Low-Voltage Technology Series · Five 9s Communications
A latching relay (also known as a bistable relay, keep relay, stay relay, or impulse relay) is a two-position electromechanical switch that maintains its contact state without continuous power applied to the coil. Once triggered by a brief pulse, the relay locks into its new position and holds it indefinitely — even if power to the coil is completely removed.
This behavior contrasts sharply with a standard electromagnetic relay, which requires continuous coil current to remain energized. The moment a standard relay loses coil power, its contacts spring back to their default position. A latching relay has no such requirement — it remembers its last state.
| Feature | Standard (Non-Latching) Relay | Latching Relay |
|---|---|---|
| Coil power to stay energized | Continuous — must be held ON | None — pulse to switch, then zero |
| State after coil power removed | Returns to default (spring reset) | Holds last position |
| Power consumption (holding state) | Constant coil draw (watts) | Zero (0 W) |
| Heat generation | Continuous coil heating | None during hold |
| Memory on power loss | No — resets to default | Yes — maintains last state |
| Number of coils | One | One (single-coil) or Two (dual-coil) |
| Typical applications | Motor starters, alarms, simple switching | Lighting control, power systems, metering |
Understanding what is inside a latching relay helps explain why it behaves so differently from a standard relay. The key addition in a latching relay is either a permanent magnet or a mechanical latch mechanism — neither of which requires electricity to hold position.
Receives a brief pulse of current. Generates a magnetic field strong enough to move the armature. Once the pulse ends, the coil draws zero current.
A movable iron lever or plate that responds to the coil's magnetic field. Pivots or shifts to open or close the contacts when energized.
The "memory" of the relay. After the coil pulse ends, the permanent magnet holds the armature in its last position. No electricity required — pure magnetic force.
The electrical connection points that control the external circuit. Can be Normally Open (NO), Normally Closed (NC), or both (changeover/SPDT).
In mechanical-type relays: a ratchet gear, pawl, or spring locking mechanism physically holds the armature in place instead of a permanent magnet.
Input connection points where the control pulse is applied. Dual-coil types have two sets: one for SET, one for RESET. Single-coil types use reversed polarity.
Output connection points that connect the relay to the load circuit. The switched contacts control current delivery to lights, motors, or other devices.
Protective housing shielding internal components from dust, moisture, and vibration. Critical for long-term reliability in field installations.
The standard latching relay circuit uses two push-buttons. Button 1 (B1) is a Normally Open (NO) switch that energizes (SETs) the relay. Button 2 (B2) is a Normally Closed (NC) switch that de-energizes (RESETs) the relay. Press the buttons below to see the circuit operate.
After pressing SET, notice the self-hold path (contact B → + rail) keeps the relay latched even after B1 is released. Pressing RESET (B2) breaks the supply rail, dropping the coil and unlatching the relay.
The most common type. Uses a permanent magnet to hold the armature after switching. The magnet pivots above a U-shaped electromagnet and can stick to either end of the core. A reverse voltage pulse or second coil pulse reverses the polarity and flips the armature. Available in single-phase and three-phase versions. Contact switching current can reach 150A. Control coil voltages typically 9V DC or 12V DC.
Uses a ratchet gear, pawl, or spring locking mechanism to physically hold the armature instead of a magnet. When the coil energizes, the armature moves and clicks into the ratchet lock. A reset pulse releases the ratchet. Mechanical types are less common today due to bulkier construction and slower operation, but they are extremely robust in high-vibration environments.
Also called a step relay. Changes its contact position with each successive pulse — ON, OFF, ON, OFF, etc. — using an internal position-detection circuit that automatically determines which coil to energize. Commonly used in lighting circuits where one push-button location needs to toggle a circuit ON and OFF repeatedly. Very popular in European building automation.
Has one coil only. SET and RESET are achieved by reversing the polarity of the applied voltage. Positive voltage = SET; negative voltage = RESET. Simpler construction, fewer terminals. Requires a driver circuit capable of reversing polarity.
Has two separate coils — one for SET, one for RESET. Applying a pulse to the SET coil closes the contacts and latches. Applying a pulse to the RESET coil opens the contacts and latches. Simpler control logic (no polarity reversal required). The most common type in building automation and low-voltage control panels.
Not a traditional electromechanical relay. Uses semiconductor devices (transistors, SCRs, or flip-flop logic) to latch a state without any moving parts. No physical armature or contacts — current is switched through semiconductor junctions. Extremely fast switching, silent operation, long life. Used in high-speed automation and control systems.
Latching relays excel wherever a circuit needs to remain in a state — ON or OFF — for long periods without wasting energy, or wherever a device needs to remember its last state after a power outage.
Any SET button (S1, S2, S3 — wired in parallel) turns the lights ON. Any RESET button (R1, R2 — also in parallel) turns them OFF. No 3-way or 4-way switch wiring required. Additional control locations can be added by simply wiring more push-buttons in parallel on each bus.