A time off delay relay (often called an OFF-delay timer) is a control component where the output contacts change state instantly when the coil is energized, but delay returning to their normal state for a preset duration after the coil power is removed. Unlike standard ON-delay timers, off-delay relays require either a continuous power supply to run their internal clock after the trigger drops, or a massive internal supercapacitor to sustain the timing circuit. They are the standard solution for run-on cooling fans, conveyor belt clearing cycles, and motor braking delays.
The Anatomy of an Off-Delay: Coil, Contacts, and the Internal Capacitor
The most common bench mistake with off-delay timers is wiring them like a standard ice-cube relay. To understand why they fail when miswired, you have to look at how they sustain power during the "off" phase.
There are two primary architectures you will encounter in 2026:
- True Off-Delay (3-Wire / Continuous Power): Terminals A1 and A2 receive continuous line voltage. A separate trigger terminal (often labeled Y1 or S) receives the switch signal. When Y1 is energized, contacts flip. When Y1 drops, the internal microcontroller uses the continuous A1/A2 power to count down the delay before resetting the contacts. The Macromatic TR-601 series is a classic example of this architecture.
- Capacitive Off-Delay (2-Wire): Power and trigger share the same A1/A2 terminals. When power is removed, an internal bank of supercapacitors keeps the solid-state timing circuit alive for the duration of the dial setting. These are cheaper but limited to lower contact ratings and shorter maximum delays (usually under 10 minutes).
Load Ratings and Selection: Which Column Governs Your Circuit?
Relays do not have a single "amp rating." A relay rated for 10A resistive will weld its contacts shut if you use it to switch a 10A inductive motor. You must look at the utilization category defined by IEC 60947-5-1 to find the governing column for your specific load.
Below is the spec-sheet breakdown for a standard industrial off-delay timer, such as the Schneider Electric Zelio RE17 series or equivalent 10A solid-state hybrid timers.
| Parameter | Resistive (AC-1) | Inductive (AC-15) | Motor (AC-3) | DC Breaking (L/R=7ms) |
|---|---|---|---|---|
| Nominal Voltage | 240V AC | 240V AC | 240V AC | 24V DC |
| Max Continuous Current | 10 A | 10 A | 10 A | 10 A |
| Governing Breaking Capacity | 10 A | 3 A | 1.5 HP / 9.8 A LRA | 2 A |
| Electrical Life (Ops) | 100,000 | 50,000 | 20,000 | 30,000 |
Which column governs? If you are using the relay to switch a 120V AC contactor coil (which is highly inductive), the 10A resistive rating is irrelevant. You must use the AC-15 Inductive column, which limits you to 3A. If your contactor coil draws 4A, this relay will suffer contact welding within a few hundred cycles.
Selection Decision Path by Load Type
| Load Type | Examples | Governing Spec Column | Action / Sizing Rule |
|---|---|---|---|
| Resistive | Heaters, incandescent lamps, PLC inputs | AC-1 / DC-1 | Size at 100% of relay max continuous current. |
| Inductive | Solenoids, contactor coils, transformers | AC-15 / DC-13 | Derate to 30% of resistive max. Use snubber circuits across loads. |
| Motor | Fractional HP fans, conveyor belts, pumps | AC-3 / HP Rating | Must meet Locked Rotor Amps (LRA) spec. Never use standard relays for >1HP without an intervening contactor. |
| Capacitive | LED drivers, SMPS power supplies | Inrush / Tungsten | Check inrush rating. LED drivers can pull 50x nominal current for 2ms, requiring specialized zero-cross relays. |
Wiring the Coil and Contacts (Including DC Flyback Protection)
Proper wiring requires treating the coil (control) side and the contact (load) side as entirely separate circuits.
Coil Side Wiring and DC Protection
For a 3-wire true off-delay relay, wire your continuous 24VDC or 120VAC to A1 and A2. Wire your trigger switch (limit switch, PLC output, or pushbutton) to Y1 and A2.
Contact Side Wiring and Overcurrent Protection
The output contacts (typically labeled 15/18 for Normally Open, and 15/16 for Normally Closed) switch the load. Always wire the line (source) voltage to the common terminal (15) and the load to the NO/NC terminals.
When sizing the branch circuit protection for the contact side, do not treat fuses and breakers as interchangeable without considering their time-current curves. A standard thermal-magnetic breaker (like a Square D Homeline) has a magnetic trip curve designed to protect wiring from dead shorts, but it will tolerate a 600% inrush current for a few milliseconds. A fast-acting semiconductor fuse (like a Bussmann Fusetron) will clear the same fault instantly. If your off-delay relay is switching a highly inductive load that arcs heavily on break-open, pair it with a fast-acting fuse sized exactly to the relay's AC-15 breaking capacity to prevent the relay from becoming a bomb in the event of a contact weld.
Bench Testing and the Repair-vs-Replace Verdict
When an off-delay circuit fails to drop out, or drops out instantly without timing, you need to isolate whether the fault is in the timer, the trigger, or the load.
How to Test Dead (Power Off)
- Verify De-energization: Use a CAT III multimeter to confirm 0V across A1/A2 and the contact terminals.
- Coil Resistance Check: Set your meter to Ohms. Measure across A1 and A2. A healthy 120VAC coil should read between 10kΩ and 15kΩ. A 24VDC coil will read much lower (typically 2kΩ to 4kΩ). If it reads infinite (OL), the internal winding or PCB trace is open. If it reads near 0Ω, it is shorted.
- Contact Continuity: Measure across 15 and 16 (NC). It should read < 1 ohm. Measure across 15 and 18 (NO). It should read OL. If the NC reads high resistance, the contacts are pitted or carbon-fouled from switching inductive loads.
How to Test Live (Power On)
Warning: Only perform live testing if you are qualified and using properly rated test leads.
- Apply continuous power to A1/A2. The relay should remain in its normal state (NO open, NC closed).
- Apply trigger voltage to Y1. You should hear an immediate internal click (or see the LED change). Verify voltage passes through 15 to 18.
- The Drop-Out Test: Remove the trigger from Y1 while leaving A1/A2 powered. Start a stopwatch. The voltage across 15 and 18 must remain stable until the dialed time elapses, at which point it should instantly drop to 0V. If it drops out the millisecond Y1 is removed, the internal timing capacitor or microcontroller has failed.
When to Repair vs. Replace
In modern industrial and DIY contexts, the verdict is almost always replace. Modern off-delay relays (like the $45–$85 solid-state hybrid units from Macromatic, Omron, or Schneider) are potted in epoxy or housed in sealed DIN-rail enclosures. You cannot open them to replace a failed timing capacitor or a burnt triac without destroying the housing and compromising the dielectric insulation.
The only exception is legacy pneumatic off-delay relays (which use a physical air-dashpot and bellows to create the delay). If a pneumatic unit fails to time out, you can sometimes rebuild it by cleaning the brass needle valve and replacing the neoprene diaphragm. However, pneumatic timers are highly susceptible to ambient temperature changes and dust. If you are maintaining a panel with pneumatic off-delays in 2026, the most reliable upgrade path is to swap them for solid-state digital off-delay timers, which offer timing accuracy of ±0.5% regardless of workshop temperature or humidity.
