[CIRCUIT IDEAS] DEEPAK GUPTA Latchable Relay Frontline Electronics, Pvt. Ltd., Salem Armature relays are made of coils and contacts. When the coil is energized, the induced magnetic field moves the armature, which opens or closes the contacts. There are 2 types of electromechanical relays: • latching • non-latching. A non-latching relay has an initial position of normally closed (NC) maintained by the force of a spring or permanent magnet while no current flows. The normally open (NO) contact is maintained by the force of a magnetic field while current flows through the coil. When the current stops, the relay reverts back to its initial NC position. A normal relay is "on" when and only when power is applies to the coil. It is "off" then no power is applied. Non-latching electromechanical relays are useful in control applications when the switch must return to a known state if power is lost. A latching relay can have 1 or 2 coils. Latching relays have no default position and remain in their last position when the drive current stops flowing. While the relays themselves may be latching, their reset position in a module is based on the control circuitry Latching relays are useful in applications where power consumption and dissipation must be limited because, once actuated, they require no current flow to maintain their position. A latching relay has two coils. Whey you first get the relay it may be ON 6 4 8 9 RLY1 11 R1 2.2k 13 +12V 1 2 RESET SET 15 C1 22uF 25V + - 16 A A K K D2 D1 + - S1 D1,D2: 1N4004 A K or OFF we don't know. Apply power to the on-coil for more than 25mS will cause the relay to be ON. It will stay ON for years with out power. To turn off the relay apply power to the off-coil for a short period of time and now the relay is OFF. Sending power to the off-coil again will not change anything. This circuit allows an SPST momentary pushbutton to act as a push-on push-off switch, using a DPDT latching (bi-stable) relay. It was originally intended to allow a single pushbutton switch on the dash of a vintage car to provide a latched function. The relay only draws current when it is being switched. At other times, the only current drain on the 12V supply is the leakage current of one 22µF capacitor, which is very low. It works as follows. Assume that initially the latching relay is in the reset state, with pins 4 and 6 connected together. In this state, C2 charges up to +12V via 2.2kO resistor R2 while capacitor C1 remains discharged as it is not connected to the 12V supply. If S1 is pressed, C2 discharges via the relay’s “set” coil, diode D2 and S1. This switches the relay into its set position, connecting pins 4 and 8. C1 then begins to charge via R1. While S1 is being held down, the relay does not return to the reset position because the current supplied via R1 is insufficient for the coil to latch the armature. As soon as S1 is released, current no longer flows though the coil so C1 can finish charging, ready for the next button CON1 press. Once the relay has switched and C1 has finished charging, pressing S1 again causes the relay to switch back to the reset R2 state via the same process. The unused set 2.2k of relay contacts can be used as an SPST or SPDT switch. The circuit as shown has been tested with the Jaycar SY4060 relay. It will C2 work with other DPDT twin-coil latching 22uF 25V relays but the resistor and capacitor values may need to be adjusted to suit. Relays with lower resistance coils will need larger value capacitors and smaller value resistors.