Efficient way of driving latching relays

If you need to keep relay turned on or off for a long time, then standard relays may not be best solution. First of all in order to keep relay switched, power must be applied. If you are using mechanical relay, then coil must be constantly energized. This way relay draws power and for battery operated equipment this is not acceptable. SSR relays might be a better solution, but signal still need to be applied in order to keep it turned on. In such cases there are latching relays used. These relays keep their state after switched and power is removed. They are common where relay need to be on or off for long time.

Controlling such relays is a bit trickier than regular ones. First of all relay can be turned on and of by changing current direction on the coil. One of solution could be H-bridge as for DC motor. But this is not very efficient and economical solution because relay doesn’t require constant current flow. It works with pulses. Normally there are special IC’s for controlling latching relays that work in push-pull mode where charge is directed from external capacitor from or towards coil. Kerry D. Wong demonstrates how one of four half-bridges of SN754410 can be used to drive such relay. So with single H-bridge IC you can drive four latching relays.

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