True Off-Delay Without Control Voltage: How It Works
What is a true off-delay timing relay? A true off-delay (delay-on-release) timer stores enough energy in an internal capacitor to complete its set delay after its own supply is removed, instead of needing that supply present for the whole timing period, per IEC 61812. Skip the energy-storage element and a standard off-delay relay drops its output the instant power disappears, losing the whole point of the delay. This article covers how the storage circuit charges and discharges, how a true off-delay differs from a control-signal off-delay, 2-wire versus 3-wire hookup, the practical hold-time ceiling set by the capacitor, and where the function actually earns its place in a panel.
How the Energy-Storage Circuit Produces the Delay
A true off-delay module runs on two terminals that serve double duty: they supply the electronics and they are the trigger. While power is applied, an internal capacitor charges alongside the timing circuit. The instant that supply is interrupted, the output relay would drop immediately on a plain electronic circuit — the capacitor is what keeps the logic and the output coil alive long enough to run the countdown the dial was set to. When the capacitor's stored charge finally drops below the circuit's operating threshold, the output changes back to its rest state, and the delay is over.
Charge Path vs Discharge Path
Two paths matter here, not one. The charge path runs while the supply is present, topping the capacitor up to its working voltage. The discharge path only opens once the supply is cut, and it is sized to bleed down slowly enough to cover the set delay, not so slowly that the relay drifts. Off-delay circuits share this capacitor-and-threshold pattern with the pneumatic bellows timers they replaced: air bled through a needle valve instead of charge bleeding through a resistor, same basic idea, different physics.
What we see in the field: a true off-delay unit that has only been powered for a second or two before the trigger disappears won't deliver the full set delay, because the capacitor never finished charging. Panel builders who bounce a supply on and off during commissioning tests sometimes read this as a faulty timer when it's just an undercharged capacitor.
True Off-Delay vs Control-Signal Off-Delay
The distinction is about which wire disappears. In a true off-delay, the supply itself is the trigger, so the capacitor has to substitute for the missing power. In a control-signal off-delay, the timer's own power never goes away — a dry contact, a PLC output, or a pushbutton opens or closes a separate input terminal, and the timer counts down on its own clock while comfortably powered the whole time. Multifunction, multi-range timers such as Schneider Zelio Time RE17/RE22 or ABB's timing and control relays lineup often expose both behaviors as selectable functions on the same body, so the function switch on the front, not the part number alone, decides which one you get.
Wiring: 2-Wire and 3-Wire Trigger Circuits
A true off-delay is inherently a 2-wire device at the trigger level: the incoming supply line is the signal the relay is watching. Break that line and the timing starts immediately, running off the capacitor reservoir until it depletes. A control-signal off-delay needs a third path — dedicated supply terminals stay wired to the control transformer or 24V DC bus, and a separate normally-open or normally-closed contact feeds the trigger terminal. Mixing these up on a drawing is a common source of "the timer isn't delaying" service calls: wire a control-signal off-delay as if the supply itself were the trigger, and it never sees a transition to time from.
How Long Can a True Off-Delay Hold the Contact?
The capacitor sets a ceiling. Multifunction timers that include a true off-delay setting alongside other functions typically publish a shorter maximum time for that specific function than for their supply-present functions, because the reservoir can only store so much energy in a 17.5 mm or 22.5 mm housing. Push the dial toward the top of a true off-delay range and the repeat accuracy usually loosens too, since the discharge curve gets harder to read precisely near its tail.
Formula: Timing Error from Repeat Accuracy — Source: IEC 61812 (repeat accuracy definition)
Δt = ±a × tset
| Symbol | Description | Unit |
|---|---|---|
| Δt | Deviation of the actual delivered delay from the dial setting | s |
| a | Repeat accuracy factor of the timer (commonly in the ±0.5% to ±1% class) | % |
| tset | Delay time set on the dial or rotary switch | s |
This depends on how the manufacturer defines the true off-delay function's range relative to the unit's other functions — some multifunction timers cap it at a fraction of their multifunction maximum, others don't separate it out at all. Check the function-specific range line in the datasheet table, not just the headline range printed on the front label.
Where the True Off-Delay Function Matters in a Panel
The function earns its keep whenever the shutdown sequence has to keep running after the thing that triggered it is already gone. A door interlock that has to hold a warning beacon on for a few seconds after the operator pulls the isolator handle is a clean example — the isolator action removes power to the circuit, and only stored energy in the timer can keep the beacon logic alive to finish its countdown. Standby lighting hold-on after a mains dip, and motor coast-down indication after the starter drops out, follow the same pattern: the event that starts the timing is the same event that removes the timer's own power.
Some panel builders still spec a control-signal off-delay with a separately backed-up auxiliary supply instead of chasing a true off-delay model, particularly when the required hold time runs longer than a small capacitor can reasonably deliver. It's a fair trade: a slightly more complex wiring diagram in exchange for a delay that doesn't depend on how charged a capacitor happened to be.
Selecting and Specifying a True Off-Delay Timer
Start from the function, not the brand. Confirm the datasheet lists a true off-delay (sometimes labeled delay-on-release without auxiliary supply, or energy-storage off-delay) as a distinct function from a generic off-delay, since the two terms get used loosely in casual conversation. Check the maximum time for that specific function, the repeat accuracy class, the contact rating against the load it's switching, and whether the terminal layout is 2-wire or 3-wire. Multi-voltage models cut inventory but confirm the true off-delay range doesn't shrink further on the wide-voltage variant — some designs trade accuracy or maximum time for supply flexibility.
For a walk-through of the full selection checklist across all timing functions, see how to select a timing relay, and for the general contrast between on-delay and off-delay behavior, see on-delay timing relay and off-delay timing relay functions. Both Schneider and ABB catalog true off-delay as one of several selectable functions on their DIN-rail multifunction ranges rather than as a single dedicated part, so the model number alone won't tell a buyer which function is active — the rotary function selector on the front does.
| Criteria | True Off-Delay | Control-Signal Off-Delay | PLC Software Timer |
|---|---|---|---|
| Delay continues after supply loss | Yes, from stored capacitor charge | No — supply stays energized throughout | No — CPU and I/O must stay powered |
| Terminal layout | 2-wire (supply doubles as trigger) | 3-wire (separate supply and trigger) | Wired to a PLC input, no dedicated timer body |
| Typical hold-time ceiling | Bounded by capacitor size, shorter than the unit's other ranges | Set only by the dial range, not stored energy | Set only by program logic and scan time |
| Failure mode on undercharge | Delivers less than the set delay | Not applicable — supply is present | Not applicable — logic runs from stored program |
Frequently Asked Questions
Is a true off-delay relay the same as a normal off-delay relay?
No. Both revert the output the set time after a trigger, but a normal (control-signal) off-delay needs its own supply present through the whole delay, while a true off-delay stores energy in a capacitor so it can finish the delay even after its supply is cut.
Why does my true off-delay timer give a shorter delay than I set it for?
The most common cause is an undercharged capacitor — the unit was only powered briefly before the trigger disappeared, so the reservoir hadn't reached full charge. Repeat accuracy also loosens toward the top of the range, which shows up as a shorter-than-expected delivered time.
Can a true off-delay function replace a UPS-backed control circuit?
Only for short hold times bounded by the capacitor's capacity, typically a fraction of the timer's headline range. For longer guaranteed hold-on periods, a control-signal off-delay run from a backed-up auxiliary supply is the more predictable choice.
Is true off-delay wiring 2-wire or 3-wire?
True off-delay is 2-wire at the trigger level — the same terminals that power the unit are the ones being monitored. A control-signal off-delay needs a third terminal for a separate trigger input, since its own supply terminals stay energized.
Do Schneider and ABB timers offer a true off-delay function?
Yes. Multifunction, multi-range DIN-rail timers such as Schneider Zelio Time RE17/RE22 and ABB's CT-D range typically include a true off-delay (energy-storage) function as one of several selectable functions on the same body, alongside on-delay, control-signal off-delay, and other modes.
Conclusion
A true off-delay relay is a normal off-delay function with a capacitor added so the timing survives the loss of its own supply. That single design choice sets the wiring pattern (2-wire, supply as trigger), bounds the achievable delay (capacitor size), and defines exactly where the function belongs in a panel — anywhere the trigger event and the loss of the timer's own power happen at the same moment. For everything else, a control-signal off-delay or the timing relay engineering guide overview of the full function set is the better starting point.
