Suppressing Relay Coil Transients
When a 28 Vdc relay coil is turned off, the inductive energy stored in it can create surge voltages to 1500 volts on a DC power line. With the increased use of solid state devices which are sensitive to spikes, relay coils must be suppressed to limit voltage spikes to a maximum of 50 to 80 volts.
The measure of successful coil suppression depends on the degree to which the method affects the operation of the relay. Improper or excessive suppression can cause the relay to suffer from a long release time, slow contact transfer, and contact bounce on break. All of these conditions will increase contact arcing when load switching, which will reduce relay life dramatically.
There are a number of common ways for a relay user to suppress relay coil transients and each has advantages and disadvantages. However, the most widely used methods utilize zener-diode and/or zener-zener combinations. These combinations are compact, provide excellent suppression, and do not affect relay release-time or contact life. Figure 48 illustrates some of the more common configurations.