An on delay timer is a control relay that waits for a preset time interval after receiving an input voltage before energizing its output contacts. In a real circuit or installation, it changes a simultaneous 'power-on' event into a staggered sequence, preventing inrush current spikes or allowing upstream systems to stabilize before downstream loads engage. Builders and technicians commonly confuse it with an off-delay timer (which drops out after a delay when power is removed) or an interval timer (which cycles for a set time then stops regardless of continuous input).

The Internal Logic and Timing Sequence

Modern on delay timers (often called 'on-delay time delay relays') use either an internal microcontroller or an analog RC (resistor-capacitor) charging network to track time. When the input voltage is applied to the power terminals (typically labeled A1 and A2), the internal clock starts. The output relay coil remains de-energized during this countdown.

Once the preset time elapses, the internal circuit drives the output coil. This causes the Normally Open (NO) contacts to close and the Normally Closed (NC) contacts to open. The critical operational rule of an on delay timer is its drop-out behavior: when input power is removed, the output contacts revert to their resting state instantly. There is no delay on release.

The Traffic Light Analogy: Think of an on delay timer like a traffic light sensor. The inductive loop detects a car waiting at a red light (input applied). The controller waits a programmed 5 seconds to let cross-traffic clear (the delay period). Finally, the light turns green (output energizes). If the car runs the red light and leaves the sensor zone before the 5 seconds are up (input removed), the light instantly stays red (output drops out immediately).

Worked Numeric Example: Preventing HVAC Contactor Chatter

Theory is useful, but voltage sag on a jobsite is where on delay timers earn their keep. Consider a 5-ton commercial rooftop HVAC unit. The control circuit is powered by a 40VA, 24VAC control transformer. When the thermostat calls for cooling, it simultaneously sends 24VAC to the compressor contactor coil and the condenser fan motor contactor.

The compressor contactor coil requires 15VA to pull in. If both contactors attempt to pull in at the exact same millisecond, the combined inrush current causes the 40VA transformer to sag. The control voltage drops from 24VAC down to 16VAC. Because standard contactors typically require at least 18VAC to seal in, the contacts chatter violently, arc, and eventually weld shut or burn out the coil.

The Fix: We install a Macromatic TR-60122 on-delay timer (retailing around $65) on the DIN rail.

  1. Input Wiring: We wire the 24VAC control feed to the timer's A1 and A2 terminals. The TR-60122 has a wide input range (12-240VAC) and will hold in reliably even if the voltage sags to 16VAC.
  2. Time Setting: We set the multi-turn potentiometer dial to a 3.0-second delay.
  3. Output Wiring: We wire the 24VAC hot feed into the timer's common terminal (15), and run the switched output from the NO terminal (18) to the condenser fan contactor coil.

The Result: When the thermostat calls for cool, the compressor starts immediately. The TR-60122 receives 24VAC and begins its 3-second count. The compressor contactor pulls in, causing the transformer to sag momentarily. Three seconds later, the transformer has stabilized, the timer's internal logic triggers, and the NO contacts close. The fan contactor pulls in smoothly at a stable 23VAC. When the thermostat satisfies, power drops from A1/A2, and the fan drops out instantly.

Where You Meet This in Practice

Beyond HVAC voltage sag mitigation, on delay timers are foundational in several specific industrial and commercial applications:

  • Motor Star-Delta Starting: In large 3-phase induction motors, an on delay timer is used to transition the motor from a star (wye) configuration to a delta configuration. The timer waits 5 to 10 seconds for the motor to reach near-rated RPM on reduced voltage before switching the contactors to full line voltage, preventing massive mechanical shock and electrical spikes.
  • Audio/Video Rack Sequencing: Turning on a rack of 15 high-wattage audio amplifiers simultaneously will trip a 20A branch breaker. An on delay timer (or a cascaded series of them) staggers the amplifier turn-on by 2 seconds each, keeping the inrush current under the breaker's magnetic trip threshold.
  • Alarm System Entry Delays: Security panels use on delay logic to give the homeowner 30 seconds to enter the disarm code after opening the front door before the siren output is energized.
Failure Mode Warning: Ghost Timing. If you use a solid-state (triac-based) on delay timer to switch a high-impedance PLC input, the timer's internal snubber circuit may leak 2mA to 5mA of current even when 'off'. This leakage can be enough to keep a sensitive PLC input logically 'high'. Always use an electromechanical relay output (like the Omron H3Y-2) for PLC logic, or wire a 10k-ohm bleeder resistor across the PLC input terminals to drain the leakage current.

On-Delay vs. Off-Delay vs. Interval

Ordering the wrong time delay function is a common mistake that leads to returning parts to the electrical supply house. Use this matrix to verify you have the correct logic for your circuit.

Function Trigger Event Output State During Delay Output State After Delay Drop-Out Behavior
On-Delay Input Voltage Applied De-energized (Resting) Energized Instant drop-out when input is removed
Off-Delay Input Voltage Removed Energized (while input is present) Energized (during countdown) Drops out after time elapses
Interval (One-Shot) Input Voltage Applied Energized De-energized Drops out when time elapses, ignores continued input
Repeat Cycle Input Voltage Applied Cycles On/Off continuously Cycles On/Off continuously Stops cycling instantly when input is removed

Frequently Asked Questions

Can I wire an on delay timer to work as an off delay timer?

No, the internal logic is hardcoded by the manufacturer. You cannot rewire a standard analog or digital on delay timer to perform an off-delay function. To achieve off-delay logic, you must purchase a dedicated off-delay timer, a multi-function timer (which includes a DIP switch or dial to select the mode), or program a smart relay/PLC to handle the logic in software. Attempting to force off-delay behavior with external diodes or capacitors on an AC circuit is dangerous and violates NEC control circuit standards.

Why does my on delay timer output chatter or bounce?

Chatter at the exact moment the timer's output contacts close is almost always caused by voltage starvation. When the timer's internal relay pulls in, it draws a small amount of current. If it is simultaneously switching a heavy load (like a large contactor coil) on the same control transformer, the combined VA draw pulls the system voltage below the timer's minimum hold-in threshold (often around 12VAC for a 24VAC nominal system). The timer drops out, the voltage recovers, the timer starts counting again, and the cycle repeats. Fix this by upgrading the control transformer VA rating, or by powering the timer from a separate, dedicated power supply.

What is the difference between an on delay timer and a soft starter?

They solve similar problems but use entirely different methods. An on delay timer provides binary logic: the output is 100% OFF for X seconds, then snaps to 100% ON. A soft starter, conversely, uses phase-angle-fired SCRs (silicon-controlled rectifiers) to continuously ramp the voltage and current from 0% to 100% over a period of seconds. If you need to prevent a mechanical belt from snapping under sudden torque, you need a soft starter or a VFD (variable frequency drive). If you just need to sequence two distinct electrical loads so they don't pull inrush current simultaneously, an on delay timer is the correct, much cheaper choice.