Impulse, Flasher and Cyclic Timing Relays
What do impulse, flasher and cyclic timing relays do? An impulse (single-shot) timer fires one fixed-duration output pulse per trigger event, independent of how long the trigger stays applied, while a flasher (cyclic) timer repeats an on/off cycle continuously at a set period — both are function classes covered under the same IEC 61812 timing-relay standard as on-delay and off-delay. Pick the wrong one and a beacon that should blink stays lit solid, or a single feed pulse turns into a repeating jam. This article covers pulse-versus-cycle behavior, symmetrical vs asymmetrical flasher setting, the period and duty-cycle math, typical applications, and where these two functions sit inside multifunction and multi-range timing relays.
Impulse (Single-Shot) Timing: One Pulse, No Repeat
An impulse relay changes its output on trigger and holds that state for a fixed dwell time, then reverts automatically — regardless of whether the trigger signal is still present. That is the key difference from on-delay timing: an on-delay output stays changed for as long as the trigger is held after the delay elapses, while an impulse output reverts on its own clock. Most single-function impulse timers are non-retriggerable: a second trigger arriving mid-pulse is ignored, and the output completes its set dwell before it can fire again. Some multifunction devices offer a retriggerable variant that extends the pulse on each new trigger, which is a different behavior worth checking in the catalogue before specifying.
What we see in the field: panel builders often reach for an impulse timer when what they actually need is an off-delay timer, because both produce a limited-duration output. The difference is what starts the clock — an off-delay times from the trigger being removed, an impulse times from the trigger arriving.
Flasher (Cyclic) Timing: Symmetrical vs Asymmetrical
A flasher, also called a cyclic or clocking timer, repeats an on/off cycle for as long as supply and trigger conditions are satisfied, rather than settling into a final state. Two sub-types cover most stocked ranges. A symmetrical flasher uses one time setting for equal ON and OFF dwell — the output spends the same number of seconds energized as de-energized. An asymmetrical flasher (sometimes listed as flicker or dual-range cyclic) uses two independent potentiometers, one for ON dwell and one for OFF dwell, so a beacon can flash briefly on and rest much longer off, or the reverse.
Startup state matters for safety circuits: depending on the model, a flasher can begin its cycle ON or OFF at power-up. Confirm the default state in the datasheet before wiring an alarm or annunciator that must fail into a defined condition — an output that happens to power up OFF looks identical to a stuck relay until the first cycle completes.
Setting the Period: Formula and Duty Cycle
Flasher timing reduces to two dwell settings and a resulting period. On a symmetrical unit both dwells share one dial; on an asymmetrical unit they are set independently, which changes the duty cycle away from 50%.
Formula: Cyclic (Flasher) Period and Duty Cycle — Source: IEC 61812, timing-relay functional definitions
Tcycle = Ton + Toff ; D = (Ton / Tcycle) × 100%
| Symbol | Description | Unit |
|---|---|---|
| Ton | ON dwell time | s |
| Toff | OFF dwell time | s |
| Tcycle | Total period, one full ON+OFF cycle | s |
| D | Duty cycle, share of the period spent ON | % |
On a symmetrical timer, D is fixed at 50% by design — the single dial sets Tcycle and both dwells split it evenly. On an asymmetrical timer, D can sit anywhere the range allows, which is what lets one relay drive a beacon with a short bright pulse and a long rest instead of an even blink. Repeat accuracy for the cycle is specified the same way as for a single-shot delay — a percentage of the set dwell, not of the full period — so a long OFF dwell on an asymmetrical setting carries more absolute timing error than a short one at the same percentage.
Where Impulse and Flasher Functions Are Used
Impulse timers gate a downstream device for exactly one fixed window per trigger: a dosing valve, a marking or stamping solenoid, a single feed pulse into a conveyor infeed, or a priming pulse to a pump before the main run signal takes over. The output does the same thing every time a trigger arrives, which is the point — no drift from how long an operator holds a pushbutton.
Flasher functions show up wherever a signal needs to draw attention rather than just report a state: a beacon or pilot lamp cycling before a machine starts (pre-start warning), an alarm horn set to pulse instead of sound continuously, or a blinking indicator layered on top of a steady lamp to distinguish "running normally" from "running in a warning condition." This depends on the field device's own duty rating — driving a filament beacon or a horn continuously at a 50% symmetrical cycle can shorten its life against the intermittent duty it was designed for, so check the lamp or horn spec against the flasher's period before final selection.
Neither function is what runs a star-delta motor start sequence — that needs a single ON-delay-with-transition behavior, not a repeating cycle — but a flasher is a common companion device on the same panel, driving the run/fault indicator next to the starter.
Impulse and Flasher as Functions Inside Multifunction Timers
Few panel builders stock a dedicated single-function impulse relay or a dedicated flasher relay anymore. A multifunction, multi-range timer — Schneider Zelio Time RE22 or ABB CT-MFD class — puts impulse, symmetrical flasher, asymmetrical flasher, on-delay, off-delay and often star-delta on one 17.5-22.5 mm DIN-rail module, selected by a rotary switch, with the time range picked the same way. One stocked part number covers several functions that would otherwise need several separate timing relay types on the shelf.
The trade-off is setting complexity: a multifunction unit has more rotary positions to get wrong during commissioning than a single-purpose flasher, so the function and range should be checked against the wiring diagram before power-up, not assumed from the box label. For a walkthrough of that selection process, see how to select a timing relay; for the full function family this article sits inside, see the timing relay engineering guide. Stocked ranges from both brands are listed under timing and control relays.
| Criteria | Impulse (single-shot) | Symmetrical flasher | Asymmetrical flasher |
|---|---|---|---|
| Output after set time | Reverts once, stays reverted | Reverts, then re-triggers itself continuously | Reverts, then re-triggers itself continuously |
| Number of time settings | One (pulse dwell) | One (shared ON/OFF dwell) | Two (independent ON and OFF dwell) |
| Duty cycle | Not applicable — single event | Fixed at 50% | Set by the two dwell ratios |
| Typical use | Dosing pulse, single feed stroke, priming pulse | Even blink beacon, alarm pulse | Uneven flash pattern, short flash / long rest |
Frequently Asked Questions
What is the difference between an impulse timer and an off-delay timer?
An impulse timer reverts its output automatically after a fixed internal dwell, independent of the trigger. An off-delay timer keeps its output changed until the trigger is removed, then starts its delay and reverts after that set time. If the trigger is held indefinitely, an impulse output still reverts — an off-delay output does not.
Does a flasher timer need the trigger held continuously to keep cycling?
On most designs, yes — the flasher cycles as long as its supply or control-voltage trigger is present, and stops (usually in the OFF state) once the trigger drops. Check the specific model, since some multifunction units cycle from a momentary start pulse instead.
What is the difference between symmetrical and asymmetrical flasher functions?
Symmetrical uses one time setting, giving equal ON and OFF dwell (a fixed 50% duty cycle). Asymmetrical uses two independent settings, so the ON and OFF dwells can differ, giving any duty cycle within the timer's range.
Can one multifunction timer replace both a dedicated impulse relay and a dedicated flasher relay?
Generally yes. Multifunction, multi-range timers such as Schneider Zelio Time RE22 or ABB CT-MFD class devices include impulse and both flasher variants as selectable functions on the same module, alongside on-delay and off-delay.
Why does my flasher output start ON instead of OFF at power-up?
Startup state is model-dependent — some flasher functions default to ON on power-up, others to OFF. This matters for alarm circuits that must fail into a known state; confirm the default in the datasheet rather than assuming it from a similar device.
Is a flasher timing relay the same as a PLC-generated blink output?
Functionally similar, but not the same reliability class. A flasher relay keeps cycling on its own hardware clock even if the PLC program stalls or the CPU is in fault, which is why panel indicator and alarm circuits often use a hardwired flasher relay rather than a PLC output bit for that function.
Conclusion
Impulse and flasher functions solve two different problems that both happen to produce a limited-duration output. Impulse gives one fixed pulse per trigger and is the right choice for dosing, marking and priming signals that must not drift with operator behavior. Flasher gives a repeating on/off cycle and is the right choice for anything meant to draw attention — a warning beacon, an alarm horn, a fault indicator layered on a run lamp. Symmetrical flasher settings are simplest to specify; asymmetrical settings earn their extra dial when the application needs an uneven flash pattern rather than a plain 50% blink. On a panel that already carries a contactor and a timer for motor starting, adding these functions from an existing multifunction timer rather than a new dedicated relay keeps the bill of materials and the spares drawer smaller.
