Relay Coil Energy Saver

Some relays will become
warm if they remain energized 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 I 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
energized 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.

Circuit diagram:

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.
Author: Myo Min – Copyright: Elektor July-August 2004

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