- Latching Relay Definition: A latching relay is a type of relay that maintains its contact position without continuous power application, allowing for efficient control of circuits.
- Circuit Diagram: The latching relay circuit diagram shows how Button-1 and Button-2 control the energizing and de-energizing of the relay, respectively.
- Operation Mechanism: By pressing Button-1, the relay is energized and remains so even after the button is released, until Button-2 is pressed.
- Efficiency and Use: Latching relays are energy efficient as they require power only for changing states, not for maintaining them.
- Practical Application: They are commonly used in setups requiring the circuit to remain active without continuous power, such as home lighting systems and industrial conveyors.
What is a Latching Relay?
A latching relay is a two-position electromechanical switch. A magnetic or mechanical latching type holds its contacts after a short coil pulse, so the coil does not need power to stay in that state. A standard relay wired with a holding contact is also called a latching circuit, even though that coil stays powered while the contacts remain closed.
Use either form when a small coil current must steer a larger load current. A magnetic latching coil draws power only during the set or reset pulse. The latching relay circuit diagram below is the other form: a seal-in circuit that keeps the coil powered through its own contact after the start button is released.
A magnetic latching relay behaves like a double-throw toggle switch. Once the toggle is pushed to one side, it stays there until it is pushed the other way.
Once a magnetic latching relay is set electrically, it stays in that position until a reset pulse or a second coil drives it back.
The same magnetic device is also sold as an impulse relay, a bistable relay or a stay relay.
What is an Impulse Relay?
An impulse relay is a latching relay that changes contact state on a short coil pulse and is often listed as a bistable relay.
A dual-coil type uses one coil to set the contacts and the other to reset them. After the pulse ends, a magnet or mechanical latch holds the armature, so the contacts stay put with coil power removed.
A single-coil impulse type toggles on each successive pulse. Dual-coil types do not toggle from simply restoring supply; they move only when the matching set or reset coil is pulsed.
Impulse and magnetic latching relays suit ON/OFF loads started and stopped from several momentary push-buttons, such as lighting or a conveyor controlled from more than one location.
Latching Relay Circuit Diagram
The diagram is a seal-in (electrically latched) circuit around an ordinary relay, not a magnetic latching relay. Button-1 (B1) is the start (make) push-button. Button-2 (B2) is the stop (break) push-button.
Pressing button-1 energizes the relay coil. That closes the contacts from A to B and from C to D.
Contact A-B then parallels B1, so coil current continues after button-1 is released. The coil stays powered for as long as that holding path remains.
To drop the circuit, the coil must be de-energized. Pressing button-2 opens the holding path and the armature returns.
How Does a Latching Relay Work?
Button-1 is a normally open (NO) switch. Button-2 is a normally closed (NC) switch. At rest, Button-1 is open and Button-2 is closed.
Press Button-1 to start. Current then flows from (+Ve) through B1, A, B and back to (-Ve).
That current energizes the coil. Contacts close so A meets B and C meets D.
After B1 is released, the coil stays energized through the holding path (+Ve)-B2-B-A-(-Ve). The holding path is electrical latching. The coil still draws current while the circuit is ON.
To stop, break that holding path so the coil de-energizes.
Pressing NC push-button B2 opens the path, drops the coil and the contacts return.
Relays are built with many contact arrangements, so the same seal-in idea can be wired on different pole counts.
How to Make a Latching Relay Circuit
The steps below build that seal-in circuit from a standard relay, two push-buttons and a DC supply.
Step-1 Connect the relay with a push-button and a DC supply as shown in the figure below.
The push-button is a normally open (NO) switch, so it is open at rest. Press it and the relay picks up. Release it and the relay drops. The result is ordinary momentary control.
For a latching (seal-in) circuit, the relay must stay ON after the start button is released.
Step-2 For seal-in operation, the common point of the relay must connect with the source via the holding contact, as shown in the figure below.
In this wiring, pressing the push-button picks the relay up. After the button is released, a contact still feeds the coil.
Releasing the button opens the original feed through the push-button. The coil then stays fed from the DC line through that closed contact.
After that first press, the coil stays ON. There is no stop path yet, so you cannot turn it OFF from the start button.
Step-3 Connect an extra normally closed (NC) push-button in the DC line to the relay, as shown in the figure below.
The extra button is a normally closed (NC) stop in series with the DC feed. Pressing it opens the coil circuit.
Use push-button 1 to turn the circuit ON and push-button 2 to turn it OFF.
