Latching Circuit Diagram

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Detailed seal-in relay latching circuit reference covering the hold contact, start/stop logic, push-button control, and typical industrial applications with full component breakdown.

A latching circuit (also called a seal-in circuit, hold circuit, or self-holding circuit) is a relay control arrangement where the relay holds itself energised after the initiating pushbutton is released. It is one of the most fundamental building blocks of industrial control, underpinning motor starters, alarm latches, and process interlock panels.

The circuit operates as follows: when the momentary START pushbutton (normally-open) is pressed, current flows through the relay coil, energising it. The relay closes its auxiliary normally-open contact, which is wired in parallel with the START button. When the START button is released, current continues to flow through this auxiliary contact — the relay holds itself in (seals in). The relay remains latched until the STOP pushbutton (normally-closed, in series with the coil supply) is pressed, opening the coil supply and dropping the relay out. Once dropped, the auxiliary contact opens and the relay stays de-energised until START is pressed again.

The STOP button is always wired in series with the coil, not in parallel with anything. This is a safety-critical arrangement: a wire-off or open-circuit fault in the STOP button causes the relay to drop out — a fail-safe condition. Wiring STOP buttons in parallel with the coil would be a grave error; it would mean a fault could prevent stopping.

In motor starter applications, the latching relay is typically a contactor (a high-current-rated relay), and the circuit is expanded with: overload relay contacts in series with the STOP button (so a motor overload causes an automatic stop); phase failure detection; and additional interlocks. The auxiliary hold contact in a contactor is often labelled 13/14 per IEC 60617 conventions.

A latching circuit is distinct from a bistable relay (or mechanical latching relay), which uses two coils — Set and Reset — or a permanent magnet mechanism to hold its state without continuous coil energisation. The seal-in circuit described here requires continuous coil current to maintain the latched state.

How to wire latching circuit diagram

Define the control circuit supply voltage Determine the control circuit voltage. Common choices are 230 V AC (from phase-to-neutral of the power supply), 24 V AC, 24 V DC (from a transformer and rectifier), or 110 V AC (preferred in some industrial and mining environments for reduced shock risk). Select a relay coil rated for the chosen control voltage.
Connect the STOP button in series with the coil supply Run the control supply live through the STOP pushbutton first. The STOP button must be a normally-closed (NC) momentary pushbutton, wired in series so that pressing it breaks the coil supply. All additional STOP buttons and protective contacts (overload relay, emergency stop) are also wired in series in this chain.
Connect the START button in series with the remaining coil path From the output of the STOP button (and any other series-connected NC contacts), wire the START pushbutton (normally-open, NO) in series with the relay coil. The other side of the coil connects to the control supply neutral or negative. This gives the basic path: Supply → STOP (NC) → START (NO) → Coil → Return.
Add the auxiliary hold (seal-in) contact in parallel with START Connect a normally-open auxiliary contact of the relay (terminal pair 13/14 in IEC convention, or as labelled on the relay) in parallel with the START button — between the output of the STOP button and the coil input. When the relay energises via START, this contact closes and provides the parallel hold path, maintaining coil energisation after START is released.
Connect the main load contacts Wire the relay's main (power) contacts in the load circuit they are intended to control — for example, in series with a motor contactor, a lamp, a solenoid valve, or another relay coil. The load contacts are electrically independent from the control circuit and can be rated for a different voltage and current.
Add protective contacts in series with the STOP path (if required) For motor control applications, add the overload relay NC contact (95–96 terminals) in series with the STOP button chain. Add emergency stop NC contacts, phase failure relay NC contacts, and any other required interlock contacts in the same series chain. All of these will drop the circuit out when operated.
Test and commission the circuit Energise the control circuit. Press START — the relay should energise and hold. Release START — the relay should remain latched. Press STOP — the relay should drop out immediately. Verify that operating any protective contact in the series chain also drops the relay out. Test normal operation of the load circuit through the main contacts.

Specifications

Circuit type	Seal-in (self-holding) relay latching circuit using auxiliary normally-open hold contact
Typical control supply voltages	24 V DC, 24 V AC, 110 V AC, 230 V AC
START button	Normally-open (NO) momentary pushbutton, connected in parallel with the seal-in contact
STOP button	Normally-closed (NC) momentary pushbutton, connected in series with the coil supply
Hold contact rating	Must carry the full relay coil current; typically the same rating as other auxiliary contacts (e.g. 10 A / 250 V AC)
Power-loss behaviour	Drops to de-energised (stopped) state — does not self-restart on supply restoration
Applicable standard (machinery)	IEC 60204-1 (Safety of Machinery — Electrical Equipment of Machines)
IEC auxiliary contact designation (contactor)	13 (NO) / 14 (NO) for sealing contact; 21 (NC) / 22 (NC) for interlocking

Safety warnings

Always isolate and verify dead the control panel supply before working on relay control circuits. Control circuits at 230 V AC are potentially lethal; even 24 V DC circuits can cause severe burns from short-circuit fault current. Use a calibrated voltage tester to confirm de-energisation. Work to IEC 60364, BS 7671, NEC/NFPA 70, or the applicable national standard.
STOP buttons and emergency stop devices must always be wired in series (normally-closed in the coil supply path), never in parallel. Wiring a stop device in parallel with a relay coil or auxiliary contact is a safety-critical error that could prevent the circuit from stopping and is prohibited under IEC 60204-1 (Safety of Machinery — Electrical Equipment).
A latching circuit does not provide protection against unintended restart after a power failure. In machinery applications, ensure an appropriate restart prevention scheme is implemented per IEC 60204-1 Clause 7 — the standard seal-in circuit already provides this by dropping out on power loss, but verify this is the required behaviour for the specific application.
Relay coil suppression components (diodes for DC, RC snubbers for AC) must be fitted. Without suppression, the inductive back-EMF spike when the coil is de-energised can damage PLCs, transistor outputs, proximity switches, and other electronic control components sharing the control supply.
This diagram is for educational and illustrative reference only. Control circuit design for machinery must comply with IEC 60204-1 and must be designed, documented, and verified by a competent electrical engineer. Installation must be inspected by a licensed electrician.

Tools needed

Insulated screwdrivers (flat-blade 2.5 mm and 4 mm for terminal block and relay base screws)
Calibrated multimeter (DC/AC voltage, continuity, resistance)
Ferrule crimping tool (0.5–2.5 mm² range for control cable terminations)
Wire stripper for 0.75–1.5 mm² flexible cable
Cable labels or heat-shrink markers for conductor identification
Calibrated voltage tester (non-contact and two-probe)
Logic probe or test lamp (for tracing live/dead states in low-voltage DC control circuits)

Common mistakes

Connecting the seal-in contact in series with the START button instead of in parallel — in series, releasing START still drops the relay; only a parallel connection provides the hold path.
Wiring the STOP button as normally-open — a normally-open STOP button that breaks when pressed will prevent starting (not stopping) because the contact is open by default, blocking the coil supply at rest.
Omitting the coil suppression component — in PLC-controlled circuits, the inductive spike from the relay coil de-energising can cause PLC output card damage or nuisance PLC resets.
Using the relay's main contacts for both the seal-in function and the load — if the main contacts are used for hold and the load simultaneously, an overload that opens the contacts on load also releases the seal-in, which is correct behaviour; but the contact wear from the load current is significantly higher than from the low-current control hold duty, shortening contact life.
Not testing the circuit with all protective contacts operated — a latching circuit in a motor starter must be tested to confirm that opening any NC protective contact (overload relay, emergency stop) drops the relay out cleanly.

Troubleshooting

Relay energises when START is pressed but drops out when START is released: Cause: The seal-in (hold) contact is not wired in parallel with the START button — it may be missing, wired in the wrong position, or the auxiliary contact is not making Fix: With the relay energised (hold START pressed), use a multimeter in continuity mode to verify the auxiliary NO contact (13/14 or as labelled) closes when the relay picks up. If the contact closes but does not hold the circuit, verify its wiring — trace from the STOP button output to the coil input and confirm the auxiliary contact terminals are in parallel with the START button.
Relay cannot be stopped by pressing the STOP button: Cause: STOP button wired in parallel with the coil or auxiliary contact instead of in series with the coil supply; or the STOP button contact has failed in the closed position Fix: Verify the STOP button wiring — it must be in series (the coil supply current flows through the STOP button NC contact). If correctly wired, measure across the STOP button contacts while pressing the button; a correctly wired NC button should show an open circuit when pressed. If it remains closed, the button contact has failed and must be replaced.
Relay chatters (rapidly picks up and drops out) when START is pressed: Cause: Control supply voltage is below the relay coil minimum pick-up voltage, or the auxiliary contact has insufficient contact force to carry the coil current reliably Fix: Measure the control supply voltage directly at the relay coil terminals A1/A2 while the relay is attempting to pick up. If voltage is below 85 % of rated coil voltage, investigate the supply — check transformer rating, fuse, and wiring resistance. If voltage is correct, the relay or its auxiliary contact block may be faulty and should be replaced.

Frequently asked questions

What is a seal-in contact in a relay latching circuit?

A seal-in contact (also called a hold contact or auxiliary contact) is a normally-open contact on the relay that is wired in parallel with the START pushbutton. When the relay energises, this contact closes and provides an alternative current path for the coil — maintaining energisation after the momentary START button is released. This is what allows the relay to 'hold itself in'.

Why is the STOP button wired normally-closed and in series, not in parallel?

Wiring the STOP button in series with the coil means that opening it (pressing STOP, or a wire breaking) always de-energises the relay — a fail-safe condition. If it were wired in parallel, a fault could prevent the stop command from working. This series normally-closed arrangement for stop functions is a fundamental industrial control safety principle.

What is the difference between a latching relay circuit and a bistable relay?

A seal-in latching circuit requires continuous current through the coil to maintain the latched state — remove the coil supply and it drops out. A bistable (mechanically latching) relay uses a Set coil to latch and a Reset coil to unlatch, with a mechanical mechanism holding position without continuous current — power loss does not change its state.

Can multiple START buttons be added to a latching circuit?

Yes. Additional START buttons (all normally-open) are simply wired in parallel with the existing START button and the seal-in contact. Pressing any one of them will energise the relay. Additional STOP buttons (normally-closed) are wired in series with each other and with the existing STOP button — pressing any one stops the relay.

What happens to a latching circuit when power is lost and restored?

A standard seal-in latching circuit does not retain state on power loss. When supply power is interrupted, the coil de-energises and the hold contact opens. On supply restoration, the circuit is in the de-energised (stopped) state, regardless of its condition before the power loss. This is a desirable safety feature preventing automatic restart after power interruptions.

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