What a Delay On Delay Off Relay Actually Does (and When to Use One)
A delay on delay off relay—often called a dual-function timer or multifunction time-delay relay—combines two distinct timing modes into a single DIN-rail or plug-in package. It provides a Delay-On-Make (DOM) function, which waits a set time after power is applied before closing the output contacts, and a Delay-Off-Break (DOB) function, which keeps the contacts closed for a set time after the trigger signal is removed.
The classic bench example is an HVAC blower fan. When the thermostat calls for cooling, the relay receives 24VAC. The DOM function holds the blower off for 30 seconds, allowing the compressor to equalize head pressure and the evaporator coil to get cold before blowing air. When the thermostat drops the call, the DOB function keeps the blower running for 60 seconds to scavenge residual cooling from the ductwork before shutting down. If you only need one of these functions, buy a single-function timer; dual-function units cost roughly 20% more ($35–$60 vs $25–$45) but save a DIN-rail slot and simplify the control wiring.
The Rating Table: Which Column Governs Your Load?
The most common mistake makers and junior techs make is sizing the relay contacts based on the maximum resistive amperage printed on the casing. If a relay says '10A 250VAC', that is almost always the AC-1 (resistive) rating. If you switch a motor with it, the contacts will weld shut on the first inrush cycle. You must look at the utilization categories defined by IEC standards.
| Specification | AC-1 (Resistive) | AC-15 (Inductive) | AC-3 (Motor) |
|---|---|---|---|
| Typical Load | Heaters, Incandescent lamps | Contactor coils, Solenoids | Squirrel-cage motors, Compressors |
| Inrush Multiplier | 1.0x to 1.2x | 6x to 10x | 6x to 8x (LRA) |
| Governing Column for... | Lighting & Heating panels | Control circuit switching | Direct-on-line (DOL) motor loads |
| Example 10A Relay Rating | 10A @ 250VAC | 3A @ 250VAC | 2A @ 240VAC (approx 0.5 HP) |
Which rating column governs your load? The column that matches the physical nature of the load's inrush current. For any load with a magnetic field (motors, transformers, heavy contactor coils), the AC-3 or AC-15 column governs. Always derate a standard 10A timer relay to 2A or 3A when switching motors directly.
Wiring the Coil vs. the Contacts (and the DC Flyback Rule)
Electromechanical timer relays separate the control circuit (the coil and internal microcontroller) from the load circuit (the output contacts).
- Coil Side (A1 / A2): This is where your trigger voltage goes. On a dual timer, applying voltage to A1/A2 starts the Delay-On timer. Removing it starts the Delay-Off timer.
- Contact Side (15/16/18 or COM/NC/NO): Pin 15 is typically Common, 16 is Normally Closed (NC), and 18 is Normally Open (NO). Your load switches through these dry contacts.
If you are driving the A1/A2 coil with a DC voltage (e.g., 12VDC or 24VDC from a PLC transistor output or an ESP32/Arduino driver circuit), the collapsing magnetic field when the coil de-energizes will generate a massive reverse-voltage spike. This spike will destroy your driving transistor or PLC output. You must wire a flyback diode (like a 1N4007) in reverse bias directly across A1 and A2 (cathode to positive, anode to negative). AC coils do not require this, as the sine wave naturally crosses zero.
When protecting the coil circuit, never substitute a fast-acting glass fuse for a thermal-magnetic breaker without checking the time-current curve. Inductive coil inrush can easily pop a fast-blow 1A fuse, whereas a 1A B-curve breaker will tolerate the 20ms inrush spike without tripping.
Selection Decision Path: Sizing by Load Type
Use this decision tree to select the exact part number for your workbench or panel build. Do not guess; match the load to the row.
| Load Scenario | Governing Rating | Decision Rule | Concrete Part Pick (2026 Standard) |
|---|---|---|---|
| Resistive: Duct heaters, incandescent stairwell lighting | AC-1 | If load < 8A continuous, use internal contacts. If > 8A, use relay to trigger an external contactor. | Finder 80.01.0.240.0000 (Multifunction, 16A AC-1, ~$45) |
| Inductive: Switching large 3-phase motor contactor coils | AC-15 | Ensure contact rating exceeds the VA rating of the contactor coil. Add RC snubber across contactor coil. | Schneider Zelio RE17RAMMW (Multifunction, 8A AC-15, ~$55) |
| Motor: Direct switching of small HVAC blower fans or sump pumps (< 0.5 HP) | AC-3 | Derate heavily. A '10A' relay is only good for ~2A motor load. If motor LRA > 12A, you MUST use an external contactor. | Omron H3Y-2 AC120 (Twin-contact, 3A AC-3 rating, ~$38) |
Default Recommendation: For 90% of DIY and light commercial HVAC/maker projects where you are switching a 120VAC blower fan under 3 Amps, buy the Omron H3Y-2 AC120. It is bulletproof, widely available, and the twin-contact DPDT output gives you flexibility for both NO and NC logic without adding auxiliary relays.
Dead and Live Testing: Bench to Panel
Before energizing a newly wired control panel, verify the circuit dead. Once verified, test live to confirm the timing logic.
Dead Testing (Power Off, Locked Out)
- Coil Resistance: Set your multimeter to Ohms. Measure across A1 and A2. A 24VDC coil should read between 100Ω and 500Ω. A 120VAC coil will read much higher (typically 2kΩ to 5kΩ). If it reads OL (open), the internal coil or thermal fuse is blown.
- Contact Continuity: Measure across COM (15) and NC (16). It should read < 1Ω. Measure COM (15) and NO (18). It should read OL. If the NO contact shows continuity dead, the internal mechanical armature is jammed or contacts are welded.
Live Testing (Energized)
For deeper diagnostics on solid-state timer outputs, hook an oscilloscope across the load. You are looking for clean, square voltage transitions. If you see a slow voltage ramp or high-frequency chatter during the transition, your contacts are pitted and bouncing, or a TRIAC is failing in a solid-state variant.
Repair vs. Replace: The 15-Minute Rule
Timer relays are not like heavy-duty contactors where you can easily swap out arc chutes and contact pads. They contain surface-mount microcontrollers, timing capacitors, and miniature PCB relays sealed inside a plastic housing.
When to Repair: Only consider board-level repair if the unit is a specialized, high-current solid-state timer module costing $200+, and you have identified a visibly blown MOV or TRIAC on an accessible board. Even then, factor in your bench rate.
When to Replace: For any standard DIN-rail electromechanical delay on delay off relay (like the Omron, Finder, or Schneider models listed above, priced between $35 and $60), the rule is absolute: Replace the entire unit. If the timing drifts by more than 10% of the dial setting, the internal electrolytic timing capacitor has dried out. If the output contacts chatter, the mechanical spring has fatigued. Spending 15 minutes trying to desolder and replace a $0.50 capacitor on a $40 sealed relay is a false economy that risks a field callback when your DIY solder joint fails under panel vibration.
Always keep two spare multifunction timers in your shop inventory. When a unit fails, swap it in 30 seconds, put the dead one on the bench, and diagnose it later without holding up the machine or HVAC system.
