Latching Relay Diagram: How to Wire a Bistable Set/Reset Relay
This is a free printable latching relay diagram: download the diagram as SVG or open it and print to paper or PDF.
A latching relay holds its switched state without continuous coil power, making it ideal for memory functions, pulse-triggered switching, and low-power control circuits.
A latching relay — also called a bistable relay or impulse relay — differs fundamentally from a standard monostable relay. Where a normal relay returns to its de-energised state the moment coil current is removed, a latching relay retains whichever state it was last driven into. It requires only a brief pulse of coil current to change state, then holds that position mechanically (or magnetically) with no continuous power consumption.
Two principal designs exist. The dual-coil latching relay has separate SET and RESET coils. A pulse to the SET coil energises and locks the contacts in the energised position; a pulse to the RESET coil releases them back to rest. The two coils must never be energised simultaneously or the relay mechanism will fight itself — always interlock them in software or hardware logic.
The single-coil (impulse) latching relay uses polarity reversal or a single coil with a toggle mechanism. Each successive pulse to the same coil alternates the state. Some single-coil designs use a permanent magnet to hold the armature; reversing current collapses the magnetic hold and re-latches in the opposite position.
In a dual-coil circuit, the SET coil is typically labelled S1/S2 and the RESET coil R1/R2 on the relay body. The common contact (COM), normally open (NO) and normally closed (NC) load contacts are separate from the coil terminals entirely. Coil voltage ratings commonly range from 5 V DC through 12 V DC, 24 V DC, 48 V DC, and 120/240 V AC depending on application.
Latching relays appear in lighting control panels (stairwell timers), home-automation modules, industrial memory rungs in ladder logic, battery isolation circuits, and anywhere that the coil supply is intermittent but the switched state must persist. Their low standby power draw makes them particularly attractive in solar and battery-backed systems where energy budget is tight.
How to wire latching relay diagram
- Identify relay terminals Locate the coil terminals (S1/S2 for SET, R1/R2 for RESET on dual-coil types) and the load contact terminals (COM, NO, NC). These are clearly separated on the relay body; do not confuse coil terminals with contact terminals. Consult the relay datasheet for your specific part number.
- Connect flyback diodes Solder a flyback diode (e.g. 1N4007) across each coil: cathode (banded end) to the positive coil terminal, anode to the negative terminal. This suppresses the inductive voltage spike that occurs when coil current is interrupted and protects the driving transistor or microcontroller GPIO.
- Wire the SET coil control circuit Connect one end of the SET coil (S1) to your positive DC supply rail. Connect the other end (S2) through a current-limiting element (if required) to the switching device — a momentary push-button, transistor output, or microcontroller GPIO via a driver transistor rated for the coil current.
- Wire the RESET coil control circuit Mirror the SET wiring for the RESET coil (R1/R2), using a separate switching path. Add a hardware interlock — such as a normally closed contact from the SET driver feeding into the RESET driver path — to prevent both coils energising simultaneously.
- Connect the load contacts Wire your load circuit from the supply, through the relay COM terminal, and out from either NO (closed after SET pulse) or NC (closed in reset state) to the load return. Ensure the load voltage and current are within the relay's contact rating. Observe polarity if the load is DC.
- Test set and reset operation Apply a SET pulse of at least 20–50 ms. Verify the load contact closes (if using NO) and that the relay remains latched when coil power is removed. Then apply a RESET pulse. Verify the contact opens and holds. Cycle several times to confirm reliable latching in both directions.
- Verify power-up state and add initialisation if required Power cycle the complete circuit and observe which state the relay starts in. If your application requires a known-good initial state, add an RC-timed one-shot or firmware routine to issue a RESET pulse within the first 100 ms of power-up, before normal operation begins.
Specifications
| Typical coil voltage range (DC) | 5 V, 12 V, 24 V, 48 V DC (relay-specific; verify datasheet) |
|---|---|
| Minimum SET/RESET pulse width | 20–50 ms (typical; confirm with relay datasheet) |
| Coil resistance (24 V DC type, typical) | 400–1200 Ω per coil (verify datasheet) |
| Contact rating (common industrial type) | 10 A / 250 V AC or 10 A / 30 V DC (resistive load) |
| Mechanical life (no-load switching) | ≥ 10 million operations (typical for quality relays) |
| Electrical life (rated load) | ≥ 100 000 operations at full resistive rating |
| Operating temperature range | −40 °C to +85 °C (typical industrial grade) |
| Insulation class (coil-to-contact) | 4 kV AC rms (1 min) — verify relay datasheet |
Safety warnings
- Isolate and verify dead before working on any mains-voltage load circuit switched by the relay contacts. Use an approved voltage tester on all poles. This work must comply with IEC 60364, BS 7671, NEC/NFPA 70, or AS/NZS 3000 as applicable. In most jurisdictions, fixed mains wiring must be performed or inspected by a licensed electrician.
- Never energise the SET and RESET coils of a dual-coil relay simultaneously. Doing so creates opposing magnetic fields that can damage the coil windings, overheat the mechanism, and cause unpredictable contact behaviour. Always implement a hardware or software interlock.
- Coil inductive kick can destroy transistor drivers and microcontroller outputs if flyback diodes are omitted. Always fit a correctly oriented diode across every coil before powering the circuit.
- Verify that the relay's contact voltage and current ratings exceed the maximum load requirements with a safety margin. Exceeding contact ratings causes arcing, contact welding, and fire risk.
- In automotive or mobile applications, ensure all wiring is properly fused close to the supply and protected against chafing. Verify the relay coil voltage matches the vehicle's actual supply voltage, which may vary between 12 V and 16 V depending on charging state.
Tools needed
- Digital multimeter (continuity, DC voltage, resistance)
- Soldering iron and solder (for PCB or terminal connections)
- Wire strippers and crimping tool
- Insulated screwdrivers (flat and Pozidriv)
- Voltage tester (non-contact or probe type) for mains circuits
- Oscilloscope (optional, for verifying pulse width and flyback suppression)
Common mistakes
- Reversing flyback diode polarity — installs diode backwards so it conducts continuously and blows the fuse or driver instead of suppressing the spike.
- Using continuous DC instead of a pulse — applying sustained voltage to one coil of a bistable relay draws unnecessary current and may overheat the coil, which is rated for intermittent pulse duty only.
- Omitting the hardware interlock between SET and RESET driver paths — relies solely on software logic, which can fail during initialisation or fault conditions and destroy the coil.
- Selecting a relay with inadequate contact current rating — a 5 A contact relay used on a 10 A motor load will arc, weld contacts closed, and eventually fail dangerously.
- Ignoring the power-up state — assuming the relay starts in a safe position without verifying or forcing an initialisation pulse, leading to unexpected load activation on power-up.
Troubleshooting
- Relay latches on SET pulse but immediately resets when pulse is released
- Cause: Coil pulse is too short (below the relay's minimum operate time), or supply voltage is marginal and dips below the must-operate voltage during the pulse Fix: Extend the pulse duration to at least 50 ms. Measure supply voltage under load with a multimeter; if it sags below 80 % of rated coil voltage, add bulk capacitance to the supply rail or increase the supply current capacity.
- Both coils remain energised and relay buzzes or overheats
- Cause: Interlock logic has failed or is absent, allowing SET and RESET coils to be driven simultaneously Fix: Power down immediately. Review the control circuit and add a hardware interlock (NC contact from SET driver in series with RESET driver path and vice versa). Inspect the coil windings for heat damage before reapplying power.
- Driver transistor fails repeatedly
- Cause: Flyback diode is absent, incorrectly oriented, or has failed; the inductive voltage spike on coil de-energisation exceeds the transistor's collector-emitter breakdown voltage Fix: Replace the transistor and fit a new 1N4007 diode with correct polarity (cathode to V+). Verify suppression with an oscilloscope: the peak collector voltage should not exceed the supply rail by more than 1–2 V.
- Relay contacts fail to switch load despite coil operating correctly
- Cause: Contact oxidation or welding from previous overload; or load is connected to wrong contact terminal (NC instead of NO or vice versa) Fix: Use a multimeter in continuity mode to map which contact closes after a SET pulse and which closes after RESET. If contacts are welded shut, the relay must be replaced — never attempt to force open welded contacts.
- Relay resets spontaneously during operation
- Cause: Electrical noise or voltage spikes on the supply rail coupling into the RESET coil terminal and inducing a false reset pulse Fix: Add a 100 nF ceramic decoupling capacitor close to each coil terminal. Screen control wiring from power wiring. Add a Schmitt-trigger input buffer on the reset control line if driven from a microcontroller output.
Frequently asked questions
What is the difference between a latching relay and a standard relay?
A standard (monostable) relay requires continuous coil current to hold its contacts in the energised position and springs back when power is removed. A latching (bistable) relay changes state on a brief coil pulse and remains in that state indefinitely without any holding current, consuming power only during the transition pulse.
Can I wire a dual-coil latching relay with a single control switch?
Yes, using a momentary DPDT switch. One throw connects battery positive to the SET coil; the other throw connects to the RESET coil. Each press energises only one coil for the duration of the press, then both coils are de-energised at rest. Never wire both coils to the same switch pole simultaneously.
Why does my latching relay reset itself unexpectedly?
The most common causes are: a voltage spike on the coil rail coupling into the reset coil, insufficient pulse width (too short for the mechanism to latch fully), or mechanical vibration in automotive or industrial environments. Fit a flyback diode across each coil, ensure your pulse is at least 20–50 ms, and verify the supply voltage is within specification under load.
Do latching relays have a defined state on first power-up?
Not necessarily. A latching relay retains whatever position it was in when power was last removed. If a defined initial state is required, include a power-up RESET pulse in your control circuit — either from a dedicated initialisation routine in a microcontroller or a simple RC-delayed one-shot circuit on the RESET coil.
What flyback protection is needed for latching relay coils?
Fit a 1N4007 (or equivalent) flyback suppression diode across each coil, cathode toward the positive supply rail. On dual-coil types, each coil needs its own diode. For AC coils, use a metal-oxide varistor (MOV) rated above the peak AC voltage rather than a diode.
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