Timer Relay Functions: On-Delay, Off-Delay and the Rest
Short answer
- Timer relays differ by which control-voltage edge starts the count: an on-delay timer waits a preset interval after the coil is energized before changing its output, while an off-delay timer changes its output immediately and holds it for a preset interval after voltage is removed.
- Pulse, repeating, star-delta and multifunction units add one-shot pulses, alternating on and off times, and motor-starting changeover, typically across a 0.01 s to 60 min range.
- Hardware timer relays suit safety-adjacent and standalone loops where a visible, hard-wired element simplifies verification, while PLC timing suits process control that needs logging and adjustment.
What does a delay timer actually mean on a timer relay?
Timer relays are often described with shorthand that hides the actual switching condition. The most common source of confusion is the phrase “delay timer” without indicating whether the delay occurs when the coil is energized or when it is released. In practice the device’s internal logic is simple: a timing capacitor (or digital counter) starts counting at a defined edge of the control voltage, and the output changes state after the preset interval. Understanding the edge—rising or falling—lets you select the right part without trial‑and‑error.
All of the functions discussed below are available in standard timer relays. The same hardware can be packaged with auxiliary contacts to form a control relay module, which is useful when the timing function must drive a higher‑current load.
How does an on-delay timer relay work?
An on‑delay timer, often labeled TON, waits a preset interval after the control voltage appears on the coil before closing its output contact. The timing diagram is:
- t = 0 s: Control voltage applied → timer starts counting.
- 0 s < t < Tpreset: Output remains open (or in its default state).
- t = Tpreset: Output changes state (normally closes).
Key practical points:
- Typical timing range: 0.01 s to 60 min, with most manufacturers offering 0.1 s, 1 s, 10 s, 1 min, 10 min steps.
- Accuracy is usually quoted as ±1 % of the set time for the mid‑range (1 s–10 min). Repeatability—how close successive cycles are to each other—is often tighter, ±0.5 %.
- Control voltage can be 24 VDC, 120 VAC, or 230 VAC, depending on the device rating. The voltage must be present for the entire delay; loss of power resets the timer.
- Because the output does not change until the delay expires, on‑delay timers are ideal for sequencing motor starts, preventing short‑circuit re‑closures, or providing a “soft‑start” interlock.
How does an off-delay timer relay work?
An off‑delay timer, labeled TOF, does the opposite: the output changes state immediately when the coil is energized, then remains in that state for a preset interval after the control voltage is removed. The timing diagram reads:
- t = 0 s: Control voltage applied → output changes (usually closes).
- Control voltage removed → timer starts counting.
- 0 s < t < Tpreset: Output stays in the energized state.
- t = Tpreset: Output returns to its default (open) state.
Two practical nuances distinguish off‑delay operation:
- During the off‑delay interval the timer must retain enough energy to continue counting. This is achieved either by a small internal battery, a capacitor bank, or by keeping the control supply present on a separate “hold‑in” terminal. If the supply collapses, the timer may reset and the delay shortens.
- Typical timing ranges mirror on‑delay (0.01 s to 60 min), but the upper limit can be lower for devices that rely solely on a capacitor because leakage becomes significant beyond a few minutes.
Off‑delay timers are commonly used to keep a pump running for a few seconds after a level switch drops, to maintain a valve in a latched position while a pilot light fades, or to provide a “cool‑down” period after a motor stop.
What do pulse, one-shot and repeating timers do?
Beyond simple delays, many timer relays include a pulse or one‑shot function. When the control voltage makes a qualifying edge (rising for a TON‑type pulse, falling for a TOF‑type pulse), the timer produces a single output pulse of a fixed width, independent of how long the input stays active. The width is set by the same preset range used for delays.
Typical use cases:
- Generating a 0.5 s start‑pulse for a solenoid that requires a brief energization.
- Providing a fixed “reset” interval for a latch circuit after a fault condition clears.
Repeating or flasher timers add a second timing parameter: the off‑time. The device alternates between on and off states for as long as the control voltage is present. Two common configurations exist:
- Equal on/off times – a single potentiometer sets both intervals, useful for indicator lamps that blink at a constant rate.
- Independent on/off times – separate adjustments for the high (on) time and low (off) time, allowing a 0.2 s flash followed by a 2 s dark period, for example.
Repeating timers typically operate from 0.1 s to 30 min per half‑cycle, with a duty‑cycle accuracy of ±2 % because the internal oscillator is shared between the two intervals.
What do star-delta and multifunction timer relays do?
Star‑delta starters are a classic application where a timer relay must coordinate two distinct contact sets: first, it connects the motor windings in star for a reduced‑voltage start, then after a preset interval it switches to delta for full voltage. The timer portion is essentially an on‑delay that controls a change‑over contact block.
Multifunction timer relays combine several of the basic modes in a single chassis. A typical unit may offer:
- On‑delay and off‑delay selectable via a mode jumper.
- Pulse (one‑shot) output on either edge.
- Flasher mode with independent on/off adjustments.
- Auxiliary contacts for star‑delta switching or for sequencing multiple loads.
Because the internal counter is shared, the timing range for each mode is usually the same (0.01 s to 60 min). Accuracy remains within the same ±1 % band, but the more functions you enable, the higher the chance of cumulative error if the device is used near its limits. When precise timing is critical—e.g., a motor‑starting sequence that must not exceed a thermal limit—verify the timer’s repeatability at the chosen setting.
Should I use a timer relay or PLC-based timing?
Modern PLC CPUs and systems can generate delays in software with a single instruction. The advantages are obvious: one program can replace dozens of hardware timers, and the timing parameters are changed by editing code rather than swapping modules.
Nevertheless, hardware timer relays retain a niche for several reasons:
- Safety‑adjacent circuits – When a timer must be auditable on a lockout‑tagout (LOTO) inspection, a physical timer provides a visible, hard‑wired element that can be verified without accessing the PLC program.
- Power‑loss resilience – An on‑delay timer that loses its control voltage resets, which can be desirable for safety interlocks. A PLC may retain its internal timer value in non‑volatile memory, potentially causing an unintended continuation of a timed action after power restoration.
- EMI‑sensitive environments – In high‑frequency motor drives, a small, isolated timer relay can avoid the jitter introduced by a PLC’s scan cycle.
- Diagnostic simplicity – A timer relay’s status is observable on its contacts; a PLC requires software diagnostics or additional I/O to expose the same information.
When the application is purely process‑control—e.g., a batch timer that can be logged, adjusted, and coordinated with other logic—PLC timing is usually preferable. The PLC’s scan time (typically 1–10 ms) is far finer than the ±1 % accuracy of most electromechanical timers, and the software can implement complex sequences without extra wiring.
For projects that involve IDEC or other reputable manufacturers, you will find both discrete timer relay modules and integrated control‑relay packages that meet IEC‑60947‑5‑1 standards. Selecting the right form factor depends on whether you need a standalone, auditable timer (IDEC offers several options) or a PLC‑based solution that lives in the control cabinet.
In practice, a balanced approach works best: use PLC timing for the bulk of the process, and reserve hardware timer relays for safety‑critical or standalone loops where a physical, self‑contained timing element simplifies verification and maintenance.
If you need help locating a timer relay that matches a specific on‑delay, off‑delay, or multifunction requirement, or you want to cross‑reference a control‑relay solution, feel free to reach out for sourcing assistance.
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What is the difference between an on-delay and off-delay timer relay?
An on-delay timer, labeled TON, waits a preset interval after control voltage appears on the coil before closing its output contact. An off-delay timer, labeled TOF, changes output immediately when the coil is energized, then holds that state for a preset interval after the control voltage is removed. The difference lies in whether the delay starts on the rising or falling edge.
How accurate are timer relays?
Accuracy is usually quoted as ±1 % of the set time for the mid-range, 1 s to 10 min. Repeatability, meaning how close successive cycles are to each other, is often tighter at ±0.5 %. Repeating timers have a duty-cycle accuracy of ±2 % because the internal oscillator is shared between the two intervals.
When should I use a hardware timer relay instead of a PLC timer?
Hardware timer relays suit safety-adjacent circuits where a physical timer can be verified during a lockout-tagout inspection without accessing the PLC program. They also reset when control voltage is lost, which can suit safety interlocks, while a PLC may retain its timer value in non-volatile memory. Their status is also observable directly on their contacts.
What does a star-delta timer relay do?
A star-delta timer relay coordinates two contact sets for a motor starter. It first connects the motor windings in star for a reduced-voltage start, then after a preset interval it switches to delta for full voltage. The timer portion is essentially an on-delay that controls a change-over contact block.