Analog Switch Lowers Relay Pow

Abstract: Design idea outlining a method to use an analog switch and discrete components to reduce the power dissipated in relay actuaTIon.

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. You can lower this dissipation by adding an analog switch that allows the relay to operate at a lower voltage (Figure 1).


Figure 1. Analog switch lowers relay power dissipation.

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 5V. Note that the voltage required to turn a relay on (pick-up voltage) is greater than that required to keep it on (dropout voltage). The relay shown has a 3.5V pick-up voltage and a 1.5V dropout voltage, yet the circuit allows it to operate from an intermediate supply voltage of 2.5V. Table 1 compares the relay's power dissipation with fixed operating voltages across it, and with the Figure 1 circuit in place.

Table 1. Power Dissipated By Relay
Voltage