Bistable relays (BR) are widely used in battery powered equipments or applications with “memory function”. No doubt, when it comes to an average electronics hobbyist, bistable relay is a mysterious component. In principle, for remote and automatic controls there is often a need for switching contacts which have two stable contact positions, even in the defunct state. This requirement is fulfilled by bistable realys. The application of alternate control pulses to the relay coils cause the contacts to change from the one state to the other. If the supply is interrupted, the contacts remain in their previous position, even when the voltage is restored.
As the bistable (latching) relay maintains its state after being actuated, it has no default position and remains in its last position when the drive current stops flowing. A bistable relay can have one or two coils. In the single-coil type, the direction of current flow determines the position of the armature. In the dual-coil type, the coil in which the current flows determines the position of the armature.
Although are are available in a wide range of voltage and contact configuration, the 12-volt, dual-coil, 1C/O type is a very popular one in electronics hobby land. Since it is better to test the operation of such a BR before the actual construction of an electronics project, here is a move in that direction to help you to carry out a “pre-flight” test. The bistable realy tester circuit presented here is a portable, battery-operated unit, built around a handful of inexpensive discrete components.
As stated, it can be used as a semi-permanent memory because the contact position is retained when the power fails or is switched off. However, since only a short voltage pulse is needed to switch the relay to its desired position, a special drive circuitry is manadatory. Every dual-coil bistable relay has a set (S) and reset (R) winding. In the tester circuit, two differentiator networks (C2/R2 and C3/R5) have been used to generate necessary pulses for the windings of the BR under test (RUT). The relay can be tested with two button swiches (S1 and S2). The 12-volt type bistabe relays operate fine on 9 – volt (just above 70% of the maximum rated voltage). So a standard 9V battery pack is used here as the power source. This makes the unit nice and compact and also obviates the requirement of an external power supply. The visual indicators (LED1 and LED2) in the circuit can be used to test the transition of the BR under test.
The whole circuit can be easily assembled on a piece of prototyping board. Try to use a small circuit board so that it fits exactly inside a small prototype enclosure. For the test probes, it is possible to use standard IC test hooks (a total of 7 pieces) as shown here. Note that pin-header J1 in the circuit is reserved for relay coils , and J2 is for relay contacts.
It was observed that dual-coil bistable relays available now have two different coil configurations, ie, with 3-pin and 4-pin connections. In the 3-pin version, one connection (VCC) is common for both relay coils, and in the 4-pin version two independent coil connections (VCC x 2) are available. In the later type, both coils can be used as either set or reset coils, which is not possible with the 3-pin version.
Lab Note: Prototype was tested with a Tyco BR (www.te.com), lifted from a discarded electronic gadget. In India, bistable relays are available from Element 14 (http://in.element14.com)