Reducing Relay Power Consumption


Relays are often used as
electrically controlled switches. Unlike transistors, their switch
contacts are electrically isolated from the control input. On the other
hand, the power dissipation in a relay coil may be unattractive for
battery-operated applications. Adding an analogue switch lowers the
dissipation, allowing the relay to operate at a lower voltage. The
circuit diagram shows the principle. Power consumed by the relay coil
equals V2/RCOIL. The circuit lowers this dissipation (after actuation)
by applying less than the normal operating voltage of 5 V. Note that the
voltage required to turn a relay on (pickup voltage)is usually greater
than that to keep it on (dropout voltage).

In this respect the relay shown has specifications of 3.5 and 1.5 V
respectively, yet the circuit allows it to operate from an intermediate
supply voltage of 2.5 V. Table 1 compares the relay’s power dissipation
with fixed operating voltages across it, and with the circuit shown here
in place. The power savings are significant. When SW1 is closed, current
flows through the relay coil, and C1 and C2 begin to charge. The relay
remains inactive because the supply voltage is less than its pickup
voltage. The RC time constants are such that C1 charges almost
completely before the voltage across C2 reaches the logic threshold of
the analogue switch inside the MAX4624 IC.

When C2 reaches that threshold, the on-chip switch connects C1 in
series with the 2.5 V supply and the relay coil. This action causes the
relay to be turned on because its coil voltage is then raised to 5 V,
i.e., twice the supply voltage. As C1 discharges through the coil, the
coil voltage drops back to 2.5 V minus the drop across D1. However, the
relay remains on because the resultant voltage is still above the
dropout level (1.5 V). Component values for this circuit depend on the
relay characteristics and the supply voltage. The value of R1, which
protects the analogue switch from the initial current surge through C1,
should be sufficiently small to allow C1 to charge rapidly, but large
enough to prevent the surge current from exceeding the specified peak
current for the analogue switch.

The switch’s peak current (U1) is 400 mA, and the peak surge current is IPEAK = (VIN – VD1) / R1 + RON) where RON
is the on-resistance of the analogue switch (typically 1.2 Ω). The
value of C1 will depend on the relay characteristics and on the
difference between VIN and the pickup voltage.
Relays that need more turn-on time requires larger values for C1. The
values for R2 and C2 are selected to allow C1 to charge almost
completely before C2’s voltage reaches the logic threshold of the
analogue switch. In this case, the time constant R2C2 is about seven
times C1(R1 + RON). Larger time constants
increase the delay between switch closure and relay activation. The
switches in the MAX4624 are described as ‘guaranteed break before make’.
The opposite function, ‘make-before break’ is available from the
MAX4625. The full datasheets of these interesting ICs may be found at http://pdfserv.maxim-ic.com/arpdf/MAX4624-MAX4625.pdf


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