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Hints and Tips - Electical Latching Circuit

Electical Latching Circuit

Electic Latching circuits are a common and useful way of switching circuits on and off with momentary Pushbuttons. Used in Equipment such as Motor Starters, they
have the beneficial safety function in that they drop out if the supply is lost and will not re-energise until the start button is pressed again.

Common types of Relay

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Using the Circuit Design below it is possible to create a latching relay using a bog standard DPDT or DPST Relay.
The user can press a button to close the relay, and press a second button to open the relay again.
The relay remembers which button was pressed last (see description below).

 

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The diagram above shows how such a relay can be used to make a latching relay circuit. The circuit has two buttons – one is a push to make which closes when pressed but is otherwise open, and the other is a push to break which opens when pressed but is otherwise closed. When the push to make button is pressed, 12V goes across the coil energising the relay. This closes the two internal relay switches connecting A to B, and C to D. With A connected to B, the positive from the 12V power source feeds through A and B to arrive at the relay coil keeping it energised even when the push to make button is released. Therefore the relay has been latched.
In order to de-energise the relay, the push to break button needs to be pressed which cuts the positive connection feeding through A and B and energising the coil. With that button pressed, there is no voltage across the relay coil and so it de-energises. While the relay is latched, C and D are connected and therefore can be used to switch on whatever device or appliance is to be controlled by this latching relay system.

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