Timer Relay Wiring Diagram: On-Delay, Off-Delay, and Pulse Timer Circuits Explained

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A timer relay diagram shows how a time-delay relay connects to a control circuit to energize or de-energize a load after a preset interval, used in motor starters, lighting control, and process automation.

A timer relay (also called a time-delay relay or TDR) is a control device that introduces a deliberate, adjustable time delay between the control input signal and the output contact action. Understanding the wiring diagram requires knowing which of the four main functional types you are working with.

An on-delay timer (NOTC — normally-open, timed-to-close) energizes the coil immediately when the input is applied, but the output contact remains open until the preset time period elapses. Typical applications include star-delta motor starters, where the timer controls the changeover from star to delta after the motor accelerates, and compressor delay-on-start circuits that stagger multiple units to limit peak demand current.

An off-delay timer (NCTO — normally-closed, timed-to-open or NOTO — normally-open, timed-to-open) energizes the output contact immediately when the coil is energized, then holds the contact closed for the preset period after the coil is de-energized. Off-delay timers are used in exhaust fan controls (fan continues running for 10 minutes after the light switch is turned off), lubrication systems, and cooling circuits.

A pulse (or one-shot) timer output contact closes for a defined pulse duration and then opens, regardless of how long the input signal is applied. This is used for solenoid valve actuation, alarm horn silencers, and production counters.

A flicker (oscillating) timer alternately closes and opens its output contact at defined on-time and off-time intervals, used for indicator lamps, alarm annunciators, and sequential process steps.

In a wiring diagram, timer relays are represented with a standard coil symbol (rectangle with the timer function letter — TON for on-delay, TOF for off-delay — per IEC 60617 or NEMA/ANSI standards). Contact symbols carry a small arc or 't' annotation to distinguish timed contacts from instantaneous contacts. Modern solid-state timers use the same wiring conventions as electromechanical types, but their internal timing is derived from RC networks, digital counters, or microcontrollers rather than pneumatic bellows or synchronous motors.

Timer relay wiring diagrams cover a wide range of devices — from basic ON-delay and OFF-delay industrial relays to specialised automotive units such as REDARC's dual-battery timers and wiper delay relays. The core wiring structure is consistent: a supply voltage to the module, a trigger or enable input, and switched output contacts. Brand-specific modules like the REDARC BCDC series or electronic wiper-delay relays add extra control pins, so consulting the specific wiring diagram for your model is essential. Map out your timer relay circuit free at Circuit Diagram Maker.

How to wire timer relay diagram

Identify the timer type and required timing mode Before wiring, confirm whether you need on-delay, off-delay, pulse, or flicker operation. Consult the equipment sequence of operations or process timing diagram. If using a multi-function timer, set the mode selector to the required function before installing it in the panel.
Identify and verify terminal assignments from the datasheet Locate the terminal diagram in the manufacturer's datasheet for the specific timer model. Do not rely on generic diagrams — terminal numbering and the meaning of each terminal varies significantly between manufacturers and product families. Note the coil terminals (A1/A2), common contact (COM), normally-open contact (NO), and normally-closed contact (NC).
Connect the coil supply (A1/A2) Wire the control voltage supply to terminal A1 through the initiating contact (e.g., a pushbutton, float switch, or PLC output). Wire the neutral (or negative for DC) to terminal A2. Ensure the control voltage source matches the rated coil voltage exactly. For AC coils, confirm the frequency matches (50 Hz or 60 Hz), as some electromechanical timer motors are frequency-dependent.
Wire the timed output contact to the load circuit Connect the load supply to the common (COM) terminal of the timed contact set. Wire the load (motor starter coil, contactor, solenoid, or lamp) between the appropriate timed contact — NO for on-delay (closes after delay) or NC for off-delay hold (opens after delay). Verify the contact current and voltage ratings against the actual load. If the load is inductive (motor or solenoid coil), size contacts accordingly and add a surge suppressor across the load.
Set the timing range and time setpoint Most timer relays have a range selector (e.g., 0.1–10 s, 1–100 s, 0.1–10 min) and a calibrated dial or potentiometer. Set the range selector to the range that includes your required time, then adjust the dial to the setpoint. For critical timing, use a stopwatch to verify the actual delay under power before commissioning.
Test the timing sequence before energizing the load With the panel isolated from the load, apply control voltage and observe the timer LED or mechanical indicator. Confirm the correct delay period elapses before the contact changes state. Check that de-energizing the coil resets the timer correctly (output contact returns to its normal de-energized state on completion of timing for off-delay; immediately for on-delay).

Specifications

Coil voltage (typical ranges available)	24 V DC, 24 V AC, 110 V AC, 230 V AC
Timing range (typical DIN rail unit)	0.1 s to 100 hours (range-selectable)
Contact rating (typical)	8–16 A, 250 V AC (resistive)
Timing accuracy (solid-state RC)	±5–10% of setpoint
Timing accuracy (digital/microcontroller)	±0.1% or better
Operating temperature range	-20 °C to +55 °C (typical)
Mounting	DIN rail (35 mm) or surface/panel mount

Safety warnings

Control panels containing timer relays connected to mains-voltage circuits must be wired and commissioned by a licensed electrician in accordance with the applicable installation standard for your jurisdiction (NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364). Isolate the supply and verify dead with a calibrated voltage tester before opening any panel or handling conductors.
Never exceed the rated contact voltage and current of the timer relay output contacts. Exceeding these ratings causes contact arcing, welding, and fire risk. If the load exceeds the timer contact rating, interpose a suitably rated contactor or intermediate relay — the timer switches the coil of the contactor, not the load directly.
Inductive loads (motors, solenoids, transformers) generate voltage spikes that can shorten timer contact life dramatically. Always fit a surge suppressor (RC snubber for AC loads, freewheeling diode for DC loads) across each inductive load switched by the timer contacts.
When using 24 V DC coil timers with a switch-mode power supply, verify the power supply can supply the inrush current of the timer coil on energization without voltage collapse, which can cause false timing or failure to latch.

Tools needed

Digital multimeter (voltage, continuity, and resistance measurement)
Insulated screwdrivers (IEC 60900 rated for live working if applicable)
Wire stripper and ferrule crimping tool (for DIN rail panel wiring)
Non-contact voltage tester
Stopwatch or timer app (for timing verification)
Datasheet for the specific timer relay model in use
Panel wiring labels and marker pen

Common mistakes

Wiring the load directly to the timer contact without checking the contact rating — many compact DIN-rail timers are rated only 5–8 A resistive, which is insufficient for motor starter coils with high inrush.
Confusing on-delay NO and off-delay NO contacts — both appear identical on a diagram but behave oppositely during timing, leading to sequence errors that are difficult to diagnose once the panel is installed.
Connecting an AC coil timer to a DC supply, or vice versa — AC coil timers rely on the alternating flux cycle for operation; applying DC to an AC coil causes transformer saturation and coil overheating.
Setting the timing range selector to a range that does not include the required time, causing the timer to either immediately activate (dial maxed out below setpoint) or never activate within the machine cycle.
Omitting a freewheeling diode across DC relay and solenoid loads switched by the timer, causing voltage spikes that corrupt solid-state timer ICs and reduce contact life.
Neglecting to account for control transformer burden when adding multiple timers — each timer coil draws VA continuously, and overloading a control transformer causes voltage sag that affects timing accuracy and coil reliability.

Troubleshooting

Timer does not start timing when control input is applied: Cause: No voltage at coil terminals (A1/A2), blown control fuse, open initiating contact, or incorrect control voltage at the coil Fix: Measure voltage at A1 and A2 with the initiating contact closed. Verify voltage matches coil rating. Check and replace control fuse if blown. Test continuity of the initiating contact and wiring from supply to A1.
Timer times out but output contact does not change state: Cause: Failed output contact, load circuit wiring error, or timer module fault Fix: After timing completes, measure continuity between COM and NO (or NC). A healthy contact will change state. If no contact change is detected, the timer relay output has failed — replace the timer. If contact changes but load does not respond, trace the load circuit wiring.
Timing period is inconsistent or drifts over time: Cause: Supply voltage variation affecting RC timing in solid-state timers, or wear in an electromechanical timer mechanism; component aging in RC-based timers Fix: Measure supply voltage at the timer coil terminals under loaded conditions. Stabilize supply voltage if fluctuating. For electromechanical timers, inspect and clean the timing mechanism. For precision applications, upgrade to a digital microcontroller-based timer relay which is immune to supply variation.

Frequently asked questions

What is the difference between an on-delay and off-delay timer relay?

An on-delay timer waits a preset time after the coil energizes before closing (or opening) its output contact. An off-delay timer changes its output contact state immediately when the coil energizes, then holds that state for the preset period after the coil de-energizes. The key distinction is whether the delay occurs on energization or de-energization of the control input.

What do the terminal markings on a timer relay mean?

Terminal labels vary by manufacturer but common conventions are: A1/A2 — coil supply terminals; 1–8 — contact terminals (1/2 = coil side on some types); NO — normally open contact; NC — normally closed contact; COM — common contact. Always verify against the specific datasheet. IEC 60947-5-1 standardizes contact numbering: 1x = NO, 1x-1 = NC in sequence pairs.

Can a timer relay be wired for both on-delay and off-delay?

Many modern multi-function timer relays include a rotary mode selector that configures the same unit for on-delay, off-delay, pulse, flicker, or other timing modes. When using such a device, confirm the mode selector is set correctly before commissioning, as the wiring remains identical but the contact behaviour is fundamentally different depending on mode.

Why does the timer relay not hold its set time accurately?

Electromechanical (pneumatic or synchronous motor) timers can drift with temperature and wear. Solid-state RC timers are sensitive to supply voltage variation and component tolerance. Digital or microcontroller-based timers are most accurate. Also verify supply voltage to the coil is within the rated range — under-voltage can slow electromechanical timers, and some solid-state timers require a minimum voltage for accurate timing.

What control voltage options are available for timer relay coils?

Timer relay coils are available in a wide range of control voltages to suit different panel designs: 24 V DC and 24 V AC are standard for modern control panels; 110 V AC and 230 V AC are common in older or mains-voltage control circuits. Always match the coil voltage to the control circuit supply, and verify whether the timer is AC-only, DC-only, or universal AC/DC rated.

What is a timer delay relay wiring diagram and why does it show a 'rapid backfire' connection?

A timer delay relay adds a timed delay before its output contacts open or close. The 'rapid backfire' label that sometimes appears in online wiring diagrams refers to an incorrect or unintended connection that causes the relay coil to retrigger itself, creating a rapid oscillation or chatter — it is a fault condition, not a normal function. Correct wiring has the trigger input isolated from the relay output to prevent self-retriggering.

How do I wire an electronic timer relay?

Electronic timer relays (DIN-rail or panel-mount) typically have four to six terminals: A1 and A2 for coil supply voltage, a set of output contacts (NO, NC, and common labelled 11, 12, 14 or similar), and sometimes an external trigger input. Connect A1/A2 to the correct supply voltage (match the relay's rated voltage), wire the load between the output contact and neutral/earth, and use the timing adjustment potentiometer or DIP switches to set the delay period.

How do I wire a REDARC timer relay?

REDARC timer relays (such as the SBI12 or BCDC series) include clearly labelled terminals in the supplied wiring diagram. The common approach is: connect the battery input to the relay's supply terminal, the ignition sense wire to the trigger input, and the output terminal to the load (secondary battery or device). The timer holds the circuit closed for a set period after the ignition is turned off, preventing premature disconnection of charging equipment.

How do I wire a wiper timer relay (intermittent wiper relay)?

A wiper timer relay inserts between the wiper switch and wiper motor to provide an adjustable intermittent (pause) function. It requires: a permanent 12 V supply, a switched 12 V from the wiper park switch (so the relay knows when the wiper has returned to park), and an output to the wiper motor's low-speed terminal. The internal timer circuit holds the output open for the set delay, then closes it to start the next wipe cycle.

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