Circuit to protect slow acting sensor relay contact - uses two auxiliary relays giving time delay to cover sensor stabilising time

摘要

The application is to circuits containing a switching element with a creeping contact, or one which does not immediately take up its final position, e.g. a thermal relay or float switch. The addition of a pair of auxiliary relays (1, 2) one with a timer (2) and and make before break contact (2a), ensures that the primary element (3) contact (3a) does not have to make and break current whilst it is in an undefined state. - In a thermal application the temperature sensor (3) closes in a cool state, the auxiliary relay (1) picks up and switches on the heater (4) through one contact (1b). Another contact (1a) transiently energises the time delay relay (2). The contact (2a) of the later moves to bridge, and finally change, the supply point to the auxiliary relay. The time delay relay then drops off and its contact reverts to the original position to keep the heater circuit closed.