ABSTRACT
This study gives details on how to make relays operate faster than at first appears to be possible. This is done by the simple and almost universal practice of protecting relay driver transistors from damage at turn-off by putting a reverse-biased diode across the relay coil slows down its opening drastically, as it permits current to keep circulating and hold the relay in. The main dynamic parameters of a relay are the pull-in and drop-out time. The delay required at amplifier turn-on depends on the amplifier characteristics. The relay drop-out must be as fast as possible. If a relay is powered directly from the supply rails, then it will drop out eventually as the rails collapse. Detecting the loss of DC supply technique involves a subsidiary supply rail with a small reservoir capacitor. Mechanical detection is a mains switch that closes or opens a control circuit before the mains power contacts are opened.
