RELAY PROTECTION

RELAY PROTECTION CIRCUITS

Sometime it is observed, that in some equipments relay(s) energised or remain in on state continuously, for example Time Delay circuit and Reverse Battery protection circuit.  The relay coil heats up and burn sometime.  These two circuits prevent relay from damage, specially if the voltage across the relay coil, crosses the specified limit for longer period.

Both circuits are very simple to understand.  First circuit (fig.1) is suitable for switching relay according to logic given at the base of transistor Q1.  As soon as high logic given to base of transistor Q1 the relay draws maximum current through capacitor C1.  As the capacitor is charged resistance R1 maintains the hold current for relay.  Relay required 25% less current to hold latch state .

Fig2 is a reverse battery protection circuit.  When connected to power the transistor Q1 gets base biasing voltage through capacitor C1 & resistor R2 and the relay is energised through collector-emitter path.  As the capacitor C1 is charged the transistor Q1 is cut off and relay gets hold current through resistor R1.