Timer Diagram: How Electrical Timer Relays Work and How to Wire Them

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A comprehensive reference covering electrical timer relay types, wiring configurations, and timing circuit diagrams for industrial, commercial, and domestic applications.

An electrical timer — whether an electromechanical relay timer, solid-state timer module, or programmable logic controller (PLC) timer instruction — is a device that introduces a controlled time delay into a control circuit. Timers are fundamental to automation and control, used to delay the start of a process, limit how long a process runs, create timed cycles, and coordinate multiple events in sequence.

The most commonly encountered discrete timer device in industrial panel building is the DIN rail-mounted timer relay. Timer relays are categorised by their timing function: on-delay (TON), off-delay (TOF), one-shot (pulse), interval, and recycle (flasher) timers are the most frequently used types.

An on-delay (TON) timer begins timing when its control input is energised. Its output remains de-energised during the timing period and then actuates when the set time elapses — as long as the input remains energised. Used to delay the start of a motor or process after a command is given.

An off-delay (TOF) timer's output energises immediately when the control input is energised. When the input is de-energised, the output remains energised for the set timing period before dropping out. Used to keep a fan or lubrication pump running for a fixed period after a machine stops.

A one-shot (pulse or monostable) timer produces an output pulse of fixed duration when triggered by an input. The output remains active for the set time regardless of what the input does after triggering.

A recycle (flasher or astable) timer alternates its output on and off repeatedly, controlled by adjustable on-time and off-time settings. Used for flashing indicator lights and cyclic processes.

Wiring a timer relay correctly requires connecting the correct supply voltage to the coil supply terminals (A1 and A2 in IEC notation), the control (trigger) input where applicable, and the load circuit through the timer's output contacts (typically a changeover contact: NO, NC, and COM). Always verify the timer's coil voltage and contact current rating against the application. Refer to the manufacturer's wiring diagram for the specific timer model — terminal designations can vary between manufacturers.

This page is a generic educational reference. Always comply with applicable wiring standards and engage qualified personnel for electrical installations.

How to wire timer diagram

Define the required timing function Before selecting a timer, determine the required behaviour: Do you need to delay the start of an output after an input (on-delay/TON)? Delay the stop of an output after the input drops (off-delay/TOF)? Produce a single timed output pulse (one-shot)? Create a repeating timed cycle (recycle/flasher)? The timing function determines the timer type to specify.
Select the correct timer relay Specify the timer by: timing function (TON, TOF, one-shot, recycle), coil supply voltage (match the control circuit supply — commonly 24 V DC, 24 V AC, 110 V AC, or 230 V AC), time range (must encompass the required set time — e.g., a 0–60 second range for a 30-second delay), output contact rating (verify the contact can switch the load current and voltage — typically 5–10 A at 250 V AC for DIN rail timer relays), and contact configuration (NO, NC, or changeover CO).
Wire the coil supply Connect the control circuit supply to the A1 and A2 coil terminals of the timer. A1 connects to the supply positive or line (L), A2 to the supply negative or neutral (N). In many on-delay configurations, A1 is connected through the control start contact (pushbutton or output from a PLC or controller) so that the timer coil energises only when commanded. Verify the coil voltage matches the supply — an incorrect coil voltage will result in the timer not operating or being damaged.
Wire the control input (if separate from the coil supply) Some timer relay models have a separate control (trigger) input terminal in addition to the coil supply. In these designs, the coil is permanently energised and the control input triggers the timing cycle. Consult the specific timer's wiring diagram — not all timers use this arrangement. Most common DIN rail timers use the coil supply terminals as the sole control input.
Wire the output contacts to the load circuit Connect the load circuit through the appropriate timer output contact. For an on-delay application: use the NO (Normally Open) contact to open the load circuit during the timing period and close it once the delay has elapsed. Connect the supply to the COM terminal and the load through the NO terminal. For an off-delay application where the load must remain on after the input drops, use the NC contact if the load should be on when the timer is de-energised, or the NO contact for the timed-on behaviour — confirm the specific timing diagram from the timer datasheet.
Set the time range and set point Locate the time range selector (if present) and set the correct range dial position (e.g., 0.1–10 s, 1–100 s, 0.1–10 min). Then adjust the set point potentiometer or switch to the required time value. Analogue timers have a scale printed on the set point knob or label — rotate to align the desired time with the indicator mark. Digital timers use a numeric display and pushbutton entry.
Commission and verify timing Energise the control circuit. Trigger the timer's input. Use a stopwatch to measure the actual output delay from input energisation (on-delay) or de-energisation (off-delay) to output actuation. Compare the measured time against the set point. Adjust the set point if the measured time is outside the acceptable tolerance. Document the final timer setting in the panel schedule or electrical drawings.

Specifications

Common timing functions	On-delay (TON), Off-delay (TOF), One-shot (pulse/monostable), Recycle (flasher/astable), Interval
Typical coil supply voltages	24 V DC, 24 V AC, 48 V AC, 110 V AC, 230 V AC — specify when ordering; verify against control circuit supply
Typical time ranges	0.1–10 s, 1–100 s, 0.1–10 min, 1–100 min — check datasheet for available ranges for each model
Typical output contact rating	5–10 A at 250 V AC (resistive load) — verify in manufacturer datasheet; inductive loads derate the contact rating
Contact configuration	Typically 1 changeover (CO): COM, NO, NC — verify for specific timer model
Mounting format (industrial)	35 mm DIN rail (EN 60715), direct or via plug-in socket base
Applicable control circuit standard	IEC 60947-5-1 (control circuit devices and switching elements); IEC 60204-1 (safety of machinery — electrical equipment)
PLC timer instruction equivalents (IEC 61131-3)	TON (on-delay timer), TOF (off-delay timer), TP (pulse/one-shot timer)

Safety warnings

ISOLATION BEFORE WIRING: Always de-energise, isolate, lock out, and verify the control circuit and any associated power circuits are dead before wiring or adjusting a timer relay. Control panels can contain 24 V DC, 110 V AC, and 230 V AC simultaneously — verify all voltage levels present in the panel before touching any conductors.
QUALIFIED PERSONNEL ONLY: Installation of electrical control circuits, including timer relays in industrial panels, must be carried out by qualified electricians or electrical technicians who are competent and authorised to work on industrial electrical equipment in accordance with applicable codes (NEC/NFPA 70, IEC 60364, BS 7671, or local equivalent). Industrial panel wiring also falls under machinery directive requirements in many jurisdictions (IEC 60204-1).
CONTACT RATING: Never use a timer relay's output contact to switch a load that exceeds the contact's rated current or voltage. Overloaded contacts arc excessively, weld closed, and can cause fires. If the load exceeds the contact rating, use the timer contact to switch an appropriately rated contactor or power relay that then switches the load.
COIL VOLTAGE MATCHING: Connecting a timer coil to a supply voltage significantly above its rating (e.g., connecting a 24 V coil to 230 V) will immediately and permanently damage the timer. Verify coil voltage before connecting.
TIMING DOCUMENTATION: Whenever a timer set point is adjusted — even temporarily for a test — record and restore the correct set point before returning the machine or system to service. An incorrect timer setting in a production process can cause product quality failures, equipment damage, or safety interlocks to operate incorrectly.

Tools needed

Calibrated digital multimeter (AC and DC voltage; continuity testing of output contacts)
Insulated screwdrivers (flathead for timer terminal screws; typically 0.6 x 3.5 mm and 0.8 x 4.0 mm)
Wire strippers and ferrule crimping tool (for stranded conductor preparation)
Stopwatch or timer (for verifying timer set point accuracy)
Personal lockout lock and danger tag (for control circuit isolation)
Panel test probe or DIN rail test lead set (for safe in-panel voltage measurement)
Timer manufacturer's datasheet or wiring diagram (for the specific timer relay model)

Common mistakes

Selecting the wrong timing function — specifying an on-delay timer when an off-delay is required, resulting in a control circuit that behaves opposite to the design intent.
Connecting the coil supply voltage to the output contact terminals, or the output contact circuit to the coil terminals — always identify A1, A2 (coil) and COM, NO, NC (contacts) before wiring; the labelling varies between manufacturers.
Failing to size the time range correctly — a timer with a 1–100 second range cannot accurately produce a 3-second delay because the set point must be set to approximately 3% of full scale, where even small mechanical variations cause significant timing error. Select a timer range where the set point is in the 30–70% region.
Using a single-pole timer contact to switch a three-phase load directly — timer relay contacts are typically rated for single-phase switching; use the timer contact to switch a three-phase contactor.
Not using ferrules on stranded conductors at terminal screws — bare stranded wire under a screw terminal can have individual strands pushed out of the terminal on tightening, leaving a reduced cross-section contact and a potential loose connection.
Adjusting a timer set point with the panel energised and live conductors exposed — always isolate the panel before making internal adjustments, or use a tested 'safe to work energised' procedure where justified and permitted.

Troubleshooting

Timer output does not actuate after the set time period: Cause: Timer coil not energised (no supply at A1/A2), time range set incorrectly (set point beyond actual range), timer relay fault, or output contact welded or failed open Fix: With the circuit energised and the timer input activated, measure voltage at A1 and A2 — should read the timer's rated coil voltage. If coil voltage is correct, verify the time range selector is on the correct range. Listen for the timer to 'click' at the expected time (electromechanical types). If no click and coil supply is confirmed correct, the timer relay is faulty — substitute a known-good timer.
Timer output actuates immediately without any delay: Cause: Timer set point wound to minimum (zero or near-zero time set), timer wired as an off-delay but behaviour expected is on-delay, or timer contact type is NC instead of NO Fix: Verify the time range and set point are correctly configured. Confirm the timer timing function (TON vs TOF) matches the circuit requirement. Verify the output is taken from the NO contact (for on-delay where output closes after delay) and not the NC contact (which is closed immediately and opens after delay). Consult the timer's function diagram in the datasheet.
Timer set time is inconsistent or drifts over multiple cycles: Cause: Analogue set point potentiometer worn or contaminated, supply voltage fluctuating outside the timer's specified operating range, or timing capacitor deteriorating (electromechanical timers) Fix: For analogue timers: clean the set point potentiometer with electronic contact cleaner (with the timer de-energised). Measure the supply voltage and verify it is within the timer's specified operating range. For consistent high-precision timing requirements, replace analogue timers with digital or solid-state equivalents.
Control circuit fuse blows when timer coil is energised: Cause: Timer coil connected to incorrect voltage (coil impedance too low for the supply), wiring short circuit at the coil terminals, or multiple timer coils exceeding the fuse rating Fix: Isolate the circuit. Verify the coil voltage rating matches the supply (read the timer label). Measure resistance between A1 and A2 with the coil disconnected — a very low reading (near zero) indicates the coil is shorted. Calculate total coil current if multiple timers share one fuse; resize the fuse or redistribute timers across multiple fused circuits.
Output contact does not switch the load even though the timer actuates: Cause: Load connected to the wrong contact terminal (NC instead of NO, or NO instead of NC), contact rating exceeded and contact has welded open, or load wiring connected to wrong terminal pin on the socket base Fix: With the timer actuated, measure continuity between COM and NO — should show continuity. Measure between COM and NC — should show open. If results are reversed, swap the load connection to the correct contact terminal. If the contact shows continuity in both states, it is welded — replace the timer relay. For plug-in timers, verify the socket base wiring against the socket base terminal diagram.

Frequently asked questions

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

An on-delay (TON) timer waits for the set period after its input is energised before actuating its output contact. An off-delay (TOF) timer actuates its output immediately when energised, and holds the output active for the set period after the input is de-energised. On-delay delays starting; off-delay delays stopping.

What do A1 and A2 terminals mean on a timer relay?

A1 and A2 are the IEC 60947-5-1 standard designations for the coil supply terminals of a control relay or timer relay. A1 is typically the positive or line terminal, and A2 is the negative or neutral return. The coil voltage rating (e.g., 24 V DC, 110 V AC, 230 V AC) must match the supply connected between A1 and A2.

What does 'NO' and 'NC' mean on a timer relay output contact?

NO (Normally Open) is a contact that is open (no continuity) when the timer output is de-energised, and closes (makes contact) when the timer output is actuated. NC (Normally Closed) is the opposite — closed when de-energised, open when actuated. Most timer relays provide a changeover (CO) contact with COM, NO, and NC terminals, giving you both options.

How is a timer relay wired to control a motor starter?

In a typical on-delay star-delta application: the timer coil (A1, A2) is energised from the control circuit supply when the start command is issued. The timer's NO contact, when it closes after the set delay, energises the delta contactor coil and simultaneously de-energises the star contactor — transitioning the motor from star to delta. The exact ladder diagram depends on the specific control circuit design.

Can I use a PLC timer instead of a discrete timer relay?

Yes. PLC timer instructions (TON, TOF, RTO in IEC 61131-3 function block language) replicate the behaviour of discrete timer relays in software. PLC timers offer greater flexibility — timing values can be changed in software, multiple timers can be configured for different time ranges, and timer status can be monitored in real time. For simple standalone circuits, discrete timer relays are often more cost-effective.

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