In general, drive circuits for latching relays are more complex than those for other relays. Typically, a half-bridge circuit is needed if dual supply voltages are available, and a full-bridge will be required if just one supply is available. Discrete circuits may be developed for the control of latching relays, but it is increasingly common to employ dedicated driver ICs. Whichever approach an engineer chooses, key design criteria are likely to include minimum circuit size, high-efficiency operation and the implementation of appropriate protection functionality.
Because the requirements for controlling latching relays – bridge circuitry, low-power operation, the need for protection, compact design – are similar to the drive requirements of brushed motors, it is also possible to deploy dedicated motor control driver (MCD) ICs in latching relay control circuitry. What’s more, because many smart meters have two latching relays, and because some motor drive ICs combine dual control with high levels of integrated protection functionality, this approach can help to simplify circuit design and reduce component count.
Take, for example, Toshiba’s TC78H610FNG. Supplied in a compact SSOP 16 package measuring just 5.0mm x 4.4mm, this direct driver IC incorporates two MOSFET-based H-bridges. As a result, it is ideally suited to driving the two single-coil latching relays found in modern smart meters. Electric current direction is easily controlled by the application of appropriate input signals to the IC.