EN
We know that the contact protection of relay is more cruel than MOSFET, and the load of general relay is much larger than MOSFET.
Common DC heavy load DC motor, DC clutch and DC solenoid valve, these inductive load switches are closed, and the surge caused by the back electromotive force of hundreds or even thousands of volts will reduce or even completely damage the contact life. Of course, if the current is small, such as near 1a, the back electromotive force will cause arc discharge, which will cause metal oxide to pollute the contact, cause contact failure and increase contact resistance.
It should be mentioned here that the relay will always fail. The main purpose of our protection is to extend the service life of the relay, because the contact will always deposit carbon and age, and its surface is not as clean as it was originally. The contact resistance of the relay will increase rapidly at the end of its life.
Under normal temperature and pressure, the critical dielectric breakdown voltage in the air is 200-300v. Therefore, our goal is to control the voltage below 200V or less.
Figure 1:
We generally have the following centralized methods to suppress:
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Suppression method |
Circuit diagram |
Characteristic |
Selection of devices |
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Resistance capacitance |
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If the load and time are related, the initial leakage current may cause the load to misoperation . |
The function of the capacitor C is to suppress excessive voltage when the inductor discharges. The resistor R is used to measure the current when the power supply is started. Their values are determined by the test.The breakdown voltage of the capacitor is 200 ~ 300V . |
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If the load is a relay or a solenoid valve, the release time will be extended. When the contact power supply voltage is 24 or 48V ,Piezoresistors are effective,The voltage across the load is 100 to 200V . | ||
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Diode |
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The diode acts as a relay to disconnect the subsequent flow diode, which serves as a path for the coil to release energy and dissipate heat. Compared with RC circuit, it will significantly change the release time of relay (2 ~ 5 times). |
Selection of diode parameters: The reverse breakdown voltage is at least 10 times of the power supply voltage, and the forward current is at least equivalent to the load. When the voltage of the diode is not high, it can generally be used for about 2 reverse breakdown voltages and 3 times the power supply voltage. |
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Diode zener |
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It is very effective to prevent the diode from affecting the release time of the relay. |
The breakdown voltage of the zener tube should be consistent with the power supply voltage of the relay. |
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Varistor |
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Using the characteristics of voltage stabilization of varistors, this circuit can prevent the contact voltage from being too high, and it will slightly delay the release time of the relay. When the contact power supply voltage is 24 or 48V, the varistor is very effective, and the voltage across the load is 100 ~ 200V. |
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Standard diodes can significantly prolong the return time, and connecting conventional diodes in series with zener diodes will not affect the return time too much. If it is an inductive load, when the contacts are separated, the longer return time will prolong the arc generation time and shorten the contact life. For example, a relay with a diode connected to a coil takes 9.8ms to release the contact. Combining zener diode with small signal diode can shorten the time to 1.9ms. The return time of the relay with no diode connected to the coil is 1.5ms.
Although inductive load is more difficult to handle than resistive load, good protection will make the performance better.
In the actual circuit, there is a certain distance limit between the protection device (diode, resistor, capacitor, varistor, etc.) and the load. If the two are too far apart, the effect of the protective device may be weakened. Generally, the distance between the two should be within 50cm.
Switching at higher frequency under DC load will cause abnormal high corrosion (generation of electric spark)
When the DC solenoid valve or clutch is controlled at a high frequency, blue green corrosion may occur on the contacts. The reason for this is that when an electric spark (arc discharge) is generated, nitrogen and oxygen react in the air.
