Low-Consumption Monostable Relay

Posted on Feb 4, 2013

A monostable relay has two states: operative when a large enough current flows through its coil and quiesc

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ent when no current flows. A relay contact that assumes a certain position after the supply voltage has been switched on is required in many applications. Of course, many relays operate in that manner. However, most of these relays require an energizing current of 50 mA or more and that normally precludes a battery supply. The circuit presented here, which uses a bistable relay, can solve that problem. The contact of a bistable relay normally remains in the position it is in after the supply is switched off. This circuit, however, makes the bistable relay behave like a monostable type, at a modest current. When the supply voltage is switched on, CI charges via Dl and the relay coil. The current then flowing through the coil causes the relay contact to assume one of two positions. The forward drop across Dl ensures that the base of Tl On this condition) is more positive than its emitter so that Tl, and thus T2, is switched off. When the supply voltage is switched off, the emitter of Tl is connected to the positive terminal of CI, while the base is connected to the negative terminal of the capacitor via Rl and the relay coil. This results in Tl, and thus T2, switching on so that CI discharges via T4 and the relay. The current flows through the relay coil, then flows in an opposite direction and this causes the contact to change over. The bistable relay thus behaves exactly as a monostable with the advantage, however, that the operational current is determined by Rl, which amounts to only 130 . To ensure reliable operation, the rating of the relay coil should be 65 to 75% of the supply voltage. In the prototype, a 9-V relay was used with a battery supply voltage of 12 V.

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