Contact Protection and Arc Suppression Methods for Mechanical Relays
Mechanical relays are a very common and versatile method for switching a variety of electrical signals and power. The mechanical contacts in relays provide good isolation between the circuits activating them and the signals being switched, and can often handle large amounts of current. However, there are some effects related to mechanical contacts that must be considered when designing a system using relays, particularly when they are switching power.
(The contacts on manually operated mechanical switches experience these effects too, but since these switches are usually operated less frequently for uses such as switching equipment power on and off, these effects are usually less of a concern.)
Contact Wear, Arcing, and NoiseEvery time the contacts of a mechanical relay or switch are closed or open, there is a certain amount of wear. For a very brief time, only a small section of the contact is touching, and all of the current must go through this part of the contact. If the current being switched is large, part of the contact is degraded or destroyed. (The manufacturer of the component will include life expectancy information in their product specifications.)
Also, because there is a very small gap in the contacts for a brief time when the contacts are broken, an electrical arc may be generated across the gap if the voltage is high enough. This arc will produce a certain amount of radio frequency interference (RFI) through the air, and noise in the system power supply, and may affect nearby electronic circuits in a number of ways.
How much wear on the contacts and how much RFI and noise generated depends on:
Protection and Suppression CircuitryBecause of these effects it is sometimes desirable to include contact protection and arc suppression circuitry to relays, however, most companies that manufacture products that can use their relays in a variety of applications do not include this circuitry. Why? Well, unless it is known that the relays will be switching particular types of power signals, adding this circuitry would reduce the versatility of the product by preventing it from being used to switch other signals properly.
For this reason it is sometimes up to the system designer to add protection and suppression circuitry appropriate to the power being switched if the system experiences any problems due to the effects of this switching.
Example CircuitryThe figure below shows one of the most common ways of protecting relay contacts and suppressing noise and arcing caused by the contacts. Note that there are many ways of doing this, and that the circuit shown below is only one of many methods. In particular, RFI and power supply noise problems may be difficult to eliminate and may require some experimentation or the use of more advanced suppression circuitry.