Some relays will become warm if they remain energised for some time. The circuit shown here will actuate the relay as before but then reduce the ‘hold’ current through the relay coil current by about 50%, thus considerably reducing the amount of heat dissipation and wasted power. The circuit is only suitable for relays that remain on for long periods. The following equations will enable the circuit to be dimensioned for the relay on hand:
R3 = 0.7 / I
Charge time = 0.5 × R2 × C1
Where is the relay coil current. After the relay has been switched off, a short delay should be allowed for the relay current to return to maximum so the relay can be energised again at full power. To make the delay as short as possible, keep C1 as small as possible.
In practice, a minimum delay of about 5 seconds should be allowed but this is open to experimentation. The action of C2 causes the full supply voltage to appear briefly across the relay coil, which helps to activate the relay as fast as possible. Via T2, a delay network consisting of C1 and R2 controls the relay coil current flowing through T1 and R3, effectively reducing it to half the ‘pull in’ current. Diode D2 discharges C1 when the control voltage is Low. Around one second will be needed to completely discharge C1. T2 shunts the bias current of T1 when the delay has elapsed. Diode D1 helps to discharge C1 as quickly as possible. The relay shown in the circuit was specified at 12 V / 400 ohms. All component values for guidance only.