Over-voltage and under-voltage protection circuit for single-phase alternating-current power supply

Abstract

The utility model relates to a single-phase alternating-current power supply overvoltage and undervoltage protection circuit which comprises a first driving module, a first relay, a first switch piece, a second driving module, a second relay and a second switch piece, the two ends of the first driving module are connected with a live wire and a zero line respectively, and the output end of the first driving module is connected with the control end of the first switch piece. One end of the first relay is connected with the live wire, the other end of the first relay is connected with the input end of the first switch piece, the two ends of the second driving module are connected with the live wire and the zero line respectively, and the output end of the second driving module is connected with the control end of the second switch piece. The other end of the second relay is connected with the input end of the second switch piece, the output end of the first switch piece and the output end of the second switch piece are connected with a null line, and a normally-open contact of the first relay and a normally-closed contact of the second relay are connected in series in a load and a loop. According to the invention, the safety and stability of the circuit are improved.

Description

Technical Field

[0001] This application relates to the field of circuit protection, and in particular to a single-phase AC power supply overvoltage and undervoltage protection circuit. Background Technology

[0002] In power systems, AC power supplies provide power to loads, and the stability of the power supply voltage is crucial for the normal operation of the load equipment. However, due to grid fluctuations, equipment failures, and other reasons, the power supply voltage may become too high or too low, which can damage the load equipment and even cause safety accidents. Therefore, a protective circuit that can automatically disconnect the power supply to the load when the power supply voltage is abnormal is needed. Utility Model Content

[0003] To improve the safety and stability of the circuit, this application provides a single-phase AC power supply overvoltage and undervoltage protection circuit.

[0004] This application provides a single-phase AC power supply overvoltage and undervoltage protection circuit, which adopts the following technical solution:

[0005] A single-phase AC power supply overvoltage and undervoltage protection circuit includes a first drive module, a first relay, a first switch, a second drive module, a second relay, and a second switch. The live wire and neutral wire terminals of the first drive module are respectively connected to the live wire and neutral wire of the AC power supply. The signal output terminal of the first drive module is connected to the control terminal of the first switch. One end of the coil of the first relay is connected to the live wire of the AC power supply, and the other end of the coil is connected to the input terminal of the first switch. The output terminal of the first switch is connected to the neutral wire of the AC power supply. The live wire and neutral wire terminals of the second drive module are respectively connected to the AC power supply. The live and neutral wires of the AC power supply are connected. The signal output terminal of the second drive module is connected to the control terminal of the second switch. One end of the coil of the second relay is connected to the live wire of the AC power supply, and the other end of the coil of the second relay is connected to the input terminal of the second switch. The output terminal of the second switch is connected to the neutral wire of the AC power supply. The normally open contact of the first relay and the normally closed contact of the second relay are connected in series in the circuit between the load and the AC power supply. The first drive module is used to control the first switch to conduct when the voltage is higher than a first set value, and the second drive module is used to control the second switch to conduct when the voltage is higher than a second set value.

[0006] By adopting the above technical solution, the first setting value is the minimum value of the normal operating voltage of the load, and the second setting value is the maximum value of the normal operating voltage of the load. When the circuit is within the normal operating range, the first drive module controls the first switch to conduct, so that the coil of the first relay is energized and closes, and its normally open contact closes, so that the load is energized and works. At this time, the second drive module controls the second switch to cut off, so that the coil of the second relay is not energized and closes, and its normally closed contact does not open, ensuring normal power supply to the load. When the line voltage is too low, the first drive module controls the first switch to cut off, so that the coil of the first relay is not energized and releases, and its normally open contact opens, so that the load cannot be energized and works, achieving the purpose of undervoltage protection. When the circuit is working normally, if the line voltage suddenly rises, the second drive module controls the coil of the second relay to energize and close, and its normally closed contact opens, cutting off the power supply to the load, and the load stops working, achieving the purpose of overvoltage protection, thereby improving the safety and stability of the circuit.

[0007] Preferably, the first driving module includes a first resistor, a first potentiometer, and a first bidirectional trigger diode. One end of the first resistor is connected to the live wire of the AC power supply, the other end of the first resistor is connected to a fixed terminal of the first potentiometer, the other fixed terminal of the first potentiometer is connected to the neutral wire of the AC power supply, the sliding terminal of the first potentiometer is connected to one end of the first bidirectional trigger diode, and the other end of the first bidirectional trigger diode is connected to the control terminal of the first switching element.

[0008] By adopting the above technical solution, when the circuit is within the normal operating range, the first potentiometer is adjusted to turn on the first bidirectional trigger diode. The signal voltage of the diode triggers the first switch, causing it to turn on and energize the first relay coil. Its normally open contact closes at this time, allowing the load to operate. When the voltage is lower than the preset first set value, the voltage obtained by the voltage division of the first resistor and the first potentiometer decreases, the first bidirectional trigger diode turns off, thereby controlling the first switch to turn off, and its normally open contact opens at this time, preventing the load from operating and achieving the purpose of undervoltage protection. The first bidirectional trigger diode only turns on after a trigger signal is applied. Once it turns on, it will remain on until the current drops to a certain level (called the holding current) before it stops turning on.

[0009] Preferably, the second driving module includes a fourth resistor, a second potentiometer, and a second bidirectional trigger diode. One end of the fourth resistor is connected to the live wire of the AC power supply, and the other end of the fourth resistor is connected to a fixed terminal of the second potentiometer. The other fixed terminal of the second potentiometer is connected to the neutral wire of the AC power supply. The sliding terminal of the second potentiometer is connected to one end of the second bidirectional trigger diode, and the other end of the second bidirectional trigger diode is connected to the control terminal of the second switching device.

[0010] By adopting the above technical solution, when the circuit is working normally, if the line voltage suddenly rises, the voltage obtained by voltage division through the fourth resistor and the second potentiometer reaches the turn-on voltage of the second bidirectional trigger diode. The second bidirectional trigger diode turns on, the second switch turns on, the coil of the second relay is energized and closes, its normally closed contact opens, the power supply to the load is cut off, and RL stops working, thereby achieving the purpose of overvoltage protection.

[0011] Preferably, the first switching element includes a first bidirectional thyristor, the control electrode of the first bidirectional thyristor is connected to the end of the first bidirectional trigger diode away from the potentiometer, the end of the first relay coil away from the live wire is connected to the anode of the first bidirectional thyristor, and the cathode of the first bidirectional thyristor is used to connect to the neutral wire of the AC power supply.

[0012] By employing the above technical solution, the bidirectional thyristor responds to voltage control signals extremely quickly, completing turn-on or turn-off within microseconds. This rapid response ensures that the overcurrent protection circuit can respond promptly to current anomalies, protecting the load equipment.

[0013] Preferably, the second switching element includes a second bidirectional thyristor, the control electrode of the second bidirectional thyristor is connected to the end of the second bidirectional trigger diode away from the potentiometer, the end of the second relay coil away from the live wire is connected to the anode of the second bidirectional thyristor, and the cathode of the second bidirectional thyristor is used to connect to the neutral wire of the AC power supply.

[0014] By adopting the above technical solutions, bidirectional thyristors can efficiently control current in AC power supplies, making them suitable for switching and regulation functions and enhancing circuit reliability. When used in conjunction with bidirectional trigger diodes, they can accurately respond to voltage changes, thereby providing stable overvoltage and undervoltage protection. They also have high withstand voltage and current capabilities, making them suitable for handling large load currents and extending the service life of the circuit.

[0015] Preferably, it further includes a first indicator light and a first crystal diode. One end of the normally closed contact of the first relay is used to connect to the live wire of the AC power supply, and the other end of the normally closed contact of the first relay is connected to the cathode of the first indicator light. The anode of the first indicator light is used to connect to the neutral wire of the AC power supply. The anode of the first crystal diode is connected to the cathode of the first indicator light, and the cathode of the first crystal diode is connected to the anode of the first indicator light.

[0016] By adopting the above technical solution, the first indicator light can enable the staff to understand the status of the first relay. When the first relay is energized and engaged, the first indicator light goes out. When the first relay is de-energized and released, the first indicator light illuminates, thereby indicating whether the circuit is undervoltage. In addition, the first crystal diode is used to protect the first indicator light, preventing reverse current from damaging it and improving the reliability and stability of the indicator light.

[0017] Preferably, it also includes a second indicator light and a second crystal diode. One end of the normally open contact of the second relay is used to connect to the live wire of the AC power supply, and the other end of the normally open contact of the second relay is connected to the cathode of the second indicator light. The anode of the second indicator light is used to connect to the neutral wire of the AC power supply. The anode of the second crystal diode is connected to the cathode of the second indicator light, and the cathode of the second crystal diode is connected to the anode of the second indicator light.

[0018] By adopting the above technical solution, the second indicator light can enable the staff to understand the status of the second relay. When the second relay is energized and engaged, the second indicator light will light up. When the first relay is de-energized and released, the first indicator light will turn off, thereby indicating whether the voltage of the line is overvoltage.

[0019] Preferably, it further includes a first capacitor and a second capacitor. One end of the first capacitor is connected between the sliding terminal of the first potentiometer and one end of the first bidirectional diode, and the other end of the first capacitor is used to connect to the neutral wire of the AC power supply. One end of the second capacitor is connected between the coil of the first relay and the first bidirectional thyristor, and the other end of the second capacitor is used to connect to the neutral wire of the AC power supply.

[0020] By adopting the above technical solution, the first capacitor is used to reduce signal noise and interference; the second capacitor is used for filtering.

[0021] Preferably, the device further includes a third capacitor and a fourth capacitor. One end of the third capacitor is connected between the sliding terminal of the second potentiometer and one end of the second bidirectional diode, and the other end of the third capacitor is used to connect to the neutral wire of the AC power supply. One end of the fourth capacitor is connected between the coil of the second relay and the second bidirectional thyristor, and the other end of the fourth capacitor is used to connect to the neutral wire of the AC power supply.

[0022] By adopting the above technical solution, the third capacitor is used to reduce signal noise and interference; the fourth capacitor is used for filtering.

[0023] In summary, this application includes at least one of the following beneficial technical effects:

[0024] 1. The first setting value is the minimum value of the load's normal operating voltage, and the second setting value is the maximum value of the load's normal operating voltage. When the circuit is within the normal operating range, the first drive module controls the first switch to conduct, energizing the first relay coil and closing its normally open contact, thus energizing the load. At this time, the second drive module cuts off the second switch, preventing the second relay coil from being energized and keeping its normally closed contact open, ensuring normal power supply to the load. When the line voltage is too low, the first drive module controls the first switch to cut off, de-energizing and releasing the first relay coil, and opening its normally open contact, preventing the load from being energized and achieving undervoltage protection. When the line voltage suddenly rises during normal circuit operation, the second drive module controls the second relay coil to energize and open its normally closed contact, cutting off the power supply to the load and stopping its operation, thus achieving overvoltage protection and improving the safety and stability of the circuit.

[0025] 2. When the circuit is within the normal operating range, the first potentiometer is adjusted to turn on the first bidirectional trigger diode. The signal voltage triggers the first switching transistor, causing it to turn on and energize the first relay coil. Its normally open contact closes at this time, allowing the load to operate. When the voltage is lower than the preset first set value, the voltage obtained by the voltage division through the first resistor and the first potentiometer decreases, causing the first bidirectional trigger diode to turn off, thereby controlling the first switching device to turn off. This causes its normally open contact to open, preventing the load from operating and achieving the purpose of undervoltage protection. The first bidirectional trigger diode only turns on after a trigger signal is applied. Once turned on, it remains in the conducting state until the current drops to a certain level (called the holding current) before it stops conducting.

[0026] 3. When the circuit is working normally, if the line voltage suddenly rises, the voltage obtained by voltage division by the fourth resistor and the second potentiometer, and after being charged by the third capacitor, reaches the turn-on voltage of the second bidirectional trigger diode. The second bidirectional trigger diode turns on, the second switch turns on, the coil of the second relay is energized and closes, its normally closed contact opens, the power supply to the load is cut off, and RL stops working, thereby achieving the purpose of overvoltage protection. Attached Figure Description

[0027] Figure 1 This is the overall circuit diagram of the single-phase AC power overvoltage and undervoltage protection circuit according to an embodiment of this application.

[0028] Explanation of reference numerals in the attached drawings: 1. First drive module; 2. Second drive module. Detailed Implementation

[0029] The present application will be further described in detail below with reference to all the accompanying drawings.

[0030] This application discloses an overvoltage and undervoltage protection circuit for a single-phase AC power supply. (Refer to...) Figure 1 A single-phase AC power overvoltage and undervoltage protection circuit includes a first drive module 1, a first relay KA1, a first switch, a second drive module 2, a second relay KA2, a second switch, a first indicator light VL1, a first crystal diode VD2, a second indicator light VL2, a second crystal diode VD4, a first capacitor C1, a second capacitor C2, a third capacitor C3, and a fourth capacitor C4. The first drive module 1 includes a first resistor R1, a first potentiometer RP1, and a first bidirectional trigger diode VD1. The first switch includes a first bidirectional thyristor VS1. The second drive module 2 includes a fourth resistor R4, a second potentiometer RP2, and a second bidirectional trigger diode VD3. The second switch includes a second bidirectional thyristor VS2. The first relay KA1 includes a normally closed contact KA1-1 and a normally open contact KA1-2. The second relay KA2 includes a normally closed contact KA2-2 and a normally open contact KA2-1.

[0031] The 220V AC power supply powers the load RL, which includes the live wire L and the neutral wire N.

[0032] One end of the first resistor is connected to the live wire of the AC power supply, and the other end of the first resistor is connected to one fixed terminal of the first potentiometer. The other fixed terminal of the first potentiometer is connected to the neutral wire of the AC power supply. The sliding terminal of the first potentiometer is connected to one end of the first bidirectional trigger diode. The control electrode of the first bidirectional thyristor is connected to the end of the first bidirectional trigger diode away from the potentiometer. One end of the coil of the first relay is connected to the live wire, and the other end of the coil of the first relay is connected to the anode of the first bidirectional thyristor. The cathode of the first bidirectional thyristor is used to connect to the neutral wire of the AC power supply. One end of the normally open contact of the first relay is connected to the live wire, and the other end of the normally open contact of the first relay is connected to one end of the normally closed contact of the second relay. The other end of the normally closed contact of the second relay is connected to the positive terminal of the load, and the negative terminal of the load is connected to the neutral wire. The second set value can be obtained by the user through multiple experiments. It is the voltage value at which the first bidirectional trigger diode conducts and drives the first bidirectional thyristor to conduct when the voltage is the lowest value in the normal operating voltage range of the load. The first bidirectional trigger diode conducts when the voltage is higher than the first set value. At this time, the first bidirectional thyristor conducts, so that the coil of the first relay is energized and attracted. Additionally, one end of the first capacitor is connected between the sliding terminal of the first potentiometer and one end of the first bidirectional diode, and the other end of the first capacitor is used to connect to the neutral wire of the AC power supply. One end of the second capacitor is connected between the coil of the first relay and the first bidirectional thyristor, and the other end of the second capacitor is connected to a second resistor, the other end of which is connected to the neutral wire of the AC power supply. The first capacitor is used to smooth the voltage fluctuation of the sliding terminal of the potentiometer, reducing signal noise and interference; the second capacitor is used for filtering, reducing the current fluctuation in the relay coil, preventing electromagnetic interference from affecting the operation of the first bidirectional thyristor, and improving the overall stability and reliability of the circuit.

[0033] One end of the fourth resistor is connected to the live wire of the AC power supply, and the other end of the fourth resistor is connected to one fixed terminal of the second potentiometer. The other fixed terminal of the second potentiometer is connected to the neutral wire of the AC power supply. The sliding terminal of the second potentiometer is connected to one end of the second bidirectional trigger diode. The control electrode of the second bidirectional thyristor is connected to the other end of the second bidirectional trigger diode. One end of the second relay coil is connected to the live wire, and the other end of the second relay coil is connected to the anode of the second bidirectional thyristor. The cathode of the second bidirectional thyristor is connected to the neutral wire of the AC power supply. The second set value can be obtained by the user through multiple experiments. It is the voltage value at which the second bidirectional trigger diode conducts and drives the second bidirectional thyristor to conduct when the voltage is higher than the second set value. At this time, the second bidirectional thyristor conducts, so that the second relay coil is energized and attracted. One end of the third capacitor is connected between the sliding terminal of the second potentiometer and one end of the second bidirectional diode. The other end of the third capacitor is connected to the neutral wire of the AC power supply. One end of the fourth capacitor is connected between the coil of the second relay and the second bidirectional thyristor. The other end of the fourth capacitor is connected to the fifth resistor, and the other end of the fifth resistor is connected to the neutral wire of the AC power supply. When the circuit is working normally, the line voltage suddenly rises. The voltage obtained by voltage division by the fourth resistor and the second potentiometer, and after being charged by the third capacitor, reaches the turn-on voltage of the second bidirectional trigger diode. The second bidirectional trigger diode turns on, and the second switching device turns on.

[0034] Finally, both the first and second indicator lights use light-emitting diodes. One end of the normally closed contact of the first relay is connected to the live wire of the AC power supply, and the other end of the normally closed contact of the first relay is connected to a third resistor. The other end of the third resistor is connected to the cathode of the first indicator light. The anode of the first indicator light is used to connect to the neutral wire of the AC power supply. The first crystal diode is connected in parallel with the first indicator light, and the anode of the first crystal diode is connected to the cathode of the first indicator light. The cathode of the first crystal diode is connected to the anode of the first indicator light.

[0035] One end of the normally open contact of the second relay is connected to the live wire of the AC power supply, and the other end of the normally open contact of the second relay is connected to a sixth resistor. The other end of the sixth resistor is connected to the cathode of the second indicator light. The anode of the second indicator light is connected to the neutral wire of the AC power supply. The anode of the second crystal diode is connected to the cathode of the second indicator light, and the cathode of the second crystal diode is connected to the anode of the second indicator light. Operators can visually determine whether the current circuit voltage is undervoltage or overvoltage by observing the first and second indicator lights.

[0036] The implementation principle of a single-phase AC power supply overvoltage and undervoltage protection circuit according to an embodiment of this application is as follows: When the circuit is within the normal operating range, the first potentiometer RP1 is adjusted to turn on the first bidirectional trigger diode VD1. The signal voltage of the turn-on diode triggers the first bidirectional thyristor VS1, causing it to conduct. The first relay KA1 is energized and its normally open contact KA1-2 closes, energizing the load RL. At this time, the second potentiometer RP2 is adjusted to de-energize the second bidirectional trigger diode VD3 and the second bidirectional thyristor VS2. The second relay KA2 is not energized and its normally closed contact KA2-2 remains open, ensuring normal power supply to the load R.

[0037] When the line voltage is too low, the voltage obtained by voltage division through the first resistor R1 and the first potentiometer RP1 also decreases. After the first capacitor C1 is charged, it cannot reach the break-off voltage of the first bidirectional trigger diode VD1. The first bidirectional trigger diode VD1 is cut off, the first bidirectional thyristor VS1 is cut off, the first relay KA1 is not energized and is released, its normally open contact KA1-2 is opened, and the load RL cannot be energized to work, thus achieving the purpose of undervoltage protection.

[0038] When the circuit is working normally, if the line voltage suddenly rises, the voltage obtained by the voltage division of the fourth resistor R4 and the second potentiometer RP2, and the third capacitor C3, reaches the turn-on voltage of the second bidirectional trigger diode VD3. The second bidirectional trigger diode VD3 turns on, the second bidirectional thyristor VS2 turns on, the second relay KA2 is energized and closes, its normally closed contact KA2-2 opens, cutting off the power supply to the load RL, and the load RL stops working, thereby achieving the purpose of overvoltage protection.

[0039] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A single-phase AC power supply overvoltage and undervoltage protection circuit, characterized in that: The system includes a first drive module (1), a first relay, a first switch, a second drive module (2), a second relay, and a second switch. The live wire and neutral wire of the first drive module (1) are respectively connected to the live wire and neutral wire of the AC power supply. The signal output terminal of the first drive module (1) is connected to the control terminal of the first switch. One end of the coil of the first relay is connected to the live wire of the AC power supply, and the other end of the coil of the first relay is connected to the input terminal of the first switch. The output terminal of the first switch is connected to the neutral wire of the AC power supply. The live wire and neutral wire of the second drive module (2) are respectively connected to the live wire and neutral wire of the AC power supply. The first relay is connected to the live wire and the neutral wire. The signal output terminal of the second drive module (2) is connected to the control terminal of the second switch. One end of the second relay coil is connected to the live wire of the AC power supply. The other end of the second relay coil is connected to the input terminal of the second switch. The output terminal of the second switch is connected to the neutral wire of the AC power supply. The normally open contact of the first relay and the normally closed contact of the second relay are connected in series in the circuit between the load and the AC power supply. The first drive module (1) is used to control the first switch to conduct when the voltage is higher than the first set value. The second drive module (2) is used to control the second switch to conduct when the voltage is higher than the second set value.

2. The single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 1, characterized in that: The first driving module (1) includes a first resistor, a first potentiometer and a first bidirectional trigger diode. One end of the first resistor is connected to the live wire of the AC power supply. The other end of the first resistor is connected to a fixed end of the first potentiometer. The other fixed end of the first potentiometer is connected to the neutral wire of the AC power supply. The sliding end of the first potentiometer is connected to one end of the first bidirectional trigger diode. The other end of the first bidirectional trigger diode is connected to the control end of the first switch.

3. The single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 2, characterized in that: The second driving module (2) includes a fourth resistor, a second potentiometer, and a second bidirectional trigger diode. One end of the fourth resistor is connected to the live wire of the AC power supply, and the other end of the fourth resistor is connected to one fixed end of the second potentiometer. The other fixed end of the second potentiometer is connected to the neutral wire of the AC power supply. The sliding end of the second potentiometer is connected to one end of the second bidirectional trigger diode, and the other end of the second bidirectional trigger diode is connected to the control end of the second switch.

4. The single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 3, characterized in that: The first switching device includes a first bidirectional thyristor, the control electrode of the first bidirectional thyristor is connected to the end of the first bidirectional trigger diode away from the potentiometer, the end of the first relay coil away from the live wire is connected to the anode of the first bidirectional thyristor, and the cathode of the first bidirectional thyristor is used to connect to the neutral wire of the AC power supply.

5. A single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 4, characterized in that: The second switching device includes a second bidirectional thyristor. The control electrode of the second bidirectional thyristor is connected to the end of the second bidirectional trigger diode away from the potentiometer. The end of the second relay coil away from the live wire is connected to the anode of the second bidirectional thyristor. The cathode of the second bidirectional thyristor is used to connect to the neutral wire of the AC power supply.

6. The single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 1, characterized in that: It also includes a first indicator light and a first crystal diode. One end of the normally closed contact of the first relay is used to connect to the live wire of the AC power supply, and the other end of the normally closed contact of the first relay is connected to the cathode of the first indicator light. The anode of the first indicator light is used to connect to the neutral wire of the AC power supply. The anode of the first crystal diode is connected to the cathode of the first indicator light, and the cathode of the first crystal diode is connected to the anode of the first indicator light.

7. The single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 1, characterized in that: It also includes a second indicator light and a second crystal diode. One end of the normally open contact of the second relay is used to connect to the live wire of the AC power supply, and the other end of the normally open contact of the second relay is connected to the cathode of the second indicator light. The anode of the second indicator light is used to connect to the neutral wire of the AC power supply. The anode of the second crystal diode is connected to the cathode of the second indicator light, and the cathode of the second crystal diode is connected to the anode of the second indicator light.

8. A single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 5, characterized in that: It also includes a first capacitor and a second capacitor. One end of the first capacitor is connected between the sliding terminal of the first potentiometer and one end of the first bidirectional diode. The other end of the first capacitor is used to connect to the neutral wire of the AC power supply. One end of the second capacitor is connected between the coil of the first relay and the first bidirectional thyristor. The other end of the second capacitor is used to connect to the neutral wire of the AC power supply.

9. A single-phase AC power supply overvoltage and undervoltage protection circuit according to claim 5, characterized in that: It also includes a third capacitor and a fourth capacitor. One end of the third capacitor is connected between the sliding terminal of the second potentiometer and one end of the second bidirectional diode. The other end of the third capacitor is used to connect to the neutral wire of the AC power supply. One end of the fourth capacitor is connected between the coil of the second relay and the second bidirectional thyristor. The other end of the fourth capacitor is used to connect to the neutral wire of the AC power supply.

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