An on delay off delay timer is a control relay that inserts a user-defined time gap before turning a load on when triggered, and a second time gap before turning it off when the trigger is removed. In practical circuit design, this dual-function component changes how we manage thermal sequencing, inrush currents, and anti-short-cycle protection by replacing two separate single-function relays or an oversized PLC with a single, compact DIN-rail module. However, it is frequently confused with an 'interval' (one-shot) timer, and more dangerously, installers routinely confuse the continuous power requirements with the switched trigger signal—a mistake that instantly defeats the off-delay function.
The Core Function and the 'Off-Delay Power Trap'
To use this component correctly, you must understand the difference between the power supply and the control trigger. A true off-delay requires the timer's internal microcontroller and output relay to remain energized even after the trigger signal drops.
The most common failure mode in the field is wiring the trigger switch directly to the timer's main power terminals (A1 and A2). When the trigger switch opens, the timer loses all power and the output relay drops out instantly—giving you zero off-delay.
The Fix: A1 and A2 must receive continuous, unswitched power. The trigger signal must be wired to a dedicated dry-contact input terminal (usually labeled Y1, S, or E1, depending on the manufacturer).
Think of it like a secure airlock: you must wait for the pressurization cycle before the inner door opens (on-delay), and you must wait for the decontamination cycle before the outer door unlocks (off-delay). If you cut the main power to the airlock, both doors just fail open.
Worked Example: Sequencing an HVAC Blower Motor
Let's look at a real-world numeric scenario. You are wiring a commercial rooftop unit (RTU) where the gas heat exchanger needs time to warm up before the blower pushes air, and it needs to keep blowing after the gas valve closes to extract residual heat and prevent the high-limit switch from tripping.
- Control Voltage: 24VAC (from the RTU control transformer)
- Load: 1/3 HP blower motor contactor coil (Drawing approx. 20VA / 0.85A at 24VAC)
- On-Delay Setting: 30 seconds (Allows the heat exchanger to reach 120°F before air moves across it)
- Off-Delay Setting: 90 seconds (Extracts residual heat from the exchanger into the ductwork)
Crucial E-E-A-T Note: Never wire the 1/3 HP motor directly to the timer's internal 8A or 10A relay contacts. A 1/3 HP motor at 120VAC draws roughly 6A running, but its Locked Rotor Amps (LRA) inrush can spike to 35A-40A. This will weld the timer's internal relay contacts shut on the first start cycle. The timer must switch the coil of a definite-purpose contactor (like a Siemens 3RT2015), and the contactor handles the motor load.
Where You Meet This in Practice
Beyond HVAC blowers, dual-delay timers are the workhorses of sequential automation in environments where a full PLC is overkill or too expensive to maintain.
- Compressor Anti-Short-Cycle: In refrigeration, if a high-pressure switch trips and resets rapidly, the on-delay prevents the compressor from restarting until head pressures equalize (usually 180 seconds), while the off-delay keeps cooling fans running to clear the condenser.
- Exhaust Fan Cooldowns: In commercial kitchens or welding bays, the exhaust fan runs as long as the equipment is on, but stays running for 5 to 10 minutes after the equipment is shut off to clear lingering smoke or fumes.
- Stairwell and Corridor Lighting: Triggered by a momentary pushbutton. The on-delay is usually set to 0s (instant on), while the off-delay is set to 300 seconds (5 minutes) to keep the path illuminated after the user walks away.
Decision Tree: Picking the Right Timer Module
Not all timers are built for the same environment. Use this decision path to select the exact hardware for your panel.
| IF your application requires... | THEN choose this form factor... | Concrete Pick & Price (2026) |
|---|---|---|
| A single, fixed function with basic analog dials, 120VAC mains power, and plug-in replacement capability. | Octal plug-in analog timer (8-pin or 11-pin base). | Omron H3Y-2 (Approx. $45) |
| Wide voltage ranges (24-240V AC/DC), precise digital multi-function settings, and DIN-rail mounting for industrial panels. | Slim DIN-rail digital multi-function timer (17.5mm width). | Macromatic TR-60121 (Approx. $85) |
| Complex logic, multiple interdependent delays, remote monitoring, or integration with 3+ sensors. | Programmable Smart Relay (Mini-PLC). | Schneider Zelio SR2B121BD (Approx. $220) |
Wiring and Dialing In the Delays
When wiring a modern DIN-rail dual-function timer like the Macromatic TR-60 or the Schneider Electric Zelio timing relays, follow this exact terminal sequence:
- Power the Brain: Wire your continuous 24VAC (or 120VAC) to A1 and A2. The LED power indicator should illuminate solidly.
- Wire the Trigger: Wire your control switch (thermostat, pushbutton, or sensor relay) between your voltage source and the Y1 (or S) terminal. Do not break the A1/A2 circuit.
- Wire the Load: Wire the hot/line voltage to terminal 15 (Common). Wire your contactor coil to terminal 18 (Normally Open). Terminal 16 is Normally Closed and is rarely used in this specific application.
- Set the Mode: Use the rotary dip switch to select the 'On-Delay + Off-Delay' mode (often labeled as Function C or ON/OFF depending on the brand).
- Dial the Timers: Set the T1 (On-Delay) potentiometer to 30 seconds. Set the T2 (Off-Delay) potentiometer to 90 seconds.
Verification Step: Apply the trigger to Y1. Wait exactly 30 seconds; the internal relay should click and terminal 18 should go live. Remove the trigger from Y1. The internal relay must remain clicked for exactly 90 seconds before dropping out. If it drops out instantly, you have fallen into the off-delay power trap.
Frequently Asked Questions
Can I use a standard 'Off-Delay' timer to get both functions?
No. A dedicated single-function 'Off-Delay' timer (often labeled Function E or IEC timing diagram 'E') energizes the output instantly when the trigger is applied, and only delays the drop-out when the trigger is removed. It lacks the initial on-delay pause required for heat exchanger warm-ups or motor soft-start sequencing.
What happens if the main power (A1/A2) drops during the off-delay countdown?
The timer will instantly lose memory and power, and the output relay will drop out immediately. If your application requires the off-delay to complete even during a total facility power failure, you must use a timer with an internal battery backup or wire the A1/A2 terminals through a dedicated UPS (Uninterruptible Power Supply).
Why does my timer's off-delay seem to reset if I toggle the trigger too fast?
Most digital multi-function timers have a 'retrigger' logic. If the trigger is reapplied to Y1 before the T2 off-delay countdown finishes, the timer aborts the off-delay, resets the internal clock, and immediately re-energizes the output. If you need the off-delay to finish regardless of new triggers, you must look for a timer with a 'non-retriggerable' off-delay mode, though these are rare in standard DIN-rail modules.
