A delay-on timer (TON) waits a preset interval after receiving continuous power before closing its output contacts, while a delay-off timer (TOF) closes its contacts immediately upon receiving power but waits a preset interval after losing power before opening them. What these components change in a real circuit is the exact synchronization between a control signal and a load's physical state. Instead of snapping a load on or off the millisecond a switch is thrown, delay timers shift the switching event. This protects equipment from short-cycling, clears residual thermal or kinetic energy, and sequences multi-stage machinery safely. The most common point of confusion for builders and technicians is mixing up TOF timers with "interval" (one-shot) timers, or assuming a delay-off timer can function without a continuous power source to keep its internal timing circuit alive during the countdown.
The Core Difference: Delay-On (TON) vs. Delay-Off (TOF)
To spec the right component, you have to look at what triggers the countdown and what happens when power is interrupted. According to standard NEMA and IEC timing diagrams, the behavior is strictly defined by the coil energization state.
Delay-On (TON / Delay-on-Energize): When you apply voltage to the coil, the internal clock starts. The output contacts remain in their normal state (usually open) until the preset time elapses, at which point they close. If you remove power before the time elapses, the timer resets to zero instantly. When power is removed after the contacts have closed, they open immediately.
Delay-Off (TOF / Delay-off-De-energize): When you apply voltage to the coil, the output contacts close instantly. The magic happens when you remove power from the coil. The internal clock starts counting down, and the contacts remain closed until the preset time elapses, after which they open.
A "true" TOF timer has an internal capacitor or battery backup. It only needs a momentary pulse of power to close the contacts, and it uses its internal stored energy to run the countdown clock when main power drops. A "false" TOF timer requires a continuous 120VAC or 24VAC supply to power its internal logic board; it uses a separate, dry-contact trigger input to start the countdown. If you wire a false TOF timer like a true TOF (cutting main power to start the timer), the timer will simply die and the contacts will open instantly.
Worked Numeric Example: HVAC Blower Delay-Off
Let's look at a classic TOF application: a residential electric furnace blower motor. When the thermostat calls for heat, the 5kW heating elements energize. When the room reaches the setpoint, the thermostat drops the 24VAC call-for-heat signal. If the blower stopped instantly, the residual heat trapped in the heat exchanger would overheat the limit switch and waste energy.
Control Voltage: 24VAC from the furnace transformer.
Load: 1/3 HP blower motor drawing ~4.5A at 120VAC.
The Circuit Setup:
We use an ICM Controls ICM277 or a functional equivalent solid-state off-delay relay. The relay's coil is wired across the 24VAC transformer secondary, but the thermostat's "G" (fan) or "W" (heat) terminal acts as the trigger.
- T=0 (Call for Heat): Thermostat closes, sending 24VAC to the timer's trigger. The timer's internal relay clicks instantly, sending 120VAC to the blower motor contactor.
- T=15 mins (Setpoint Reached): Thermostat opens, dropping the 24VAC trigger. The timer's internal clock starts the 90-second countdown. The blower continues pulling 4.5A.
- T=15 mins + 90 sec: Countdown reaches zero. The timer's internal triac or mechanical relay opens, cutting 120VAC to the blower. The motor coasts to a stop.
Failure Mode to Avoid: If you use a cheap mechanical timer with a 10A resistive rating for this motor, the inductive inrush current of the 1/3 HP motor (which can be 6x the running current, or ~27A for a fraction of a second) will weld the timer's internal contacts shut. Always use a timer with an HP (horsepower) rating, or use the timer to switch the coil of a heavy-duty interposing contactor.
Where You Meet Delay Timers in Practice
Beyond HVAC, you will find TON and TOF logic embedded in both discrete relay modules and programmable logic controllers (PLCs). Here is where they solve specific physical problems on the jobsite or workbench:
- Compressor Anti-Short Cycle (TON): If a commercial walk-in cooler loses power for 2 seconds and it returns, the refrigerant pressures haven't equalized. Restarting the compressor immediately will stall the motor and trip the breaker. A 3-minute TON timer prevents the contactor from pulling in until the head pressure drops.
- Stairwell and Hallway Lighting (TOF): A momentary pushbutton applies power to a TOF timer, turning the lights on instantly. Releasing the button starts a 15-minute countdown before the lights shut off.
- Sump Pump Run-On (TOF): After the float switch drops and cuts power to the pump, a 10-second TOF timer keeps the pump running to clear the check valve and prevent water hammer when the pump finally stops.
- Industrial Motor Cooling Fans (TOF): A large VFD-driven motor generates massive heat. Even after the VFD stops outputting frequency, a TOF timer keeps the external cooling blower running for 5 minutes to prevent the motor windings from baking in their own residual thermal mass.
Selecting the Right Timer: Specs That Matter
When browsing industrial relay catalogs, looking at just the time range is a mistake. You must match the electrical environment to the timer's internal switching architecture.
| Parameter | TON Typical Spec | TOF Typical Spec | Why It Matters in Practice |
|---|---|---|---|
| Timing Range | 0.1s to 10 mins | 1s to 60 mins | Determines if you need a multi-decade DIP switch or a simple potentiometer dial. |
| Contact Rating (Resistive) | 10A @ 250VAC | 10A @ 250VAC | Only valid for heating elements or incandescent bulbs. Derate heavily for motors. |
| Contact Rating (Motor/HP) | 1/2 HP @ 120VAC | 1/2 HP @ 120VAC | Crucial for inductive loads. If absent, you must use an interposing contactor. |
| Power Supply Architecture | 2-wire (Series with load) or 3-wire | 3-wire (Continuous power + Trigger) | 2-wire TONs leak a small current (mA) when off, which can cause LED bulbs to ghost or flicker. |
| Temperature Derating | -10°C to 50°C | -10°C to 50°C | Inside a sun-baked outdoor control panel at 60°C, a 10A relay may only safely handle 6A. |
Frequently Asked Questions
How do I wire a delay on and delay off timer to a 120V AC load?
For a standard 3-wire TON or TOF timer (like a Macromatic TR-60820), you wire the L1 (hot) and L2 (neutral) directly to the timer's power supply terminals to keep its internal logic alive. You then wire your control switch (thermostat, float switch, or pushbutton) to the timer's "Trigger" or "Initiate" terminal. The timer's internal relay contacts (Common and Normally Open) are wired in series with the 120V hot leg going to your load. Never wire the load directly across the timer's coil terminals, or you will destroy the internal microcontroller.
Why is my delay-off timer resetting before the time is up?
This almost always happens because you are using a "false" off-delay timer and you cut its main power supply instead of just dropping the trigger signal. A false TOF timer needs continuous 120VAC or 24VAC to power its internal clock. When you cut the main power to start the delay, the clock loses power and the contacts drop out instantly. To fix this, provide continuous power to the timer's supply terminals and use a separate dry contact to switch the trigger terminal to ground or to the hot leg to initiate the countdown.
Can I use a delay-on timer for motor soft-starting?
No. A TON timer simply delays the application of full line voltage; it does not reduce the voltage or current during startup. When the timer expires, the motor will still experience a massive locked-rotor inrush current (often 600% of full load amps). If your goal is to reduce mechanical shock or limit inrush current to prevent voltage sag on the grid, you need a true soft-starter (which uses phase-angle fired SCRs to ramp up voltage) or a VFD, not a simple time-delay relay.
What causes the internal triac in a solid-state delay timer to fail shorted?
The most common killer of solid-state delay timers is inductive kickback. When the timer switches off a highly inductive load (like a solenoid valve, a contactor coil, or a motor), the collapsing magnetic field generates a massive reverse voltage spike (often hundreds of volts). This spike punches through the timer's internal silicon triac, permanently shorting it. If you must switch an inductive load directly with a solid-state timer, wire an RC snubber network (typically 100 ohms in series with a 0.1µF capacitor) directly across the load terminals to absorb the spike.
