BISTABLE RELAY DRIVER （1）

Published:2011/8/11 21:17:00 Author:Phyllis From:SeekIC

By B. Chiron

Although bistable relays boast significant advantages, their use appears limited to niche applications, probably because their practical use requires just a tad more attention to design details than classic relays. Those days may be over soon with the introduction of a special driver IC from Quasinil (what’s in a name?).

It appears that most manufacturers of bistable relays have now succeeded in simplifying their products to the extent that a demagnetising circuit is no longer required, and a single switching signal does the job. In some cases it is possible to realize a driver circuit based on just a few discrete components. In this article we show the intricacies of bistable relay control, analyse a few simple circuits, mention the pros and cons and, finally, introduce the dedicated drivers type DRT5 and DRL5.

Pros and cons of bistable relays

Advantages

The main advantage of a bistable relay is evident from its name: as soon as the relay has switched, it remains in that position without consuming energy. This unique characteristic has quite a few consequences: no current consumption means less heat production, hence smaller heatsinks while the (rechargeable) battery sees its load current much reduced. In some cases, the use of a bistable relay allows a circuit to be simplified.

Disadvantages

- In general, the drive circuit is a little more complex than with a classic relay. Depending on the type of bistable relay, we need to either reverse the current Row or drive just one of the two coils. If we are looking at demagnetisation circuits, then it is also necessary to limit the current in compliance with the manufacturer’s specifications.

- To be able to benefit from the bistable character of the relay the driver needs to have a timing element in the form of an RC network or an IC with a few passive components around it.

- If we’re after energy reduction only, then the bistable character can work against us. To enable the relay to toggle when the supply voltage drops, a small amount of energy needs to be stored for use by the relay circuit watching the supply level.

Classes of bistable relay

Bistable relay with two coils

This is the most commonly found type. Each coil is responsible for a certain state of the relay. Some manufacturers indicate that series connection of the two coils allows the relay to be controlled as if it had just one coil.

Bistable relay with one coil

Less frequently seen, but very interesting if you are after significant energy reductions in your circuit. The entire coil is used to energize the contacts, as opposed to just half the coil length in the case of a 2-coil relay. With relays requiring a demagnetisation circuit, the demagnetisation has to be effected by means of a special resistor. In general, reversing the current in a coil is a bit more difficult. Further on in this article, a special circuit is shown for that purpose.

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