is the most commonly used circuit diagram of NPN transistors. The common input terminal is the I/O port of the microprocessor (microcontroller, DSP, ARM, etc.). Figure 4.  NPN Transistor Take the microcontroller I/O port with 0/5V input as an example. Why must a resistor be connected in series with the base? Can it work without a resistor? Here the resistor is a current control element. When the transistor is in an amplified or saturated state, the voltage of the UBE is 0.6V, and the base current can be calculated according to the input voltage U. The calculation formula is Ib=(U-0.6 )/R1. It can also be seen from the formula that if the current limiting resistor R1 is not connected, when the input voltage is greater than 0.6V, the base current will be very large to burn the tube.Moreover, the resistor cannot be used casually. It needs to be calculated according to the input voltage and the characteristics of the tube. For example, the amplification factor β of the transistor is 50, the maximum current of the collector is 500mA, and the input voltage is 5V. If the design requires the transistor to be in a saturated state, then Ic=500mA, Ib=Ic/β=10M=mA, where the current-limiting resistance R1=(5V-0.6V)/Ib=430Ω. If it is required to input 5V, the collector current is about 200mA, then Ib=Ic/β=200mA/50=4mA can be calculated, finally the current-limiting resistance R1=(5V-0.6V)/Ib=1075Ω (1K can be selected Standard resistance). Note: The above figure is used to explain the example, but it is not very reliable. A more reliable connection method should be to connect a large resistor (such as 10K, or 20K) between the base and the ground. When there is no input, pull the base down quickly to ensure that the tube is in a stable cut-off state.If the NPN transistor in the figure above is replaced with an N-channel MOS tube, the principle is the same. When a high level is input, the tube is turned on, and when a low level is input, the tube is turned off. Figure 5. MOSFET Circuit Since the MOSFET is a voltage-controlled device, the current of the gate (G) is very small and can be ignored, so it can work normally without connecting the resistor R1.The figure after remove the resistor is shown in the below: Figure 6. MOSFET Circuit without Resistor Note: In actual applications, a resistor is generally connected in series to improve reliability. The product reliability is very important. Without current-limiting resistor, when the MOS is   damaged by voltage breakdown, the components on the control terminal will be affected easily, especially the processor, is easily damaged by high current. 3.2 Pull-down Resistor in Transistor Circuits 🔺For TransistorsThe transistor is a current-type driving component, so a current-limiting resistor is connected to the base, generally less than 10K, and the typical values are 3.3K, 4.7K, 5.1K, 6.8K, etc. What is the function of this pull-down resistor?The following figure shows the transistor 8050 switching circuit. The transistor will be turned on when the I/O port outputs a high level, and the transistor will not be turned on. If the I/O port does not output a high level, the base will always be pulled low without a 68K pull-down resistor, that is to say it is in the cut-off state. The circuit may be in an unstable state, especially when it is initialized at the moment of power-on. It is easy to generate noise and easily cause the transistor to malfunction, especially for some general input/output ports. Therefore, this resistor is actually a bias resistor, which makes the base to be pulled down when there is no driving signal, making the circuit more reliable. Figure 7.  Pull-down Resistor in Transistor Circuit Although the pull-down resistor can make the circuit more reliable, tit cannot be too large or too small. If the resistance too large, the base current will not be enough to drive the transistor. On the contrary, if it is too small, the bias voltage will be less than the transistor conduction voltage. In general, this resistance is not more than 100K.Sometimes we see that a capacitor is connected in parallel with this resistor. In fact, this is generally designed in high-speed signal switching circuits. Adding a capacitor can improve performance, as shown below: Figure 8.  RC Circuit 🔺For MOSFETUnlike transistors, MOS transistors are voltage-controlled components, which are driven by voltage. We all know that there is parasitic capacitance between the two pins of MOS transistors. In fact, the key of MOS transistors’ conduction is the charging and discharging of capacitors. Therefore, for the N-type MOS, it will be turned on when Vgs is greater than a certain value, but for the P-type MOS, it will be turned on when the value of Vgs is less than a certain value.Therefore, due to the capacitive effect between the three pins, when the MOS is constantly turned off, the parasitic capacitance voltage can be properly discharged, which is similar to the role of a bleeder resistor and is a kind of protection for the MOS. Figure 9.  MOSFET Circuit Ⅳ FAQ 1. What is the function of the resistor?A resistor has the ability to reduce voltage and current when used in a circuit. The main function of a resistor is to limit current flow. Ohm's law tells us that an increase in a resistors value will see a decrease in current.   2. How do resistors work?A conductor has low resistance, while an insulator has much higher resistance. Devices called resistors let us introduce precisely controlled amounts of resistance into electrical circuits. ... A resistor works by converting electrical energy into heat, which is dissipated into the air.   3. Why do you need resistors?A resistor controls the flow of the electrical current within a circuit. ... Resistors are essential to many electoral circuits, and they can be applied to a myriad of different applications. Protect against voltage spikes. Resistors also protect components against voltage spikes.   4. What role do resistors play in electronic devices?A resistor is a passive two-terminal electrical device that resists the flow of current. It is probably the simplest element in an electronic circuit. It is also one of the most common components as resistance is an inherent element of nearly all electronic circuits. They are usually color-coded.   5. What is a good example of a resistor?A few examples include limiting electric current, voltage division, heat generation, matching and loading circuits, gain control, and setting time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude.   6. What happens if I use a higher ohm resistor?The cases where using a higher value resistor will damage a circuit exist, but are a bit less usual than the cases where it may simply produce a weaker result than desired, or a different frequency response than desired.   7. What is a resistor simple explanation?A resistor is an electrical component that limits or regulates the flow of electrical current in an electronic circuit. Resistors can also be used to provide a specific voltage for an active device such as a transistor. ... The most common type in electronic devices and systems is the carbon-composition resistor.   8. What happens when a resistor blows?Blowing Up a Resistor. By applying too high a voltage to a resistor, the resistor will draw too much current. This causes excessive power to be dissipated in the resistor which makes it go up in flames and a cloud of smoke as this video shows.   9. How is a resistor connected in a circuit?Resistors are said to be connected in “Series”, when they are daisy chained together in a single line. Since all the current flowing through the first resistor has no other way to go it must also pass through the second resistor and the third and so on.   10. Do resistors change voltage?The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor. Ohm's Law can be used to verify voltage drop. In a DC circuit, voltage equals current multiplied by resistance. V = I R.

IntroductionThe Temperature Sensor, a measuring instrument, uses various physical properties of a substance to convert the thermal quantity into the physical quantity, including expansion, resistance, capacitance, electromotive force, magnetic properties, frequency, optical characteristics and thermal noise. Many materials and components change with temperature, so there are quite a few materials that can be used as temperature sensors. Here are four temperature sensors in detail.Temperature Sensors ExplainedCatalogIntroductionⅠ Temperature Sensor Types Overview1.1 What is Thermocouple?1.2 What is Thermistor Sensor?1.3 What is Resistance Temperature Detector (RTD)?1.4 What is IC Sensor?1.5 Temperature Sensor Cons and ProsⅡ How to Test: Measuring IndexesⅢ FAQⅠ Temperature Sensor Types OverviewThere are many types of temperature sensors, which can be divided into contact type and non-contact type according to the measurement method; thermistor and thermocouple according to the characteristics of sensor materials and electronic components.The contact temperature sensor needs to maintain thermal contact with the measured medium, so that the two can perform sufficient heat exchange to reach the same temperature. This type of sensor mainly includes resistance type, thermocouple, PN junction temperature sensor and so on. The non-contact temperature sensor does not need to be in contact with the measured medium, but achieves the purpose of temperature measurement through the heat radiation or convection of the measured medium.Here is a detailed introduction to the commonly used four: Thermocouples, Thermistors, Resistance Temperature Detector (RTD), IC Sensor.Figure 1. Temp Sensors (Resistance Changes with Temperature)1.1 What is Thermocouple?Thermocouples are the most commonly used temperature sensors in measurement. Its main advantages are wide temperature range and adaptability to various atmospheric environments, and it is strong, low in price, does not require power supply, and is also the cheapest. The thermocouple consists of two different metal wires connected at one end. When one end of the thermocouple is heated, there is an electric potential difference in the thermocouple circuit, and the measured electric potential difference can be used to calculate the temperature.Figure 2. Metal JunctionsThe thermocouple sensor has two contacts. The measurement end (sometimes called the hot end) is where the two metals connect. The reference junction (also called the cold end) is connected to the measurement circuit. When there is a temperature difference between two ends, an mV signal proportional to the temperature difference is generated. The mV value increases with increasing temperature. The relationship between mV and temperature is non-linear.The thermocouple connector can be constructed by connecting the thermal junction to the outer sheath for grounding or ungrounding (insulating from the sheath). A grounded thermocouple responds faster, but the thermocouple will contact the processing voltage. Therefore, it is important to isolate the measurement circuit to prevent the formation of ground loops and to avoid measurement errors.Figure 3. Thermocouple for Temperature MeasurementInside the temperature component, the thermocouple is usually embedded in magnesium oxide (MgO) and a metal sheath, then insert it into the thermowell or protective tube. This helps protect the sensor from environmental pollution. When magnesium oxide is contaminated with water and salt, even thermocouples that are not grounded will eventually be grounded.As above mentioned, the relationship between voltage and temperature is nonlinear, so it is necessary to make a second measurement for the reference temperature (Tref), and use the test equipment software or hardware to process the voltage-temperature conversion inside the instrument to finally obtain the thermocouple temperature (Tx). Thermocouple is the simplest and most versatile temperature sensor, but its sensitivity is relatively low, which is easy to be affected by environmental interference signals, and the temperature drift of the preamplifier. So it is not suitable for measuring small temperature changes, that is, it is not suitable for high-accurate measurement and application.In actual use, the thermocouple measuring circuit can measure any temperature except 0°C. The measuring circuit must measure the temperature of the cold junction and restore the temperature to 0°C. This kind of electrical compensation is called cold junction compensation (or reference junction compensation). Most thermocouple measurement circuits do this.If the application requires a thermocouple instead of a thermistor, a higher-grade thermocouple is better. In addition, their cost difference is small, and high-quality wire can provide higher stability.🔺Table 1: Thermocouple Types and Application RangesThermocouple TypeApplication Range (℃ / ℉)E95-900℃ (200-1650℉)J95-760℃ (200-1400℉)K95-1260℃ (200-2300℉)N95-1260℃ (200-2300℉)R870-1450℃ (1600-2640℉)S980-1450℃ (1800-2640℉)T0-350℃ (32-660℉) 1.2 What is Thermistor Sensor?The main component of the thermistor sensor is the thermistor, which absorbs heat radiation around.Thermistors are made of semiconductor materials, mostly with a negative temperature coefficient, that is, the resistance decreases with increasing temperature. Temperature changes will cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of the thermistor is extremely poor and has huge effects with the production process. So the manufacturer cannot give a standardized thermistor curve.The thermistor is very small and responds quickly to temperature changes. But it needs to use a current source, and its small size also makes it extremely sensitive to self-heating errors.Figure 4. Thermistor SensorThe thermistor measures the absolute temperature on the two lines, with better accuracy, but it is more expensive than a thermocouple, and the measurable temperature range is also smaller than that of a thermocouple. A commonly used thermistor has a resistance value of 5kΩ at 25°C, and a temperature change of 1°C causes a resistance change of 200Ω. Note that the lead resistance of 10Ω only causes a negligible error of 0.05°C. It is very suitable for current control applications that require fast and sensitive temperature measurement. Small size is benefit for applications with space requirements, but care must be taken to prevent self-heating errors.Figure 5. Resistance-TemperatureThe thermistor also has its own measurement tips. With small size, it can quickly stabilize, and will not cause thermal load. However, it is not strong enough, and large currents can cause self-heating. Since the thermistor is a resistive device, any current source will cause heat on it due to power. Power is equal to the product of current squared and resistance. Therefore, a small current source must be used. If the thermistor is exposed to high heat, it will cause permanent damage. 1.3 What is Resistance Temperature Detector (RTD)?RTD is a precision temperature sensor, made of high-purity conductive metal (such as platinum, copper or nickel) or alloy. Its resistance increases with increasing temperature and decreases with decreasing temperature, similar to a thermistor. RTD is like a thermoelectric converter, converting temperature changes into voltage changes. By passing a constant temperature current through the temperature sensor, an output voltage that increases linearly with temperature can be obtained. The most suitable metal for RTD is a pure metal that remains stable within a given temperature range. The resistance-temperature change relationship is preferably linear. The larger the temperature coefficient (it is defined as the resistance change caused by unit temperature), the better, and it must be able to resist thermal fatigue and respond sensitively to temperature changes. A typical RTD has a protective sleeve and a probe. The protective sleeve is mainly used to protect the RTD from being damaged by the measured medium, which is usually made of stainless steel, carbon steel, inconel or cast iron, and its use temperature can reach 1100°C.Figure 6. Resistance Temperature Detector (RTD)It is currently the most accurate and stable sensor, and its linearity is better than thermocouples and thermistors. However, RTD is also a temperature sensor with slow response speed and more expensive price. So it is most suitable for applications that have strict requirements on accuracy, but speed and price are not critical. 1.4 What is IC Sensor?IC sensors can work in a temperature range of -55°C to +150°C, and precise one can operate at temperatures up to +200°C. It is commonly used in fitness tracking applications, wearable products, computing systems, data loggers, and automotive applications. The most common integrated IC temperature sensors are analog output devices, digital interface devices, remote temperature sensors, and those integrated ICs with thermostat functions.Figure 7. IC Sensors (Thermometer)Analog output devices (usually output voltage, but some also output current) are most like passive solutions when they need an ADC to process the output signal. Digital interface devices most often use a two-wire interface (I2C or PMBus) and have a built-in ADC. In addition to including a local temperature sensor, remote temperature sensors also have one or more inputs to monitor the remote diode temperature—they are most often placed in highly integrated digital ICs (for example, processors or field programmable gate arrays FPGA). When reached the temperature threshold, the thermostat can provide a simple alarm.Here are the details of two common types:🔺Analog Output Temperature SensorThe integrated sensor is made using silicon semiconductor integration process, so it is also called a silicon sensor or a monolithic integrated temperature sensor. It is a dedicated IC that integrates a temperature sensor on a chip and can take temperature measurement and then output analog signals. The main features of this sensor are single function (only measuring temperature), small temperature measurement error, low price, fast response speed, long transmission distance, small size, micro power consumption, etc., which are suitable for long-distance temperature measurement, control and measurement. What’s more, non-linear calibration doesn’t required, and the peripheral circuit is simple.🔺Digital Output SensorDigital temperature sensor is the product of microelectronics technology, computer technology and automatic test technology (ATE). The intelligent temperature sensor contains temperature sensor, A/D converter, signal processor, memory (or register) and interface circuit. Some products also come with multiplexers, central controller, random access memory and read-only memory. The characteristic of the intelligent temperature sensor is that it can output temperature data and related temperature control quantities, adapts to various microcontrollers (MCU). It realizes the test function through software on the basis of hardware, and its intelligent harmony also depends the level of software development.1.5 Temperature Sensor Cons and Pros🔺Table 2: Advantages and Disadvantages of thermocouples, RTDs, thermistors and IC sensors.CriteriaThermocoupleRTDThermistorIC SensorTemperature-250℃ to +750℃-100℃ to +500℃-267℃ to +2316℃-55℃ to +200℃AccuracyBestDepends on calibrationGoodGoodLinearityGoodWorstGoodBestSensitivityLessBestWorstGoodCircuityComplexDepends on accuracy/power requirementsComplexSimplestPower ConsumptionHigh when takingLow-highLowest Ⅱ How to Test: Measuring Indexes1) Measurement accuracy: 0.01 level2) Resolution 0.1uV and 0.1mΩ3) Scan switch parasitic potential: ≤0.4μV4) Temperature range: Water tank: (room temperature +5~95)°C; Oil tank: (95 ~ 300)°C; Low & constant temperature bath: (-80 ~ 100)°C; High temperature furnace: (300~1200)°C5) Temperature control stability: better than 0.01℃/10min (oil tank, water tank, low temperature constant temperature tank); 0.2℃/min (tube type verification furnace)6) Total uncertainty: For thermocouple verification, measurement uncertainty is better than 0.7 ℃, repeatability error <0.25 ℃; For thermistor verification, measurement uncertainty is better than 50 mk, repeatability error <10 mk7) Working power supply: AC220V±10%, 50Hz, and well protected grounding.8) High temperature furnace power: about 2kW9) Constant temperature bath power: about 2kW10) Power of microcomputer measurement and control system: <500 Ⅲ FAQ1. What is temperature sensor and how it works?How do temperature sensors work? They are devices to measure temperature readings through electrical signals. The sensor is made up of two metals, which generate electrical voltage or resistance once it notices a change in temperature. ... Temperature is the most common physical measurement type in industrial applications. 2. What happens when a temperature sensor goes bad?If the coolant temperature sensor goes bad it can send a false signal to the computer and throw off the fuel and timing calculations. ... This will cause the computer to think the engine is cold, even when it is not, and as a result will use more fuel than necessary. 3. Which temperature sensor is best?The most well-known are Pt100 (with a resistance of 100 ohms at 0°C) and Pt1000 (with a resistance of 1,000 ohms at 0°C). The Pt1000 offers better accuracy and a larger tolerance to long wire lengths than the Pt100. Compared to thermocouples, resistance sensors offer better accuracy and a more linear response. 4. What is the application of temperature sensor?Within our homes, temperature sensors are used in many electrical appliances, from our refrigerators and freezers to help regulate and maintain cold temperatures as well as within stoves and ovens to ensure that they heat to the required levels for cooking, air confectioners/heaters. 5. How do I know if my temperature sensor is bad?What Signs May Signal Your Coolant Temperature Sensor May Be Failing.Poor Fuel Economy.Irregular Temperature Readings.Black Smoke from Your Exhaust.Your Engine is Overheating.Your Check Engine Light is On. 6. How important sensors are nowadays?Intelligent sensor systems are omnipresent in our everyday lives. They provide security, save lives and improve our quality of life. As more and more areas of life are automated and networked, the importance of innovative sensor technologies will also increase in the future. 7. What should I consider when choose a temperature sensor?Several factors must be considered when selecting the type of sensor to be used in a specific application: temperature range, accuracy, response time, stability, linearity, and sensitivity. 8. What is the value range of a temperature sensor?The effective operating range is -50 to 250 °C for glass encapsulated thermistors or 150°C for standard thermistors. 9. What are the pros and cons thermocouple?Advantages and disadvantages of thermocoupleAdvantages of thermocouple: Simple working principle, Short response time, Low price, Wide temperature ranges, Rugged construction, Self-powered, Small size.Disadvantages of thermocouple: Nonlinearity, Accuracy, Interference can cause errors. Old technology, Needs calibration, Corrosion. 10. What is difference between PT100 and RTD?There is no difference a PT100 is a version of a RTD (resistance temperature detector). What is an RTD? A resistance temperature detector, also known as an RTD or resistance thermometer, is a type of temperature sensor. ... A PT100 sensor is the most common type of Resistance Thermometer (RTD).

Introduction A Capacitor is a component that can store and release electricity, and it is also one of the most commonly used electronic components. Its distinguishing feature is the "Pass Alternating Current (AC), Stop Direct Current(DC)". In a DC circuit, a capacitor is equivalent to an open circuit. Based on this, we will have this question: What the differences betwen ac and dc capacitors? or Are ac and dc capacitors interchangeable? What's the difference between batteries and capacitors? or Why can't we use capacitors instead of batteries? As for capacitor calculations, what the time constant for discharging a capacitor? Here you will get the answers. Figure 1. Capacitor Symbol Catalog Introduction Ⅰ Why Can the Battery (DC) Charge the Capacitors? Ⅱ DC Capacitors vs AC Capacitors Ⅲ Battery or Capacitor? Ⅳ Calculate the Time Constant for the Discharge of the Capacitors? Ⅴ FAQ Ⅰ Why Can the Battery (DC) Charge the Capacitors? In circuit analysis, there are two types of circuit responses: zero-input response and zero-state response. The so-called zero input response means that the input signal is zero; the so-called zero-state response means that the states of all energy storage components and various power supplies in the circuit are zero.When analyzing the zero-state response, short-circuit the voltage source and open the current source. For the capacitor, at the moment of energization in the zero state response, it can be considered as a voltage source with zero voltage, so it is equivalent to a short circuit. Analyze the following figures: Figure one is the circuit structure: power supply E, internal resistance r, switch QF, capacitor C and resistance R. When the switch QF is turned on, let's take a look at the current Ic and voltage Uc flowing through the capacitor: Figure 2. Zero-input Response and Zero-state Response Curve We see that at the moment t=0, the current flowing through the capacitor is the largest. The capacitor at this time is equivalent to a voltage source with zero voltage, and the power source E must be charged to it through the internal resistance r. Therefore, we can understand that it is actually a short circuit, so the maximum charging current, that is, the charging current Icmax at t=0 is: . In the figure, we can see that after 5τ, the current has been zero and the voltage has been charged to almost E. After that, the current flowing through the capacitor will not change any more, and the capacitor at this time plays a role in isolating the direct current. As can be seen in the figure, the charging process of the capacitor is divided into two parts, one is the transient transition process, and the other is the steady state process.Next, analyze the transition process carefully. Let us first look at the opportunity RC of the resistance r and the capacitance C. Resistance is equal to voltage divided by current, and capacitance is equal to electricity divided by voltage, and electricity is equal to current divided by time, so there is: , here T is called the time constant, generally represented by τ.The current thus flowing through the capacitor is: The exponential function here, its exponent is equal to the ratio of time to the time constant, so it is a pure number. When the time is equal to 0, Ic=Icmax; when the time is equal to 5τ, the value of the exponential function is 6.738×10-3. After substituting the above formula, get the current at this time is: At this time, the expression of the capacitor voltage Uc is: Note: Theoretically, the voltage on the capacitor should be charged to ER/(R+r), but because in the steady state, the impedance of the capacitor is infinite, so its voltage can be charged to E.From here we can see that the so-called capacitor blocking DC actually refers to its steady-state characteristics. In the steady state, the equivalent impedance of the capacitor is infinite, and the DC current cannot pass through it, so the current is zero, and there is a very small leakage current at most. In the transient state, the capacitor can flow current, and in the initial stage since the current is similar to a voltage source due to the capacitance, its characteristic is almost a short circuit, so the initial value of the current is the maximum. If our power source is not a battery, but a square wave pulser, what is the voltage of the resistor R after the capacitor? Figure 3. Square Wave Pulser The Figure 3. is a case where the time constant is small, which reflects the impulse response of the capacitor; and the picture below is a case where the time constant is large, which reflects these two effects have a large number of applications. If our power supply is AC, what is the voltage on the resistor R after the capacitor?When discussing the response of the capacitor to the AC current range, we need to temporarily look back at the first picture. From the figure, we can see that when the current takes the maximum value, the voltage is the minimum value, however, when the current takes the minimum value, the voltage is the maximum value. Why is that?The capacitance is equal to the ratio of the electric quantity to the voltage, that is, C=Q/U=It/U, from which the current is obtained: I=CU/t. And the voltage Uc on the capacitor is actually constantly changing. It is a function of time, so the above formula can be written as: This formula is very important, it is the key to unlock the capacitor under the action of AC power.The AC voltage can be expressed as, put it into the expression of the capacitor current , and get: We can see that when the voltage is zero, the current has reached its maximum value. In other words, for an AC power supply, the current I flowing through the capacitor leads the voltage by 90 degrees. It reveals the expression form of capacitance under AC voltage.Capacitors are used to block direct current, but they only have this performance in steady state. When the DC power supply changes rapidly, that is, the power supply continuously changes from zero to the maximum value, and becomes zero again. In this cycle, we can see that the capacitor not only does not block the DC, but becomes a component with almost zero impedance.In fact, it can be known from the capacitive reactance () that when the frequency of the power supply increases, the capacitive reactance of the capacitor decreases linearly with the increase in frequency. When we charge the capacitor with a DC power supply enough, the charging voltage of the capacitor can reach the same level as the electromotive force of the power supply. Here the key is that the capacitor is an energy storage element. Figure 4. Various Capacitors Ⅱ DC Capacitors vs AC Capacitors 1) Whether DC capacitors and AC capacitors are polarized: AC capacitors are also called non-polarized capacitors. As can be seen from the literal meaning, it can be polarity-independent. So it can be used in AC and DC circuits. The DC current uses a polarized capacitor, which has a high capacity, and a relatively small withstand voltage. In addition, both of them will be lost over time.2) The mobility of DC capacitors and AC capacitors are different: the voltage at one end of the DC capacitor is always high, and the current will always be the same and flow in one direction. While the two lines from the AC capacitor power supply, their voltage level is changing, so that the current can flow from A to B, or from B to A, and make certain adjustments and changes over time.3) Direct current and alternating current are different in direction conversion: alternating current can be regarded as two groups of direct current in turn to exert an effect on the load, which can be understood as the vector sum of the two groups of direct current in different time periods. The alternating current can be understood as a group of direct current in one direction in a short enough time. Figure 5. Ceramic Capacitors Ⅲ Battery or Capacitor? Capacitors and batteries are both electrical components, and both are energy storage components. But battery and capacitor are two completely different concepts. The difference between them is:1) Chemical Energy vs Electrical EnergyThe battery stores chemical energy, and then converts it into electrical energy to output. Capacitors store electrical energy, which depends on the two plates to determine the capacity. The former is a chemical change, the latter is a physical change. The main physical feature of a capacitor is to store electric charge, which can be charged and discharged like a battery, but does not undergo a chemical reaction.2) CapacityBatteries store a lot of electrical energy, but capacitors store less.3) Charge and Discharge Figure 6. Charge and Discharge Curve The charging and discharging speed and the number of times are different. Generally, it only takes a few seconds or minutes to charge a capacitor, while a battery usually takes several hours. The number of charging and discharging of the capacitor is at least tens of hundreds to thousands of millions of times, and the battery is generally only a few hundred to a thousand times.4) FunctionsThe purpose of the two is different. Capacitors can be used for coupling, blocking, filtering, phase shifting, RC, LC resonance and as energy storage components for instantaneous large current discharge. The battery is only used as a power source. Replacing Bike Battery with Capacitor Ⅳ Calculate the Time Constant for the Discharge of the Capacitors? Let’s review the calculation formula for the charge and discharge time of the capacitor. Suppose there is a power supply that charges the capacitor C through the resistor R, V0 is the initial voltage value on the capacitor, Vu is the voltage value after the capacitor is fully charged, and Vt is the voltage value at any time t On the capacitor, then the following calculation formula can be obtained:Vt = V0 + (Vu – V0) * [1 – exp( -t/RC)]If the initial voltage on the capacitor is 0, the formula can be simplified to:Vt = Vu * [1 – exp( -t/RC)]...Charging formulaIt can be seen from the above formula that because the index value can only be infinitely close to 0, but it will never be equal to 0, it takes infinite time for the capacitor to be fully charged.🔺When t = RC, Vt = 0.63Vu🔺When t = 2RC, Vt = 0.86Vu🔺When t = 3RC, Vt = 0.95Vu🔺When t = 4RC, Vt = 0.98Vu🔺When t = 5RC, Vt = 0.99VuIt can be seen that after 3~5 RCs, the charging process is basically over. When the capacitor is fully charged, the power supply is short-circuited, and the capacitor C will be discharged through R. At any time t, the voltage on the capacitor is:Vt = Vu * exp( -t/RC)...Discharging formula   Ⅴ FAQ 1. Can you use a capacitor as a battery?A voltage applied across the conductors creates an electrical field in the capacitor, which stores energy. A capacitor operates like a battery in that, if a potential difference is applied across it that can cause a charge greater than its "present" charge, it will be charged up.   2. Why can't we use capacitors instead of batteries?Capacitors don't provide large amount of energy because they have less energy density than batteries. Capacitors are useful to provide short duration power requirements because they can be charged or discharged at a higher rate than the batteries.   3. What is the difference between a battery and supercapacitor?Differences Between Capacitor and BatteryBatteries excel at storing energy, while supercapacitors rate better for power. In practical terms, this means that supercapacitors are better at discharging their stored energy quickly, while batteries save more energy in the same amount of material.   4. Which is better battery or capacitor?A capacitor is able to discharge and charge faster than a battery because of this energy storage method also. ... However, in general batteries provide higher energy density for storage, while capacitors have more rapid charge and discharge capabilities (greater Power density).   5. What is discharging of capacitor?Discharging a capacitor means releasing the charge stored within the capacitor. ... Hence the capacitor current exponentially reaches zero from its initial value, and the capacitor voltage reaches exponentially to zero from its initial value during discharging.   6. Is it safe to discharge a capacitor with a screwdriver?It's often safe to discharge a capacitor using a common insulated screwdriver; however, it is usually a good idea to put together a capacitor discharge tool and use that for electronics with larger capacitors such as household appliances.   7. Can you discharge a capacitor with a multimeter?The multimeter isn't used directly to discharge the stored energy of a capacitor. Instead, people use it to measure the voltage and power of the capacitor to know whether it is fully released or not. You can use different tools such as a light bulb or a DIY discharge tool for the process.   8. Why do we need to discharge a capacitor?You must discharge the capacitors before working on power supply circuits so you won't get shocked. ... Using a screwdriver to discharge the capacitor is not recommended because you can generate a spark and damage the printed circuit board or circuitry of the power supply. You can even blow the power section.   9. What is discharging time of capacitor?RC Discharging Table. Note that as the decaying curve for a RC discharging circuit is exponential, for all practical purposes, after five time constants the voltage across the capacitor's plates is much less than 1% of its inital starting value, so the capacitor is considered to be fully discharged.   10. What is the time constant of a capacitor?The time constant of a series RC (resis-tor/capacitor) circuit is a time interval that equals the product of the resistance in ohms and the capacitance in farad and is symbolized by the greek letter tau (τ). The time in the formula is that required to charge to 63% of the voltage of the source.   11. Is the time constant the same for charging and discharging the capacitor?The time constant of a resistor-capacitor series combination is defined as the time it takes for the capacitor to deplete 36.8% (for a discharging circuit) of its charge or the time it takes to reach 63.2% (for a charging circuit) of its maximum charge capacity given that it has no initial charge.   12. What is the formula of discharging of capacitor?q=ϵC(1−eCR−t) where q is the charge on the capacitor at time t,CR is called the time constant, ϵ is the emf of the battery. Discharging: If the plates of a charged capacitor are connected through a conducting wire, the capacitor gets discharged.   13. What is energy stored in capacitor?Electrical potential energyEnergy stored in a capacitor is electrical potential energy, and it is thus related to the charge Q and voltage V on the capacitor. We must be careful when applying the equation for electrical potential energy ΔPE = qΔV to a capacitor. Remember that ΔPE is the potential energy of a charge q going through a voltage ΔV.   14. Where is energy stored in capacitor?A charged capacitor stores energy in the electrical field between its plates. As the capacitor is being charged, the electrical field builds up. When a charged capacitor is disconnected from a battery, its energy remains in the field in the space between its plates.   15. Which type of current is blocked by a capacitor?Alternating current doesn't really "flow", it just oscillates back and forth. A capacitor acts like an elastic membrane, it allows the oscillation but blocs the flow of DC current.

Introduction As a passive device, Capacitors have unique functions in electronic circuits such as tuning, bypassing, coupling, and filtering. For example, it is used in the tuning circuit of the transistor radio, and also used in the coupling circuit and bypass circuit of the color TV. With the rapid development of electronic information technology, the update speed of digital electronic products is getting faster and faster. Capacitors play an important role in consumer electronic products such as flat-panel TVs (LCD and PDP), notebook computers, digital cameras and other products. Therefore, it is very important to ensure the capacitances and test its quality. Here this article will talk about how to test/check a capacitor in detail. 3 Ways to Check Capacitors in Circuit with Meters & Testers Catalog Introduction Ⅰ Test a Capacitor Using Multimeter 1.1 Digital Multimeter Use 1.2 Capacitor Measurements Matter 1.3 Test Non-polar Capacitors 1.4 Test Polar Capacitors 1.5 Test Chip Capacitors 1.6 Test Solid State Capacitors 1.7 Test Electrolytic Capacitors 1.8 Test Variable Capacitors 1.9 Capacitor Polarity Distinction Ⅱ Test a Capacitor Using Bridge Ⅲ Test a Capacitor Using Professional Equipment Ⅳ FAQ Ⅰ Test a Capacitor Using Multimeter 1.1 Digital Multimeter Use 1.1.1 Using Capacitance GearSome digital multimeters have the function of measuring capacitance, and their ranges include five ranges: 2000p, 20n, 200n, 2μ and 20μ. During test, the two pins of the discharged capacitor can be directly inserted into the Cx jack on the meter board, and the display data can be read after selecting the appropriate range.🔺2000p range is suitable for measuring capacitances less than 2000pF.🔺20n range is suitable for measuring capacitances between 2000pF and 20nF.🔺200n range is suitable for measuring capacitances between 20nF and 200nF.🔺2μ range is suitable for measuring capacitances between 200nF and 2μF.🔺20μ gear is suitable for measuring the capacitance between 2μF and 20μF. Experiments have proved that some types of digital multimeters have large errors when measuring small capacitors below 50pF, and there is almost no reference value for measuring capacitors below 20pF. At this time, the series method can be used to measure small-value capacitors. For example: measure a capacitor of about 220pF. Test its actual capacity C1 with a digital multimeter, and then connect the small capacitor in parallel to measure its total capacity C2, then the difference between the two (C1-C2) is the capacity of the small capacitor. It is very accurate to use this method to measure small capacitance of 1-20pF. 1.1.2 Using Resistance GearIn practice, it has proved that the charging process of the capacitor can also be observed with a digital multimeter, which is actually a discrete digital quantity that reflects the change of the charging voltage. Assuming that the measurement rate of the digital multimeter is n times/second, in the process of observing the charging of the capacitor, n independent and successively increasing readings can be seen every second. According to this display values, the quality of the capacitor can be detected and the size of the capacitance can be estimated. This method is suitable for measuring large-capacity capacitors from 0.1μF to several thousand microfarads. 1.1.3 Using Voltage GearUsing a digital multimeter to detect capacitors with DC voltage is actually an indirect method. This method can measure small-capacity capacitors from 220pF to 1μF, and can accurately measure the size of the capacitor's leakage current. 1.1.4 Using BuzzerUsing the buzzer of the digital multimeter, you can quickly check the quality of the electrolytic capacitor. For example, set the digital multimeter to the buzzer position, and use two test leads to contact the two pins of the capacitor Cx to be tested. A short buzzer should be heard, then the sound stops, and the overflow symbol "1" is displayed at the same time. Then, exchange the two test leads for another measurement, the buzzer should sound again, and finally the overflow symbol "1" is displayed. This situation indicates that the measured electrolytic capacitor is basically normal. At this point, you can dial to 20MΩ or 200MΩ to measure the leakage resistance of the capacitor to judge whether it is good or bad. Figure 1. Various Capacitor Stuff 1.2 Capacitor Measurements Matter (1) Before the measurement, the two pins of the capacitor should be short-circuited and discharged, otherwise the reading process may not be observed.(2) Do not touch the capacitor electrode with two hands during the measurement process, so as to prevent the meter from jumping.(3) During the measurement process, the value of Vin(t) changes exponentially, and it drops quickly at the beginning. As time goes by, the speed of decline will become slower and slower. When the capacity of the capacitor Cx under test is less than several thousand picofarads, and the measurement rate of the meter is low, it is too late to reflect the initial voltage value, so the initial display value of the meter is lower than the battery voltage at very first.(4) When the measured capacitance value is greater than 1μF, in order to shorten the test time, the resistance gear can be used. In addition, when the capacity of the capacitor under test is less than 200pF, it is difficult to observe the charging process because the change in readings is very short.Be sure to cut off the power and discharge capacitor before measuring. The method of discharging is to find a metal object such as a screwdriver, hold the exposed part of the metal on the insulating handle with the two pins, and measure the capacitance with a digital multimeter. Locate the capacitor block and then plug the two pins into the socket for capacitance measurement, and wait for the changing reading on the meter screen to stabilize. The actual value is the capacitance of the capacitor on the side. If has leakage, an analog multimeter can be used. When measuring, the small-capacity capacitor multimeter can be placed in RX1K or RX100. The two test leads are connected to the capacitor, the pointer deflects clockwise, and then as the capacitor is fully charged, there is no current flows, finally the watch hand will reappear counterclockwise and return to infinity. The larger the angle of the watch hand, the greater the capacity. During the deflection process, the pointer must swing at a constant speed so that it can return to infinity, which preliminarily shows that the capacitor has no leakage.If the needle suddenly slows down or does not return at a certain position on the dial, it means that the capacitor is leaking in a certain period. If it is displayed as infinity at the end, it shows that there is no leakage, but this can only be a rough judgment. If you want to find an accurate value, you have to use a capacitance meter. And the observation characteristic on the capacitance leakage tester or oscilloscope, this is impossible for ordinary people to have. There are also capacitors that have withstand voltage, which is generally written on their body. However, some ceramic capacitors are not marked on it, be careful when selecting them. Figure 2. Film Capacitor (cbb21) 1.3 Test Non-polar CapacitorsIf it is a non-polar capacitor, the multimeter can be adjusted to the "diode" gear to measure the on-off state. If the multimeter displays "1", it is normal; if displays "0" or other numbers, it means the capacitor is damaged. 1.4 Test Polar CapacitorsElectrolytic capacitors with polarities have "bulging", "deformation" or "leakage" in the shell, which show they are damaged. The capacitance block of a digital multimeter can also be used to measure the quality of the capacitor:(1) According to the rated capacitance marked by the capacitor, set the multimeter to the appropriate block.(2) Insert the capacitor into the hole of the multimeter to measure the capacity.If the capacitance is within the rated value range, it means the capacitor is intact, otherwise the capacitor is damaged. 1.5 Test Chip Capacitors1) Adjust the multimeter to the appropriate ohm gear. The principle of gear selection is: 1μF capacitor uses 20K gear, 1~100μF capacitor uses 2K gear, and larger than 100μF uses 200 gear.2) Determine the polarity. First adjust the multimeter to 100 or 1K ohms. Assuming that one pole is positive, connect the black test lead to it, and the red test lead to the other pole. Note the resistance value, and then discharge the capacitor. Then change the test lead to measure the resistance. The black test lead with a large resistance value is connected to the positive electrode of the capacitor.3) Then connect the red pen of the multimeter to the positive electrode of the capacitor, and the black pen to the negative electrode of the capacitor. If the display gradually increases from 0, and the overflow symbol 1 is displayed at the end, which shows the capacitor is normal. If it is always displayed as 0, the capacitor is short-circuited. If 1 is displayed, the internal circuit of the capacitor is open. Figure 3. Chip Capacitors 1.6 Test Solid State Capacitors✔️Capacitance Greater than 20μFWith a common digital multimeter, the maximum measured value of the capacitance block is 20μF, which sometimes cannot meet the test requirements. To this end, the following simple method can be used to measure capacitance greater than 20μF, and also the maximum capacitance of several thousand microfarads can be measured. When using this method to measure large-capacity capacitors, there is no need to make any changes to the original circuit of the digital multimeter.The measurement principle of this method is based on the formula C = C1C2/(C1+C2) in series with two capacitors. Since two capacitors with different capacities are connected in series, the total capacity after series connection is smaller than the smaller capacitor. Therefore, if the capacity of the capacitor under test exceeds 20μF, only one capacitor with a capacity less than 20μF should be used. In series with it, it can be directly measured on the digital multimeter. According to the formula above mentioned, it is easy to deduce C1=C2C/(C2-C), using this formula can calculate the capacitance value of the capacitor under test. ✔️Capacitance Less than 10μFBecause the capacity of a fixed capacitor below 10pF is too small, it can only be roughly checked for leakage, internal short circuit or voltage breakdown with a multimeter. When measuring, you can choose the R×10k block, and use two test pens to connect the two pins of the capacitor arbitrarily, and the resistance should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or has internal breakdown. ✔️Capacitance between 10PF and 0.01μFDetect whether the capacitor is charging, and then judge whether it is good or bad. The multimeter selects the R×1k block. The β value of the two transistors is above 100, and the penetration current should be small. A composite tube can be used consist of silicon transistors. The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charge and discharge process of the capacitor under test is amplified, and the amplitude of the pointer of the multimeter is enlarged, which is convenient for observation. It should be noted that during the test operation, especially when measuring small-capacity capacitors, it is necessary to repeatedly exchange the contact points A and B of the tested capacitor pin to clearly see the swing of the meter pointer. ✔️Fixed Capacitance of 0.01μFFor a fixed capacitance above 0.01μF, the R×10k block of a multimeter can be used to directly test whether the capacitor has the charging process and internal short circuit or leakage, in addition, the capacitance of the capacitor can be estimated according to the magnitude of the pointer swing to the right. Figure 4. Electrolytic Capacitors 1.7 Test Electrolytic Capacitors1) Because the capacity of electrolytic capacitors is much larger than that of general fixed capacitors, ranges should be selected for different capacities when measuring. According to experience, in general, the capacitance between 1 and 47μF can be measured with the R×1k block, and the capacitance larger than 47μF can be measured with the R×100 block.2) Connect the red test lead of the multimeter to the negative pole and the black test lead to the positive pole. At the moment of contact, the pointer of the multimeter will deflect to the right by a greater degree (for the same electrical barrier, the greater the capacity, the greater the swing), and then gradually turn to the left Turn around until it stops at a certain position. The resistance value at this time is the forward leakage resistance of the electrolytic capacitor, which is slightly larger than the reverse leakage resistance. Practical experience shows that the leakage resistance of electrolytic capacitors should generally be more than several hundred kΩ, otherwise, it will not work normally. In the test, if there is no charging phenomenon in the forward and reverse directions, that is, the hand does not move, it means that the capacity has disappeared or the internal circuit is broken; if the measured resistance value is very small or zero, it means that the capacitor has a large leakage or has been broken down.3) For electrolytic capacitors with unknown positive and negative signs, the above method of measuring leakage resistance can be used to distinguish. That is to measure the leakage resistance arbitrarily, remember its size, and then exchange the test leads to measure a resistance value. The larger resistance of the two measurements is the positive connection, that is, the black test lead is connected to the positive electrode, and the red test lead is connected to the negative electrode. Use a multimeter to block electricity and charge the electrolytic capacitor. According to the magnitude of the pointer swing to the right, the capacity of the electrolytic capacitor can be estimated.When measuring electrolytic capacitors, if the measured value does not change significantly, the corresponding pins of the probe should be exchanged for multiple measurements. 1.8 Test Variable Capacitors1) Rotate the shaft gently and smoothly. When pushing the load shaft in full directions, there should be no looseness of it.2) Rotate the shaft with one hand and gently touch the outer edge of the film set with the other hand. You should not feel any looseness. The variable capacitor with poor contact between the rotating shaft and the moving plate can no longer be used.3) Place the multimeter in the R×10k gear, connect the two test leads to the moving piece and the lead end of the fixed piece of the variable capacitor with one hand, and slowly rotate the shaft several times back and forth with the other hand. The pointers of the multimeter should not move at infinity. In the process of rotating the shaft, if the pointer sometimes points to zero, it indicates that there is a short-circuit point between the moving piece and the fixed piece. If it encounters a certain angle, the multimeter reading is not infinity but a certain resistance value, indicating that the variable capacitor has a leakage phenomenon between the film and the stator. 1.9 Capacitor Polarity DistinctionThe black part with a mark on the capacitor is negative. There are two semicircles on the position of the capacitor on the PCB, and the pin corresponding to the colored semicircle is the negative electrode. The length of the pins is also useful to distinguish the polarity: the long pin is anode and the short pin is cathode.When we don't know the positive and negative poles of the capacitor, we can use a multimeter to figure them out. The medium between the two poles of the capacitor is not an absolute insulator, and its resistance is not infinite, but a finite value, generally above 1000 megohms. The resistance between the two poles of a capacitor is called insulation resistance or leakage resistance. Only when the positive electrode of the electrolytic capacitor is connected to the positive power supply (the black test lead), and the negative terminal is connected to the negative power supply (the red test lead), the leakage current is small (the leakage resistance is large), on the contrary, the leakage current increases (the leakage resistance decreases).Without knowing it, you can first assume the “+” pole of a certain pole. Using R*100 or R*1K of the multimeter, connect the test leads, and record the scale of the stop of the test needle (the resistance value of the test needle to the left is large), and the reading can be read directly for a digital multimeter. Then discharge the capacitor, then exchange the two test leads, and perform the measurement again. In the two measurements, the black test lead is connected to the positive electrode of the electrolytic capacitor when the last position of the needle is to the left (or the end with large resistance).When measuring large capacity capacitors, if you need to measure the positive and negative back and forth, discharge it to avoid damage to the multimeter. In addition, in high-frequency circuits, switching power supply circuits have many small capacitors, which ordinary multimeters cannot correctly judge whether they are good or bad. In terms of this case, it is recommended to use a dedicated digital capacitance meter to measure. Figure 5. SMD Capacitor Ⅱ Test a Capacitor Using BridgeThe data measured with a multimeter is not too accurate, and it can only measure the deviation of the capacity. For a little professional, you can use a bridge. When testing a capacitor with a digital bridge, you can clamp the lead of the capacitor to test its capacity, which can also show the loss of the capacitor, especially through the loss, it is easier to distinguish the quality of the capacitor.   Ⅲ Test a Capacitor Using Professional EquipmentIn general, capacitors have special test equipment for each performance, such as capacitor durability test, destructive test, loss angle test, inter-electrode withstand voltage test, self-healing test, charge and discharge test, pulse voltage test, spontaneous combustion test, ripple current durability test, etc., but for most users, these devices are more expensive and difficult to operate. If you really want these data, you can entrust a third party to test or ask the manufacturer for relevant information.   Ⅳ FAQ1. How do you check if a capacitor is bad?Use the multimeter and read the voltage on the capacitor leads. The voltage should read near 9 volts. The voltage will discharge rapidly to 0V because the capacitor is discharging through the multimeter. If the capacitor will not retain that voltage, it is defective and should be replaced.   2. How do you tell if a capacitor is bad with a multimeter?If the capacitance value is within the measurement range, the multimeter will display the capacitor's value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty.   3. What are the symptoms of a bad start capacitor?Start Capacitor FailureMotor run capacitor failure symptoms include warm air flowing from the vents inside the home, the air conditioner taking more time than usual to kick on or it turns off before it is programmed to, or there is a constant low hum emitting from the machine that isn't typical.   4. Can a capacitor test good and still be bad?It can, and most often does, although it is probably lower in capacitance than it originally was, but still usually within tolerance. There isn't likely to be a problem with leakage. There are two ways to test an ESR meter, a circuit unpowered or an oscilloscope.   5. How long can a capacitor last?Age. Like all things, capacitors have a limited life span. Most are designed to last approximately 20 years, but a number of factors can cause them to wear out more quickly.   6. How do you identify a capacitor?Ceramic types of capacitors generally have a 3-digit code printed onto their body to identify their capacitance value in pico-farads. Generally the first two digits indicate the capacitors value and the third digit indicates the number of zero's to be added.   7. Will a capacitor discharge on its own?Will a Capacitor Discharge On Its Own? In theory, a capacitor will gradually lose its charge. A fully charged capacitor in an ideal condition, when disconnected, discharges to 63% of its voltage after a single time constant. Thus, this capacitor will discharge up to near 0% after 5 time constants.   8. How do I know if my AC capacitor is bad?The most common signs and symptoms of a bad AC capacitor include:AC not blowing cold air.AC takes a while to start once you turn it on.Humming sound coming from your air conditioner.AC shuts off on its own.AC won't turn on.   9. What side of capacitor is positive?To tell which side is which, look for a large stripe or a minus sign (or both) on one side of the capacitor. The lead closest to that stripe or minus sign is the negative lead, and the other lead (which is unlabeled) is the positive lead.   10. Can I use a multimeter to discharge a capacitor?The multimeter isn't used directly to discharge the stored energy of a capacitor. Instead, people use it to measure the voltage and power of the capacitor to know whether it is fully released or not. You can use different tools such as a light bulb or a DIY discharge tool for the process.   11. How fast can a capacitor discharge?A fully charged capacitor discharges to 63% of its voltage after one time period. After 5 time periods, a capacitor discharges up to near 0% of all the voltage that it once had.   12. Can a bad capacitor ruin a compressor?Using the wrong capacitor rating or a poor quality capacitor can adversely affect the operation of the motor, the compressor or an entire HVAC system. ... Depending on the motor load, this may result in a reduction in the motor's overall speed.   13. Can I replace a start capacitor with a run capacitor?Run Capacitors. Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). ... A start capacitor can never be used as a run capacitor, because it cannot not handle current continuously.   14. How do you check a capacitor without a multimeter?Just connect those two ends of the capacitor to a single phase supply and switch it ON for a few seconds. Then take that two terminal and short it, you will get a spark. And so you can somewhat assume that your capacitor is in good condition.   15. What if a capacitor reads high?The high resistance across the capacitor is a sign that the capacitor is faulty. It is reading as if there is an open circuit.   16. How many ohms should a capacitor have?Make sure the capacitor is fully discharged. Set the meter on the Ohmic range (Set it at least on 1000 Ohm = 1kΩ). Connect the multimeter probes to the capacitor terminals (Negative to Negative and Positive to Positive).   17. What if a capacitor reads low?If it reads lower than nominal value, you may want to replace it. Non-polar capacitors lower than 1μF should not alter that much with aging. Capacitors in frequency sensitive circuits such as filters, time delays, should have a tighter tolerance.   18. What happens when capacitor goes bad?A bad capacitor prevents the exterior unit from properly functioning, which hinders the cooling process as a whole. Second, improper voltage delivery to exterior unit components forces the system to work harder as it attempts to perform its job. Additional components often sustain damage due to a faulty capacitor.   19. How can you tell if a capacitor is bad?Symptoms of defective capacitors may include:Excessive noise in audio or video, including 60hz audio hum or rolling bars in video.Scratchy, distorted, or missing audio.Low contrast, blurry, or distorted LCD displays.Intermittent or outright failure.   20. What does a damaged capacitor look like?A busted capacitor can be obviously broken (leaking brownish fluid, corroded, or with the leads severed), but sometimes it's subtle. The top of a blown capacitor will be slightly bent outwards in a convex shape, rather than flat or slightly indented inwards like a working capacitor.