Detailed explanation of the principles of 24 typical application circuits of capacitors

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Detailed explanation of the principles of 24 typical application circuits of capacitors [Copy link]

 

Note: Due to the replies from netizens in the middle of the post, all 24 circuits are not continuous, so you need to pay attention when reading.

Special thanks: I would like to express my gratitude to the netizens for the problems in the circuits! Thank you for correcting the errors!

1. Capacitor step-down circuit

The most common way to reduce 220V AC voltage to low voltage is to use a power transformer. Another way is a capacitor step-down circuit. Its advantages are small size, low cost and high efficiency, and its disadvantage is that it is not as safe as a power transformer.

Capacitor pressure reduction circuit in power indication

The figure below shows the capacitor voltage reduction circuit in the power indication. In the circuit, C1 is the voltage reduction capacitor, VD1 and VD2 are light-emitting diodes, R2 is the current limiting protection resistor, and R1 is the bleeder resistor.

Working principle:

Since the capacitive reactance of C1 is relatively large, the current in the loop is limited, so that the current flowing through the light-emitting diode is of appropriate size, making it emit light. In the positive half cycle of the alternating current, VD1 emits light. During the conduction period of VD1, VD2 is cut off. The figure below is a schematic diagram of the current loop when VD1 is turned on.

In the negative half cycle of the alternating current, VD2 is turned on. At this time, VD1 is cut off. The figure below is a schematic diagram of the current loop when VD2 is turned on. Although VD1 and VD2 are turned on alternately, due to the high conduction frequency and the persistence of vision of the human eye, it will feel that VD1 and VD2 are always glowing. Understanding of capacitor voltage-step-down circuit: Capacitors have capacitive reactance in AC circuits, so there is a voltage drop across the capacitors. The AC mains frequency is a relatively low frequency of 50Hz, so the capacitive reactance is large and the voltage drop across the capacitors is large, which can greatly reduce the AC output voltage. R1 is used to discharge the charge on C1 as quickly as possible. After the AC power is disconnected, the charge inside C1 is discharged through the R1 loop to release the internal charge and make the two ends of C1 without voltage. Only in this way can the safety of this circuit be guaranteed.

Capacitor step-down half-wave rectifier circuit

The figure below is a capacitor step-down half-wave rectifier circuit. C1 is a step-down capacitor, which reduces the 220V AC voltage to an appropriate level u, and then filters it through VD1 for half-wave rectification, and then obtains a DC voltage through C2 filtering. R1 is a bleeder resistor.

Capacitor step-down bridge rectifier circuit

Since the internal resistance of the half-wave rectifier circuit is relatively large, in order to provide a larger power supply current, a capacitor step-down bridge rectifier circuit with a smaller internal resistance can be used, as shown in the figure below.

C1 is a step-down capacitor, R1 is a bleeder resistor, VD1-VD4 are bridge rectifier diodes, and C2 is a filter capacitor.

Method for selecting step-down capacitors

The capacity of the step-down capacitor determines the current in the step-down circuit. The capacity of the step-down capacitor can be selected according to the load current limiting requirements.

The following table shows the relationship between current and capacitance in a 220V/50Hz capacitor step-down circuit. The current in the table is the maximum current value under a specific step-down capacitor capacity.

Method for selecting bleeder resistors

In the capacitor step-down circuit, a bleeder resistor needs to be connected in parallel at both ends of the step-down capacitor. A large bleeder resistor can reduce power consumption, but the discharge effect is poor and the step-down capacitor discharge time is long. The bleeder resistor is usually 500 kilo-ohms to 1 megohm. According to the capacity of the step-down capacitor, some fine-tuning is required to achieve a better discharge effect. The following table shows the relationship between the bleeder resistor and the step-down capacitor. When the step-down capacitor is large, the bleeder resistor is required to be small.

Important Tips

1. The capacitor step-down power supply is a non-isolated power supply, which actually introduces 220V AC into the load circuit. Pay special attention to isolation when using it to prevent electric shock. Use a 1:1 isolation transformer during debugging to ensure safety.

2. Compared with the resistor step-down circuit, the step-down capacitor has a smaller loss on the AC power, so it is better than the resistor step-down circuit.

3. The capacitive reactance of the step-down capacitor in the capacitor step-down circuit is the internal resistance of the power supply. The larger its capacitive reactance, the larger the internal resistance of the power supply, and the smaller the current that the power supply can provide. Since the internal resistance of the power supply is large, when the load current changes, the DC working voltage also changes accordingly. In order to ensure the stability of the DC working voltage, a voltage regulator diode can be set to stabilize the output DC working voltage.

4. The voltage resistance of the step-down capacitor should preferably be above 400V, and non-polar capacitors should be used. The most ideal one is an iron shell oil-immersed capacitor. 5. The capacitor step-down circuit cannot be used for high power conditions, is not suitable for dynamic load conditions, and is not suitable for capacitive and inductive loads.

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(II) Capacitor voltage divider circuit

1. Typical capacitor voltage divider circuit Resistors can form a voltage divider circuit, and capacitors can also form a voltage divider circuit, as shown in the figure below Capacitors can be used to divide the voltage of AC, because the use of resistor voltage divider circuits has a large loss for AC signals, while capacitors have little loss for AC signals while attenuating the signal amplitude. In the circuit, C1 and C2 form a capacitor voltage divider circuit. For an input signal of a certain frequency, capacitors C1 and C2 each present a capacitive reactance, and these two capacitive reactances constitute a voltage divider attenuation of the input signal, which can reduce the amplitude of the output signal. 2. Analysis of capacitor voltage divider circuit 1) Circuit characteristics The circuit characteristics of capacitor voltage divider and resistor voltage divider are the same, 2) Mainly used for voltage divider attenuation of AC signals 3) Can only be used in AC circuits. Due to the DC isolation and AC pass characteristics of capacitors, this circuit cannot be used in DC circuits.

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3. Typical capacitor filter circuits There are many types of capacitor filter circuits, such as power supply circuits using low-frequency filter capacitors and high-frequency filter circuits using high-frequency filter capacitors. As shown in the figure below 1. Circuit analysis The power filter circuit mainly uses large-capacity capacitors. The circuit in the figure below can illustrate the working principle of the capacitor filter circuit. C1 is a filter capacitor, connected between the output end of the rectifier circuit and the ground. The unidirectional pulsating DC voltage output by the rectifier circuit is added to C1, and R1 is the load resistor of the rectifier filter circuit. 1) Waveform equivalent analysis The voltage output by the rectifier diode VD1 contains pure DC voltage and pure AC voltage. According to the principle of waveform analysis, this voltage can be decomposed into a DC voltage and a group of AC voltages with different frequencies. The figure below is the waveform after the two voltages are superimposed. 2) DC current filter capacitor C1 is an open circuit for DC power, so DC power can only form a loop through load resistor R1, as shown in the figure below. In this way, DC power causes DC voltage to be obtained at both ends of the load. Typical application circuit of capacitor (2) Capacitor voltage divider circuit, capacitor filter circuit 3) AC current has a small AC capacitive impedance to the output of VD1 because of the large capacity of C1. In this way, AC power passes through C1 to the ground and cannot flow through R1, thereby achieving the purpose of filtering out the AC component. See the figure below. Typical application circuit of capacitor (2) Capacitor voltage divider circuit, capacitor filter circuit 4) Filter capacitor The filter capacitor capacity should be large and it should be a polar electrolytic capacitor. The larger the C1, the smaller the capacitive impedance to the AC component, making the AC component on R1 smaller and the filtering effect better. 2. Fault detection The simplest way to detect C1 is to measure the DC output voltage across C1. After the circuit is powered on, use the DC voltage range of a multimeter to measure, as shown in the figure below. Typical application circuits of capacitors (2) Capacitor voltage divider circuit, capacitor filter circuit [attach]389613 [/attach] 1) When the circuit is powered on, if the DC voltage across C1 is 0, it means that C1 is likely to be broken down or open circuit. 2) After the circuit is powered off, use the pointer multimeter ohm range to measure C1. If the resistance is 0, it means that C1 is broken down. If it is not broken down, it is likely to be open circuit. Connect a capacitor of the same capacity in parallel to C1. If it works normally after power is turned on, it means that C1 is open circuit. 3) If no problems are found in the above tests, then C1 is not faulty. Check other components of the circuit.

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(IV) High-frequency filtering in power supply filter circuits

As shown in the figure below, in the circuit, a large-capacity electrolytic capacitor C1 is connected in parallel with a small-capacity capacitor C2. C2 is a high-frequency filter capacitor. This circuit with two capacitors in parallel, one large and one small, is very common in power supply circuits.

1. Circuit analysis

1) High-frequency interference

Since there is a large amount of high-frequency interference in the alternating current, the high-frequency filter capacitor in the power supply circuit is used to filter out the high-frequency components

2) Theoretical capacitive reactance and actual contradiction

Theoretically, at the same frequency, a capacitor with large capacity has small capacitive reactance. If a large and a small capacitor are connected in parallel, the capacitor C2 with small capacity does not seem to have any effect. However, due to process reasons, the large-capacitance capacitor C1 has inductive reactance characteristics. Under high-frequency conditions, the impedance of C1 is the parallel connection of capacitive reactance and inductive reactance. Because the frequency is high, the inductive reactance is large, which limits the filtering effect of C1 on high-frequency interference. 3) High frequency filter capacitor To compensate for the deficiency of large capacitor C1 under high frequency conditions, a small capacitor C2 is connected in parallel. C2 has almost no inductance. When the circuit operating frequency is high, the capacitive reactance of C2 is very small, so the high frequency component is filtered to ground through C2. 4) Working state of large capacitor Most of the unidirectional pulsating DC output by the rectifier circuit is low-frequency AC component. The capacitive reactance of small capacitor to low-frequency AC component is large and equivalent to open circuit. Therefore, the large capacitor C1 is mainly responsible for the low-frequency component, as shown in the figure below. 3896215) Working state of small capacitor For high-frequency components, due to the high frequency, the large capacitor C1 is in an open circuit state due to its inductive reactance. The capacitive reactance of C2 is much smaller than the impedance of C1 and is in a working state. It filters out various high-frequency interferences. Therefore, the high-frequency components flow through C2, as shown in the following figure. 34)]

2. Fault detection method

The power supply circuit has no DC voltage output: If you suspect C2, disconnect C2 from the circuit. If the circuit returns to normal after disconnection, it means that C2 is broken down.

If you suspect that C1 leakage causes the output voltage to drop, you can determine the fault location by disconnecting C2.

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5. Capacitor protection circuit in power supply circuit

In the power supply circuit, from the perspective of filtering, the larger the capacity of the filter capacitor, the better. However, too large a filter capacitor capacity is harmful to the rectifier diode in the rectifier circuit. The circuit shown in the figure below can illustrate the harm of large-capacity filter capacitors to rectifier diodes. In the circuit, VD1 is a rectifier diode and C1 is a filter capacitor. When not powered on, the voltage across C1 is 0V. At the moment of power on, the rectifier diode is turned on and charges the filter capacitor C1. Because the voltage across C1 is originally 0V, this is equivalent to short-circuiting the negative pole of the rectifier diode VD1 to the ground. Therefore, the current flowing through VD1 at this moment, that is, the charging current of C1, is very large. Not only that, due to the large capacity of C1, its charging voltage rises slowly, which means that a large current flows through the rectifier diode for a long time, which may burn out the rectifier diode. The larger the capacity of C1, the longer the large current flows through VD1, and the greater the possibility of VD1 being damaged.

In order to solve the contradiction between the large-capacity filter capacitor and the long-term overcurrent of the rectifier diode, a multi-section RC filter circuit can be used to improve the filtering effect, so that the capacity of C1 can be appropriately reduced. In addition, a rectifier diode protection circuit can be added.

1. Circuit analysis

As shown in the figure below, it is a protection capacitor circuit. The capacity of the small capacitor in the circuit is very small. C1 protects the rectifier diode VD1

[attach]389633 [/attach]

When the circuit is connected, since the initial voltage between the two pins of C1 is 0V, C1 is equivalent to a short circuit. The maximum current (inrush current) at the moment of startup charges the filter capacitor C2 through C1. The figure below is a schematic diagram of the inrush current loop when the power is turned on. In this way, the maximum inrush current does not flow through the rectifier diode when the power is turned on, thereby achieving the purpose of protecting the rectifier diode. After the power is turned on, C1 is quickly fully charged. At this time, it is equivalent to C1 being open, and VD1 rectifies the AC voltage.

The capacitor protection circuit is as shown in the figure below

34)]

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(VI) Safety capacitor (X capacitor Y capacitor) anti-high frequency interference circuitSafety capacitors are divided into X capacitors and Y capacitors

The following figure is X capacitor

[attach] 34)]

The figure below shows the application circuit of X capacitor and Y capacitor. This is the 220V AC input current of the switching power supply, also known as transient filter circuit or EMI filter. In the circuit, R1 is a varistor. L1 and L2 are ferrite coils, and FU1 is a fuse.

The figure below shows the actual X capacitor and Y capacitor in the switching power supply

Function of EMI filter

The EMI filter is set between the 220V AC power incoming line and the rectifier circuit to filter out voltage transients and high-frequency interference in the municipal power grid. At the same time, it also prevents the high-frequency interference generated by the switching tube in the switching power supply from being transmitted to the municipal power grid, forming high-frequency interference to other electrical appliances. 1. Analysis of differential mode high frequency interference signal and X capacitor circuit There are two 220V AC power lines, one is the phase line and the other is the neutral line. Two high frequency interference signals will be generated on these two pins. And differential mode interference signal and common mode interference signal. As shown in the following figure, 389640 34)]It can be seen from the figure that the high-frequency interference signals U1 and U2 have the same direction and are equal in size. Such two signals are called common-mode signals. The high-frequency interference signals U3 and U4 have opposite directions and are equal in size. Such two signals are called differential-mode signals. After the X capacitor C3 is connected to the circuit, since the frequency of the high-frequency interference signal is relatively high, the capacitive reactance of C3 to the high-frequency interference signal is small. In this way, the differential-mode high-frequency interference signal forms a loop through the X capacitor C3, as shown in the figure below, and cannot be added to the subsequent rectifier circuit, so as to achieve the purpose of eliminating the differential-mode high-frequency interference signal.

2. Analysis of common-mode high-frequency interference signal and Y capacitor circuit

The following figure is a schematic diagram of Y capacitors to eliminate common-mode high-frequency interference signals. A common-mode high-frequency interference signal must use two Y capacitors because there are high-frequency interference signals on both the phase line and the neutral line.

The common-mode high-frequency interference signal on the phase line is connected to the ground line through the Y capacitor C1, and the common-mode high-frequency interference signal on the neutral and phase lines is connected to the ground line through the Y capacitor C2. In this way, the common-mode high-frequency interference signal cannot be added to the subsequent circuit, thereby achieving the purpose of suppressing the common-mode high-frequency interference signal.

X capacitor requirements

Y capacitor requirements

3. Safety capacitor certification mark

maychang

This circuit does not work! No matter how large the capacitance of capacitor C1 is, the output current is only a few tenths of a milliampere (actually, it is determined by resistor R1).

maychang

tiankai001 posted on 2018-11-30 10:24 5. Capacitor protection circuit in the power supply circuit In the power supply circuit, from the filtering perspective, the larger the capacity of the filter capacitor, the better, but the filter capacitor capacity is too large...

The rectifier diode does not need protection at all. Ordinary rectifier diodes such as 1N4007 can have an overcurrent capacity of dozens of times the rated current within half a power frequency cycle (10ms). The rated current of 1N4007 is 1A, and it will not be damaged even if a current of 20 to 30A passes through it within 10ms. With such a large overcurrent capacity, why protect it?

xunke

Some of the things posted are incorrect (not verified); only the following picture can work properly:

bqgup

Finally I met a great master. Thank you for sharing the profound knowledge. I didn't expect the function of capacitors to be so powerful. I will definitely study hard in the future.

tiankai001

VII. Detailed explanation of capacitor coupling circuitThe role of coupling capacitor

Add the previous stage signal to the next stage circuit as losslessly as possible, while removing unnecessary signals. For example, the coupling capacitor can separate the DC component in the previous stage circuit while coupling the AC signal from the previous stage to the next stage (the capacitor has the characteristic of isolating DC and passing AC)

1. Typical capacitor coupling circuit

As shown in the figure below, the coupling capacitor is between the previous and next stage circuits (or two unit circuits). If it is between two stages of amplifiers, it can be called an inter-stage coupling capacitor. The purpose of using coupling capacitors between two circuits is to transfer useful AC signals from the output of the previous circuit to the input of the next circuit. Due to the DC-blocking and AC-passing characteristics of capacitors, among the DC components and AC signals output by the previous circuit, only the AC signal can be added to the input of the next circuit. Since the DC component cannot be added to the next circuit, this is convenient for circuit design and maintenance. Therefore, whenever coupling capacitors are seen in a circuit, the DC circuits between the previous and next stages are isolated from each other. 3898392016[/attach]

2. Circuit Analysis The capacitor coupling circuit is called the resistor-capacitor coupling circuit in the circuit. As shown in the figure below. C1 is the inter-stage coupling capacitor. Looking from point A in the circuit, the input impedance of the amplifier is R, and C1 and R form a resistor-capacitor coupling circuit. 1) Input resistance R In the RC coupling circuit, the resistor is invisible. It is the input resistance of the next amplifier and cannot be directly seen in the circuit. Generally, the input resistance of the amplifier is relatively large. 389840 The figure below shows the input impedance of the amplifier. The input impedance of the amplifier is the impedance seen from the input end of the amplifier after the DC bias circuit is added. The value is equal to the voltage at the input end divided by the input loop current.

2) Voltage divider circuit

From the resistor-capacitor coupling circuit, it can be seen that C1 and R form a voltage divider circuit for the signal, and the signal after voltage division is added to the post-amplifier. The resistance of R is very large and the capacitance of C1 is very small, so the coupling circuit has almost no attenuation on the signal.

3) The influence of coupling capacitor on low-frequency characteristics

[color=rgb(34, 34, See the figure below. When R is constant, increasing the capacitance of C1 can improve the low-frequency characteristics. The attenuation of the low-frequency signal when passing through the RC coupling circuit is reduced. However, increasing the capacitance of C1 will increase the leakage of the coupling capacitor, thereby increasing the circuit noise, and vice versa.

4) The influence of input resistance on low-frequency characteristics

A large input resistance R of the amplifier is beneficial to improving the low-frequency characteristics of the RC coupling circuit, so many amplifiers need to increase the input resistance.

5) Selection of coupling capacitor capacity

Circuits with different operating frequencies have different requirements for coupling capacitor capacity. When the operating frequency is high and the capacitive reactance is small, the coupling capacitor capacity can be smaller, otherwise it should be larger. In circuits with the same operating frequency, when the input resistance of the subsequent circuit is high, the coupling capacitor capacity can be smaller. In multi-stage amplifier circuits, the coupling capacitor capacity of the previous circuit can be appropriately smaller to reduce the noise caused by the leakage of the coupling capacitor.

6) Application of capacitor coupling circuit

As long as there is a circuit with signal transmission, it is possible to use a capacitor coupling circuit, whether it is an amplifier, an oscillator, or an automatic control circuit.

7) Methods for identifying coupling capacitors in circuits

The store between two-stage amplifiers or two unit circuits is usually a coupling capacitor. Based on this feature, it is very convenient to find the coupling capacitor in the circuit. In the figure below, C913 is a coupling capacitor, which is connected between the output end of the front-stage integrated circuit A901 and the back-stage circuit.

3. Similar capacitor coupling circuits

There are many capacitor coupling circuits with the same function, but they are different in either the capacity of the coupling capacitor or the circuit form.

1) High-frequency capacitor coupling circuit

As shown in the figure below, the coupling capacitor is connected between the collector of VT1 and the base of VT2. Because it is a high-frequency circuit, the capacity of C1 is small. The higher the operating frequency of the circuit, the smaller the capacity of the coupling capacitor. And you should choose a high-frequency capacitor with small high-frequency loss.

2) Audio capacitor coupling circuit

As shown below. The capacitance is between 1-10 microfarads, and low-frequency polarized electrolytic capacitors are usually used.

3) Deformed audio capacitor coupling circuit

As shown in the figure below, she added a resistor R1 to the ordinary capacitor coupling circuit, and R1 is connected in series in the coupling capacitor C1 loop. R1 is used to prevent possible high-frequency oscillation and improve the working stability of the circuit. R1 is usually 2.2 kilo-ohms.

4) Integrated circuit input coupling capacitor circuit and output coupling capacitor circuit.

As shown in the figure below, the capacitor C553 connected in series to the input end of the integrated circuit A502 is a coupling capacitor. Because it is connected to the input end of the integrated circuit, it is called an input coupling capacitor.

C556 is connected in series to the output end of the integrated circuit A502, so it is called an output coupling capacitor

Fault detection method

When you suspect that the coupling capacitor is open, directly connect a capacitor of equal capacity in parallel with the original capacitor. If the circuit function is restored, it means that the original capacitor is open.

If you suspect that the capacitor is leaking, remove the original capacitor and connect another capacitor in parallel for power-on test.

34)]As shown in the figure below, the capacitor C553 connected in series to the input end of the integrated circuit A502 is a coupling capacitor. Because it is connected to the input end of the integrated circuit, it is called the input coupling capacitor.

C556 is connected in series to the output end of the integrated circuit A502, so it is called the output coupling capacitor

Fault detection method

When it is suspected that the coupling capacitor is open, directly use a capacitor of equal capacity in parallel with the original capacitor. If the circuit function is restored, it means that the original capacitor is open. If you suspect the capacitor is leaking, remove the original capacitor and connect a new capacitor in parallel to conduct a power-on test.34)]As shown in the figure below, the capacitor C553 connected in series to the input end of the integrated circuit A502 is a coupling capacitor. Because it is connected to the input end of the integrated circuit, it is called the input coupling capacitor.

C556 is connected in series to the output end of the integrated circuit A502, so it is called the output coupling capacitor

Fault detection method

When it is suspected that the coupling capacitor is open, directly use a capacitor of equal capacity in parallel with the original capacitor. If the circuit function is restored, it means that the original capacitor is open. If you suspect the capacitor is leaking, remove the original capacitor and connect a new capacitor in parallel to conduct a power-on test.

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8. Decoupling capacitor circuit in multi-stage amplifier

The decoupling circuit is usually set between two stages of amplifiers, so only multi-stage amplifiers have decoupling circuits. Decoupling circuits are used to eliminate harmful cross-connections between multi-stage amplifiers.

1. Reasons for setting up decoupling circuits

1) The influence of power supply internal resistance on the signal

The figure below shows the internal circuit of the power supply. Ideally, the DC voltage +V terminal is grounded for AC. The dotted box is a DC power supply, which is composed of a voltage source E and an internal resistor r0 in series. When current flows through this DC power supply, there is a voltage drop on the internal resistor R0. When the AC signal current flows through this internal resistor, there is also an AC signal voltage drop. This voltage drop is the root cause of harmful cross-connections in the circuit.

2) The concept of cross-connections between multi-stage amplifiers

As shown in the figure below, VT1 and VT2 constitute the first and second common emitter amplifiers respectively. The output signal voltage of the common emitter amplifier is opposite to the input signal voltage. Assuming that there is no decoupling capacitor C1 in the circuit, and assuming that the signal voltage on the base of VT1 is increasing at a certain moment, that is, +, as shown in the circuit diagram, the phase of the signal voltage on the collector of VT1 is -, the phase of the signal voltage on the base of VT2 is -, and the phase of the signal voltage on the collector of VT2 is +.

Due to the internal resistance R0 of the +V DC power supply, when the signal current of the collector of VT2 flows through R0, a signal voltage drop is generated on it, that is, there is a signal voltage at point B in the circuit, and the phase is +. The positive polarity AC signal at point B in the circuit is added to point A through R3. The signal voltage phase at point A is also +. It is added to the base of VT1 through R1, making the base signal voltage of VT1 larger. Through a series of positive feedback in the above circuit, the signal in VT1 is very large and self-excited, resulting in a howling sound. This is a circuit howling phenomenon caused by harmful cross-connections in multi-stage amplifiers. When positive feedback appears in the amplifier circuit, the circuit will oscillate.

The frequency of this oscillation is single. When this frequency falls within the audio range, a howling sound can be heard. When this oscillation frequency falls within the super-audio range, super-audio oscillation will occur. At this time, no howling sound can be heard, but the amplifier in the circuit will heat up, and in serious cases, the amplifier device will be burned out.

2. Decoupling capacitor circuit

The figure below shows a decoupling capacitor circuit. After adding a decoupling capacitor C1 between the DC voltage supply circuits of the two-stage amplifiers of the multi-stage amplifier, the positive polarity signal at point A in the circuit is bypassed to the ground by C1 and cannot be added to the base of VT1 through R1. In this way, positive feedback cannot be generated in the multi-stage amplifier, and there is no cross-connection between stages, achieving the purpose of eliminating harmful cross-connection between stages. After adding the decoupling resistor R3, the decoupling effect can be further improved. This is because the signal voltage at point B in the circuit is attenuated by the voltage divider circuit composed of R3 and C1 (capacitive reactance), which is smaller than the signal voltage at point A when R3 is not added. There is a voltage drop after the DC current flows through the decoupling resistor R3, which reduces the DC operating voltage of the previous circuit.

Tip

In a multi-stage amplifier circuit, at least one decoupling circuit should be set for every two common emitter amplifiers, because each common emitter amplifier inverts the signal voltage once, and the phase of the signal voltage becomes the same after the two amplifiers are inverted twice, which easily produces inter-stage positive feedback and self-excitation. Therefore, amplifiers with many stages are equipped with multiple decoupling circuits. In addition to its decoupling function, the decoupling capacitor also has a filtering effect on the DC working voltage.

3. Troubleshooting method

For this circuit, the main troubleshooting is to measure the DC voltage on capacitor C1, which can reflect whether C1 and R3 are normal. The following figure is a wiring diagram for measuring the DC voltage of C1.

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9. High-frequency vibration elimination circuitHigh-frequency vibration elimination capacitor circuit

In the audio negative feedback amplifier circuit, in order to eliminate the possible high-frequency self-excitation, a high-frequency vibration elimination capacitor circuit is used to eliminate the high-frequency howling that may occur in the amplifier.

The figure below shows a common high-frequency vibration elimination capacitor circuit in audio amplifiers. C1 is a high-frequency vibration elimination capacitor connected between the collector and base of the amplifier tube VT1, with a capacity of several hundred picofarads. 3898491 ... 34)]No DC negative feedback

The DC voltage on the collector of transistor VT1 cannot be negatively fed back to the base through C1, so C1 does not have DC negative feedback.

No audio negative feedback

The transistor VT1 constitutes an audio amplifier, and C1 has only 100 pF. Such a small capacitor has a large capacitive reactance for audio signals and is equivalent to an open circuit. The audio signal cannot be added to the base of VT1 through C1, so C1 does not have negative feedback on the audio signal.

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10. Capacitor circuit for eliminating radio wave interference

In some audio amplifiers, you can sometimes hear the sound of a radio station, which means that radio waves interfere with the amplifier circuit. To prevent radio waves from interfering with the audio amplifier, a capacitor circuit for eliminating radio wave interference can be set.

1) Reasons for circuit setting

The figure below shows a single-stage amplifier, which is the first stage circuit in the entire amplification system. A radio wave signal is induced at the base of VT1. This signal is added to VT1 for amplification and detection at the same time. As a result, the audio model of the radio station is output at the collector of VT1, and radio wave interference occurs.

2) Capacitor circuit for eliminating radio wave interference

As shown in the figure below, a small capacitor C1 (100 pF) is connected between the base and emitter of VT1 to eliminate the interference of radio waves on the operation of the triode.

Circuit Principle

The radio wave added to the base of VT1 is bypassed by capacitor C1 to the emitter, and then flows into the ground through R3. It is not added to VT1, so this radio wave will not be detected by VT1, and the sound of the radio station will not appear, thus achieving the purpose of eliminating radio wave interference. Since the frequency of radio waves is very high, the capacity of C1 must be small enough, usually 100 pF.

The simplest and most effective detection method for small capacitor failure in the circuit is as follows

1. Suspected breakdown.

Use the ohm range of a multimeter to directly measure the resistance between the two pins of the small capacitor. It should be 0 ohms. If there is no inductor in parallel with the small capacitor, it can be considered that the small capacitor has broken down.

2. Suspect leakage

Directly replace the small capacitor

3. Suspect open circuit

[color=rgb(34, 34, Directly connect a capacitor of equal value in parallel with the original capacitor. 4. If the performance is suspected to be poor, directly replace the small capacitor.

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11. Neutralizing capacitor circuit

Neutralizing capacitor circuit is used in the intermediate frequency amplifier of the radio circuit.

Reason for setting the neutralizing capacitor circuit

There is junction capacitance between each electrode of the transistor. In the intermediate frequency amplifier and high frequency amplifier of the radio circuit, due to the high operating frequency, the junction capacitance between the base and collector of the transistor is greatly affected, as shown in the figure below. This junction capacitance is inside the transistor, between the base and the collector. Although this junction capacitance is very small, only a few picofarads, when the operating frequency of the transistor is high, its capacitive reactance is also relatively small, which will cause part of the signal current to be output from the collector of the transistor and flow back to the base through this junction capacitance, causing parasitic oscillation and affecting the operating stability of the intermediate frequency amplifier or high frequency amplifier.

In order to suppress this harmful parasitic oscillation, a circuit called a neutralizing capacitor is needed.

Typical neutralizing capacitor circuit

The figure below is a typical neutralizing capacitor circuit, in which C3 is the neutralizing capacitor. It should be noted that in the circuit, the primary winding L1 of the intermediate frequency transformer is tapped. If the intermediate frequency transformer winding is not tapped, the neutralizing capacitor circuit form is different from this.

The working principle of the neutralizing capacitor circuit can be explained by the circuit shown in the figure below. The tap of winding L1 is connected to the DC working voltage +V. This section is grounded for AC signals. In this way, the phases of the signals at the upper and lower ends of winding L1 are opposite, that is, when the phase at the upper end of L1 is positive, the phase of the signal at the lower end is negative.

When the phase of the signal at the lower end of winding L1 is -, the signal at this end is added to the base of the transistor through the internal junction capacitance of the transistor. At the same time, the signal at the upper end of winding L1 with a phase of + is also added to the base of the transistor through the neutralizing capacitor C3. The two signals are in opposite phases and are added to the base of the transistor after subtraction. If the capacity of C3 is adjusted so that the current flowing into the base of VT1 through the C3 path is equal to the current flowing into the base of the transistor through the internal junction capacitance of the transistor, then the subtraction between the two currents is 0, indicating that the neutralizing capacitor offsets the influence of the junction capacitance and achieves the neutralization purpose.

When the phase of the signal on winding L1 is reversed, that is, the phase of the signal on the upper end of winding L1 is —— and the phase on the lower end is +, neutralization can be performed at this time, because the current through C3 is always subtracted from the current of the junction capacitor.

Another neutralizing capacitor circuit

The figure below shows another neutralizing capacitor circuit, which is obtained by using the Wheatstone bridge principle. Its characteristic is that winding L1 has no tap. At this time, the neutralizing capacitor circuit is composed of two capacitors, C3 and C4, and a resistor R2 is added. In the circuit, capacitor C6 and winding L1 form a VT1 collector resonant circuit, which is also part of the neutralizing capacitor circuit. The working principle of this circuit can be explained by its equivalent circuit. The figure below is an equivalent bridge circuit. In the circuit, junction capacitors Cbc, C3, C4, and C6 constitute the four arms of the bridge.

Because the output signal of the amplifier is obtained from both ends of winding L1, L1 is the signal source of the bridge.

The base and emitter of the transistor are the output ends of the bridge and the input ends of the transistor VT1. If the bridge is balanced, the voltage between points B and E in the circuit is zero. At this time, the internal feedback of the amplifier is neutralized and the amplifier can work stably.

The balanced state of the bridge can be achieved by adjusting the capacitance of the neutralizing capacitor, that is, as long as the following equation is established.

Not all intermediate frequency amplifiers or high frequency amplifier circuits must have a neutralizing capacitor. If an intermediate frequency amplifier tube or high frequency amplifier tube with a very small junction capacitance is used, a neutralizing capacitor is not required.

Neutralizing capacitors can improve the symmetry of the resonance curve of the intermediate frequency amplifier.

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12. Series and parallel circuits of polarized electrolytic capacitors Parallel circuits of polarized electrolytic capacitors

See the figure below. The positive poles of the two capacitors are connected and the negative poles are connected. After they are connected in parallel, they are still a polarized electrolytic capacitor, and its capacity is the sum of the capacities of the two capacitors.

[attach]389878 [/attach]

It should be noted that when two capacitors are connected in parallel, the positive pole of one capacitor cannot be connected to the negative pole of the other capacitor, otherwise an explosion will occur in the circuit due to the reverse polarity of one capacitor.

The circuit of two large electrolytic capacitors in parallel

is shown in the figure below. C1 and C2 are both 510uF electrolytic capacitors. This capacitor parallel circuit is used for power supply filtering or as a coupling capacitor at the output end in the output loop of the OTL power amplifier circuit.

The main purpose of using two large electrolytic capacitors in parallel is:

1. Improve the reliability of the circuit

When one capacitor is open-circuited, the other capacitor can still make the circuit work normally, which can reduce the occurrence rate of circuit failure. 2. Reduce the leakage current of the capacitor. The leakage current of a large-capacity capacitor is large. The leakage current of two capacitors in parallel is less than the leakage current of one capacitor. 3. Increase the capacity. When the circuit effect is not ideal after using a large capacitor, you can use another large capacitor in parallel. 4. Reduce costs The larger the capacity of a capacitor, the more expensive it is. The price of two capacitors with half the capacity is less than the price of a large capacitor. 5. Reduce the size of the capacitor A capacitor with twice the capacity will be much larger in size, but when space is limited, two capacitors can be connected in parallel.

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13. Forward series, reverse series and parallel circuits of electrolytic capacitors Reverse series circuit of polar electrolytic capacitors

The reverse series connection of polar electrolytic capacitors is mainly to obtain a non-polar electrolytic capacitor.

See the figure below.

There are two types of reverse series circuits. The following figure a shows that the positive poles of two capacitors are connected, and the following figure b shows that the negative poles of two capacitors are connected.

No matter which reverse series circuit, the circuit effect is the same. After reverse series connection, it is equivalent to a non-polar electrolytic capacitor. Its equivalent capacitance is reduced.

Polarized electrolytic capacitor forward series circuit

See the figure below. C1 and C2 are both polar electrolytic capacitors, and the negative pole of C1 is connected to the positive pole of C2. The series connection of polar electrolytic capacitors is mainly used to improve the withstand voltage of the capacitor. In general electronic circuits, since the DC voltage is not very high, it is not commonly used. The series connection circuit of polar electrolytic capacitors is mainly used in electron tube circuits.

The following figure is a series connection circuit. Because the DC working voltage in the electron tube circuit is relatively high, the voltage resistance performance of the capacitor can be improved by connecting two electrolytic capacitors with lower voltage resistance in series.

Capacitance and voltage resistance of polarized electrolytic capacitor series circuit

Whether it is series connection or reverse series connection, the voltage resistance value of its equivalent capacitor will increase, but the capacity will decrease. As shown in the figure below, the capacity and withstand voltage of C1 and C2 are the same, which is 10 microfarads and 6V. After the two capacitors are connected in series, they are added to a 12V DC voltage. At this time, the capacity of the equivalent capacitor C0 in series is reduced by half, and the withstand voltage is increased by half.

Tip:

In the series circuit of electrolytic capacitors, try to choose capacitors with equal capacity and the same withstand voltage. If the capacitance and withstand voltage of two capacitors are inconsistent, the withstand voltage of the series connection will not exceed the withstand voltage of the capacitor with smaller capacitance.

The figure below is a series circuit of two polar electrolytic capacitors with the same withstand voltage but different capacitances. One has a capacitance of 20 microfarads and the other has a capacitance of 10 microfarads. The capacitance of their equivalent capacitance C0 is equal to the sum of the reciprocals of the capacitances according to the reciprocal of the total capacitance. According to calculations, the capacitance of C0 in this circuit is about 7 microfarads.

After C1 and C2 are connected in series, the equivalent capacitor withstand voltage is 9V, which means that the withstand voltage of this equivalent capacitor is higher than that of C1 and C2, but the actual withstand voltage value will not exceed the sum of the withstand voltages of the two capacitors.

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14. Speaker frequency division capacitor circuit

In the speaker frequency division capacitor circuit, the purpose of using the frequency division capacitor is to make the tweeter work in the high frequency band and the woofer work in the low frequency band.

1. The figure below shows the frequency division capacitor circuit in the two-way frequency division circuit. In the circuit, C1 is the coupling capacitor at the output end of the power amplifier, and C2 is the non-polar frequency division capacitor. 3898921 ... 34)]

2) Treble signal transmission process

Since the frequency of the treble is relatively high, the capacitive reactance of C2 to this frequency is small, so the treble signal is smoothly added to the tweeter BL2 through C2. The following figure is a schematic diagram of the high-frequency signal current loop

[color=rgb(34, 34, Tips: 1) The woofer has poor high-frequency characteristics. Although the treble signal can be added to the woofer, the effect is not good, so the treble is mainly produced by the tweeter. 2) The crossover capacitor is a non-polar capacitor. Since the signal output from C1 is an audio signal (AC signal) with a large amplitude, the crossover capacitor must be non-polar, such as tantalum electrolytic capacitor. Polarized electrolytic capacitors cannot work properly in large-signal AC circuits because the polarity of the AC signal is constantly changing. 2. The frequency division capacitor circuit composed of polar electrolytic capacitors in reverse series is shown in the figure below. After the polar electrolytic capacitors are connected in reverse series, the originally polar pins have no polarity, so the capacitors in series can be used as non-polar electrolytic capacitors. Such a circuit is not as good as a truly non-polar electrolytic capacitor. 389888 1) Large signal positive half-cycle working condition 34)]During the positive half cycle of the large AC signal Us, its voltage polarity is consistent with the polarity of the C1 pin, as shown in the figure below. The positive polarity voltage is added to the positive pole of C1, which meets the working conditions of polarized capacitors. At this time, C1 can work normally. 389889201[/attach]

2）Large signal negative half cycle working condition During the negative half cycle, the voltage polarity of the large signal is opposite to the polarity of the C1 pin, as shown in the figure below. The negative voltage is added to the positive electrode of C1. During the negative half cycle, the voltage of the negative electrode of C1 is always higher than the positive voltage. Because C1 is a polar electrolytic capacitor, the leakage current of C1 is large at this time and it cannot work normally. 34)]

3. Working principle of polar electrolytic capacitor in AC/DC mixed circuit

The circuit shown in the figure below can illustrate the voltage conditions of the two pins when the polar electrolytic capacitor works normally.

As can be seen from the figure, the signal Us is a signal of superposition of DC and AC, and the negative peak value of the AC signal is also greater than 0V, which is also a positive voltage, as shown in the figure above.

In this way, the voltage polarity added to C1 is always consistent with the pin polarity of C1, so the polarized electrolytic capacitor can work normally.

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15. Temperature compensation capacitors in parallel, multiple small capacitors in series and parallel Temperature compensation capacitor parallel circuit

The figure below shows a parallel circuit of two small capacitors of equal capacity. The capacity of C1 and C2 is equal. This is the line timing capacitor circuit in the line oscillator circuit of a color TV. The timing capacitors C1 and C2 are connected between the 6th pin of the integrated circuit A1 and the ground. Among them, C1 is a polyester capacitor with a positive temperature coefficient capacitor; C2 is a polypropylene capacitor with a negative temperature coefficient capacitor.

[attach]389897 [/attach]

1. Circuit analysis

In the line scanning circuit of a television, since the capacity of the timing capacitor determines the oscillation frequency of the line oscillator, the capacity of the timing capacitor is required to be very stable and not change with the ambient temperature. Only in this way can the oscillation frequency of the line oscillator be stable. Therefore, capacitors with positive and negative temperature coefficients are connected in parallel for temperature compensation.

When the operating temperature rises, the capacity of C1 increases, while the capacity of C2 decreases. The total capacitance of the two capacitors in parallel is C=C1+C2. Since the capacity of one capacitor increases and the other decreases, the overall capacity remains basically unchanged. Similarly, when the temperature drops, the capacity of one capacitor decreases while the other increases. The total capacity remains basically unchanged, stabilizing the oscillation frequency and achieving the purpose of temperature compensation. 2. Circuit analysis details 1) In the oscillator circuit, the capacity of the timing capacitor determines the oscillation frequency. When the capacity of the timing capacitor changes due to temperature changes, the oscillation frequency of the oscillator will not be stable. 2) It is important to understand that capacitors made of different materials have different temperature characteristics. Only in this way can we analyze the circuit and understand why two small capacitors of equal capacity are connected in parallel. 3) Temperature complementarity is often encountered in circuit analysis. Not only do two capacitors with different temperature coefficients have temperature complementarity, but other electronic components also have temperature complementarity.