An on-delay timer waits a preset time after receiving a start signal before activating its output, while an off-delay timer activates its output immediately upon receiving a start signal but keeps it active for a preset time after the start signal is removed. If you are wiring control panels, building automation logic, or debugging motor starters, mastering these two timing modes is non-negotiable. They dictate the sequence of operations, protect sensitive solid-state components from thermal shock, and ensure machinery shuts down safely.

The Core Difference: On-Delay vs Off-Delay Timing

In a real circuit, these timers change the sequence of operations without requiring a programmable logic controller (PLC). They act as the brain for sequential switching. However, the terminology trips up a lot of hobbyists and junior technicians because of a linguistic trap.

The Linguistic Trap: People commonly confuse 'on-delay' with 'delaying the turn-off'. In human conversation, if I say 'delay the lights turning on', I mean they should wait to turn on. But in electrical nomenclature, an On-Delay (TON) delays the activation of the output, while an Off-Delay (TOF) delays the deactivation of the output. Furthermore, both are frequently confused with Interval (One-Shot) timers, which trigger for a fixed time and then ignore further inputs until reset.
Feature On-Delay Timer (TON) Off-Delay Timer (TOF)
Input Applied Output waits. Timer starts counting. Output turns ON immediately.
Time Elapses Output turns ON. Output remains ON.
Input Removed Output turns OFF immediately. Timer resets. Timer starts counting. Output stays ON.
Time Elapses N/A (already off) Output turns OFF.
Common IEC Symbol TON (Timer On Delay) TOF (Timer Off Delay)

Think of an off-delay timer like a hotel room keycard switch: you insert the card (input ON), and the lights turn on instantly. When you pull the card to leave (input OFF), the lights stay on for 30 seconds so you can see your way to the door before clicking off. That 30-second grace period is the off-delay.

Where You Meet This in Practice

You will find these timing relays in almost every industrial and commercial electrical panel. Here is where they do the heavy lifting:

  • HVAC Blower Motors (Off-Delay): When your thermostat calls for cooling, the AC compressor and the indoor blower start together. When the thermostat is satisfied, the compressor shuts off, but the blower runs for an extra 60 to 90 seconds. This off-delay pushes the residual cold air out of the ductwork, improving efficiency and preventing coil freezing.
  • Star-Delta Motor Starters (On-Delay): Large 3-phase induction motors start in a 'Star' configuration to reduce inrush current. After a set time, they switch to 'Delta' for full running torque. An on-delay timer ensures the motor stays in Star for exactly 5 to 10 seconds before triggering the Delta contactor.
  • Stairwell Lighting (Off-Delay / Interval): Pushing a button illuminates the stairs, and the lights remain on for 3 minutes after you release the button.
  • Generator Pre-Lube Systems (On-Delay): Before the starter motor cranks a massive diesel backup generator, an on-delay timer runs the oil priming pump for 15 seconds to ensure bearing lubrication.

Worked Numeric Example: Sizing a Star-Delta Transition

Let us look at a concrete bench example using a widely available DIN-rail timer, the Schneider Electric Zelio RE22 series. We are building a control circuit for a 10 HP, 480V 3-phase air compressor motor.

  1. Coil Voltage Selection: Our control circuit is powered by a 120VAC step-down transformer. We select the Zelio RE22R2AMU, which accepts a 24-240V AC/DC supply, giving us plenty of headroom for voltage sag during motor starting.
  2. Contact Rating: The internal relay is rated for 8 Amps at 250VAC. Since we are only switching the coils of the Star and Delta motor contactors (which draw about 0.5A each), the 8A rating is more than sufficient. We do not need an interposing relay.
  3. Setting the Dial: The compressor manufacturer specifies a 6-second Star starting time. We set the timer's multiplier switch to 'Seconds' and turn the potentiometer dial to exactly 6.0.
  4. Wiring Logic: The 'Start' pushbutton energizes the main contactor and the timer coil simultaneously. The timer's normally-open (NO) contact is wired in series with the Delta contactor coil. For 6 seconds, the NO contact remains open. At 6.0 seconds, it closes, pulling in the Delta contactor.

Real-World Scenario Walkthrough: The Burned-Out VFD Heatsink

To understand what happens when you choose the wrong timer mode, let us walk through a failure I diagnosed on a CNC router retrofit.

The Setup: A 15 HP Variable Frequency Drive (VFD) powered a spindle motor. The VFD had an internal heatsink that got extremely hot under load. The installer wired a 120VAC external cooling fan directly to the VFD's 'Run' dry contact. When the VFD ran the motor, the dry contact closed, and the fan blew on the heatsink. When the motor stopped, the contact opened, and the fan stopped.

The Numbers: The VFD output was 480V at 22 Amps. The cooling fan drew 1.2 Amps. The dry contact was rated for 2 Amps. The delay time on the circuit was 0 seconds.

The Outcome: After three weeks of heavy machining, the VFD threw a 'Heatsink Overtemp' fault and eventually blew the IGBT power modules, resulting in a $1,400 replacement bill.

What Went Wrong: The installer used an instantaneous response (effectively an on-delay of 0 seconds, or just a standard relay). When the VFD stopped outputting power to the motor, the 'Run' contact opened instantly. However, the massive aluminum heatsink retains thermal energy. The IGBTs were still sitting at 85°C, and without airflow, the residual heat soaked into the silicon junctions, degrading them over time until they failed.

The Fix: We removed the direct wiring and installed a Macromatic TOF (Off-Delay) timer. We wired the VFD 'Run' contact to the timer's input, and the timer's output to the cooling fan. We set the off-delay dial to 180 seconds. Now, when the spindle stops, the VFD run contact opens, but the timer keeps the cooling fan blasting the heatsink for three full minutes, safely dissipating the thermal mass.

Frequently Asked Questions

What happens if power is completely lost to an off-delay timer?
Standard electromechanical and basic electronic off-delay timers require continuous power to their coil/supply terminals to keep the internal timing circuit alive during the delay period. If the main supply power drops to zero, the output drops out instantly. If you need the timer to finish its cycle during a total blackout, you must specify a 'True Off-Delay' timer with internal battery backup or capacitor storage, or use a UPS on the control circuit.

Can I use a solid-state timer instead of a mechanical relay?
Yes, and you should if you are switching high-frequency loads. Solid-state timers (using TRIAC or MOSFET outputs) have no moving parts and will not suffer from contact welding or bounce. However, they have a small 'leakage current' (often 1-5mA) when off. If you are driving a highly sensitive low-current LED indicator or a high-impedance PLC input, that leakage might keep the load partially energized. In those cases, stick to an electromechanical relay output or add a bleeder resistor in parallel with the load.

How do I test if a timer is actually functioning on the bench?
Do not just listen for the click. Apply the rated coil voltage (e.g., 24VDC) and use a digital multimeter set to continuity or resistance mode across the output terminals (Common and NO). For an on-delay timer, you should see infinite resistance (OL) until the exact dialed time elapses, at which point it should drop to less than 0.5 ohms. If the resistance fluctuates or reads above 2 ohms when closed, the internal relay contacts are pitted and the unit must be replaced.