Timer relay
A relay that switches its contacts after a set delay, enabling sequenced and timed control.
What it is
A timer relay introduces a time delay into a control circuit. It switches its output contacts a set period after being triggered, rather than instantly.
How it works
Different functions suit different jobs. An on-delay waits the set time after it's energised, then switches. An off-delay switches immediately but waits the set time before dropping out. Other modes include single-shot pulses and cyclic on/off.
Many timers need both a supply and a separate trigger input, and have a set time plus a range multiplier — getting either wrong is a common cause of a timer that seems 'stuck'.
Where it's used
Star-delta changeover, sequenced starts, run-on fans, delayed shutdowns, and lighting time clocks. Choosing the right function (and wiring the trigger) is most of the battle.
What it looks like when it fails
Most 'faulty timers' are set or wired wrong, not broken: an on-delay fitted where an off-delay was needed, the mode dial knocked to a different function, the range multiplier on minutes instead of seconds, or the trigger input left unwired on models that want supply and trigger separately.
Real failures look like: the timing LED cycles correctly but the contacts never change (worn contacts), timing that drifts on old units, or no LED at all with supply present — a dead timer.
How to test it
Identify the mode first — it changes what 'working' looks like. An on-delay does nothing until the set time expires after energising; an off-delay switches immediately and drops out after the delay once de-energised. Then meter the output contacts through one full cycle, timing it against your phone and watching the LEDs.
Check supply at the timer's supply terminals, and the trigger signal separately if it has one. LED cycling + contacts never moving condemns the timer. No LED at all points at supply, not the timer.
Gotchas
On-delay vs off-delay confusion is the number-one timer call-out, and the symptoms swap: a star-delta that never transitions, a run-on fan that runs forever — or never runs. Check the mode selector before you check the wiring.
Multi-function timers ship on a default function — a replacement that 'doesn't behave like the old one' is nearly always on the wrong mode or range. Photograph the old unit's dials before you pull it out.
Safety first
Metering a timer through its cycle means the control circuit is live — licensed work, due care. And remember what the timer starts: the delayed device can operate while you're watching the meter.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Timer relay not switching its output
A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.
Timer behaving as the wrong type (on-delay vs off-delay)
The timer switches at the wrong point in the sequence because it's acting as the wrong function — on-delay where off-delay is needed, single-shot where cyclic is needed, and so on.
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
Timer times out too long or too short
The timer switches, but at the wrong time — the delay is much longer or shorter than it should be, throwing out the sequence.
Related definitions
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.