Low-Voltage Technology Series · Five 9s Communications
Course Duration: 45–60 minutes
Learn how latching relays hold their state without power, how they differ from the seal-in circuits built with standard relays, the main types, their trade-offs, and where they belong — and don’t belong — in building and control systems.
A latching relay (also known as a bistable relay, keep relay, or stay 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 |
Impulse relays are a type of latching relay that toggles on each pulse — covered in Section 4.
Why does a latching relay produce no coil heat while it holds its state?
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.
In a mechanical latching relay, what holds the armature in place instead of a permanent magnet?
Technicians use the word “latching” for two different things, and it’s important to keep them straight:
The classic start/stop control circuit uses a standard relay and two push-buttons. Button 1 (B1) is a Normally Open (NO) START button. Button 2 (B2) is a Normally Closed (NC) STOP button. A normally open contact on the relay itself (A–B) is wired in parallel with B1 as the holding or seal-in contact. Press the buttons below to see it operate — then try a power failure.
After pressing START, notice the seal-in path (contact A–B) keeps the coil powered after B1 is released. Pressing STOP (B2) breaks the supply, the coil drops out, and the contacts spring open. A power outage does the same thing — and when power returns, the load stays OFF.
A latching relay needs no seal-in contact and no continuous coil power. You just send it a short pulse to change state:
| Relay type | To SET (latch ON) | To RESET (latch OFF) |
|---|---|---|
| Dual-coil (double-winding) | Pulse the SET coil | Pulse the RESET coil |
| Single-coil (single-winding) | Pulse the coil with one polarity | Pulse the coil with the opposite polarity |
Both control buttons are momentary normally open push-buttons — each press sends one pulse. Between pulses the coil draws nothing, and the contacts stay put through any power outage.
A seal-in circuit built with a standard relay is running a load when the building loses power for a few seconds. What happens when power returns?
The most common type. A permanent magnet in the relay’s magnetic circuit holds the armature in whichever position it was last driven to. A pulse of the opposite polarity (single-coil) or a pulse on the other coil (dual-coil) overcomes the magnet and flips the armature. Ratings run from small signal relays to power relays — for example, Omron’s G9TB dual-coil latching relay switches 120A. Common coil voltages include 5, 12, and 24V DC, and AC-coil versions exist.
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 or ratchet relay. Changes its contact position with each successive pulse — ON, OFF, ON, OFF — using a mechanical ratchet/cam or, in electronic versions, logic that tracks the last state. Every pulse comes from the same momentary push-button, so any number of push-buttons wired in parallel can toggle the circuit. Widely used for lighting in European buildings.
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), which makes it common in control panels. Don’t energize both coils at once — Omron’s MYK datasheet, for example, says the relay ends up SET if both receive voltage together.
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. Unlike a magnetic latching relay, most electronic latches lose their state when control power is lost — a flip-flop forgets, and an SCR unlatches when its current is interrupted.
A dual-coil latching relay is latched ON. How do you turn it OFF?
You're commissioning a panel with new latching relays straight out of the box. What state are their contacts in?
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.
This uses a dual-coil latching relay. Any momentary SET button (S1, S2, S3 — wired in parallel) pulses the SET coil and turns the lights ON. Any momentary RESET button (R1, R2 — also in parallel) pulses the RESET coil and turns them OFF. No 3-way or 4-way switch wiring is required, and more control locations are added by wiring more push-buttons in parallel on each bus.
Why shouldn't a latching relay's memory be used to run a conveyor directly?
Answer all 15 questions, then submit. You need 80% (12 of 15) to pass. These questions are different from the section knowledge checks.
What is the defining characteristic of a latching relay that distinguishes it from a standard relay?
In a seal-in circuit with a standard relay, what keeps the coil energized after the START button (B1) is released?
In a seal-in circuit, what type of push-button is the STOP button (B2), and why?
Which component gives a magnetic latching relay its 'memory'?
A latching relay is called 'bistable.' What does that mean?
What is a key advantage of latching relays in a building lighting control system?
How does a single-coil latching relay switch between SET and RESET?
Which type of latching relay changes state with each successive pulse from the same push-button?
A magnetic latching relay is latched ON when building power is lost, then restored. What is its contact state now?
What is the term for a control circuit that will NOT automatically restart a load when power returns after an outage?
According to Omron's MYK latching relay datasheet, what happens if supply voltage is applied to the SET and RESET coils at the same time?
Why do latching relay datasheets specify a minimum pulse width?
In a multi-location lighting circuit using a dual-coil latching relay, how do you add another control location?
How does an electronic (solid-state) latching circuit typically differ from a magnetic latching relay during a control power loss?
A standard relay coil draws 1 W and is held on 24 hours a day, every day. About how much energy does it use in a year?
When you submit, your results are emailed automatically to the Five 9s Communications training administrator, and a copy is sent to the email address you enter here — whether or not you pass.
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Passing completes the knowledge portion. Always follow the relay manufacturer’s datasheet and your project drawings when wiring control circuits.