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Latching Relay Driver for 12V Load

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In many electronic setups, you need to control high-current loads with low-power signals. While a high-current drive switch can achieve this, it is often accomplished using a latching relay mechanism controlled by push buttons to switch a 12V 10A load ON and OFF even after you remove the control signal.

Components List

The following components are required to make this project.

Designation Parts Quantity
RLY1 12V Relay X1
Q1 2N3904 NPN Transistor X1
R1 4.7kΩ 1/4W Resistor X1
R2, R3 2.2kΩ 1/4W Resistor X2
SW1, SW2 Push-Button X2
D1, D2 1N4002 Diode X2

Circuit Diagram

The schematic of a latching relay driver circuit is shown below.

Circuit Explanation

See Also: Transistor Relay Driver Circuit.

When push button (SW1) is pressed, the input signal is applied to the base of the NPN transistor (Q1) through a current limit resistor (R1). This turns on Q1, allowing current to flow from the collector to the emitter, and energizes the relay coil (RLY1), switching the common terminal (COM) from the NC contact to the NO contact. The load get the +12V supply through the NO contact of the relay.

The relay remains energized even after the push button (SW1) button is released because the input signal get the path through the resistor (R3) to base of the transistor, and the latched state is maintained.

When the push button (SW2) is pressed, it closes the circuit to ground, bypassing the base current of Q1 through SW2. This action stops the current flow through the relay coil, de-energizing it and switching the common terminal (COM) back to the NC contact. The load is disconnected from the +12V supply as a result.

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