These relays contact protection and contact matters, everyone knows?

We know that the contact protection of relays is more cruel than Mosfet, and the load of ordinary relays is much larger than Mosfet. Typical DC large capacity DC motor, DC clutch and DC solenoid valve. These inductive load switches turn off, and surges due to back EMF of hundreds or thousands of volts may shorten or completely damage the contact life.

Of course, if the current is small (for example, near 1A), the back electromotive force will produce arc discharge, and the discharge will cause the contact to be contaminated by metal oxide, which will cause the contact to be damaged and the contact resistance will increase. It is pointed out here that the relay will fail sooner or later. We mainly protect the contacts to extend the use time of the relay, because the contacts will always be carbon deposits, aging, and the surface is not as clean as it was.

The contact resistance increases rapidly as the relay approaches the later stages of its life. Under normal room temperature and pressure, the core dielectric breakdown voltage of air is 200~300V. Therefore, our goal is usually to control the voltage at a voltage below 200V.

We generally have the following centralized methods to suppress:

1. Relay contact protection circuit-relay series RC circuit

2. Relay contact protection circuit-relay parallel RC circuit

3. Relay contact protection circuit-relay parallel diode circuit

Standard diodes greatly extend the return time, and connecting ordinary diodes and Zener diodes does not have much effect on the return time. For inductive loads, when the contacts separate, the arc generation time will be extended and the contact life will be shortened. For example, a relay with a diode connected to one coil takes 9.8 milliseconds to release the contact. If you combine a Zener diode with a small-signal diode, the time is reduced to 1.9 milliseconds. The return time of the relay with the coil not connected to the diode is 1.5 milliseconds.

Electrical relay contact protection circuit-relay series RC circuit: electrical contact protection