A relay is an electrically operated switch that uses a low-power control signal to isolate and switch a higher-power load. To check a relay switch, you must test the electromagnetic coil for continuity (dead testing) and verify the contacts for proper switching and minimal voltage drop under load (live testing). If the coil reads open or shorted, or the contacts show a voltage drop greater than 0.1V under load, the relay has failed.
Decoding Relay Ratings: Coil vs. Contact Side
Before probing with a multimeter, you need to understand the two electrically isolated halves of the component: the coil side (control) and the contact side (load). Mixing these up is the most common reason hobbyists fry their microcontrollers or miswire a control panel.
- Coil Side (A1/A2 or 85/86): This is the electromagnet. It dictates the control voltage required to pull the armature. Wiring a 12V DC coil to a 120V AC source will instantly vaporize the fine copper wire inside.
- Contact Side (COM, NO, NC or 11, 14, 12): These are the heavy-duty metal contacts that carry the load current. They are rated by their maximum continuous current and their breaking capacity (the ability to extinguish an arc when opening).
Here is a standard rating table based on a common industrial relay like the Omron G2R-1-E series:
| Parameter | Typical Value | Description & Application |
|---|---|---|
| Coil Voltage | 12V DC / 24V AC | The nominal voltage required to energize the electromagnet. Tolerance is usually ±10%. |
| Contact Rating (Resistive) | 16A @ 250V AC | Maximum continuous current for purely resistive loads (heaters, incandescent bulbs). |
| Breaking Capacity | 4000 VA (AC) | The maximum apparent power the contacts can safely interrupt without welding shut. |
| Coil Resistance | 275 Ω (for 12V DC) | Expected multimeter reading across the coil terminals when de-energized. |
How to Check a Relay Switch: Dead and Live Testing
Testing requires a standard digital multimeter. We will perform a dead test (power removed) to check internal integrity, followed by a live test (power applied) to verify performance under load.
Step 1: Dead Testing (Coil & Continuity)
Remove the relay from the circuit or ensure the panel is completely de-energized. Set your multimeter to the Ohms (Ω) setting, typically the 2k range.
- Test the Coil: Place probes on the coil terminals (A1 and A2, or 85 and 86). A healthy 12V DC relay should read between 100Ω and 400Ω. A 24V AC relay will read much lower (10Ω to 50Ω). If it reads OL (Open Loop), the internal wire is broken. If it reads 0.0Ω, the coil is shorted. Both mean the relay is dead.
- Test NC (Normally Closed) Contacts: Place probes on COM and NC. It should read close to 0.0Ω (usually under 0.5Ω).
- Test NO (Normally Open) Contacts: Place probes on COM and NO. It must read OL. If it reads continuity, the contacts are welded shut.
Step 2: Live Testing (Voltage & Drop)
Reinstall the relay and apply power. Set your multimeter to AC or DC Volts, matching your system.
- Verify Coil Voltage: Measure across the coil terminals while the control circuit is active. You should read nominal voltage (e.g., 11.5V to 12.5V on a 12V system). If voltage is present but the relay doesn't click, the coil is mechanically jammed or internally open.
- Measure Contact Voltage Drop: With the relay energized and the load running, place your multimeter probes directly on the COM and NO terminal screws. A healthy relay will show a voltage drop of less than 0.05V. If you read greater than 0.1V, the internal contacts are pitted or carbon-fouled, causing resistance and heat. Replace the relay.
Load Selection Decision Path: Resistive, Inductive, and Motor
A common mistake is looking only at the "16A" printed on the relay casing and assuming it can switch any 16A load. The governing rating column changes entirely based on the load type due to inrush currents and inductive arcing. According to standard multimeter and component testing practices, matching the relay to the load curve is critical for longevity.
| Load Type | Inrush Current | Governing Rating Column | Example Application & Relay Choice |
|---|---|---|---|
| Resistive (AC-1) | 1x Rated | Resistive Amps / Thermal Current | Space heaters, toasters. A standard 16A relay handles a 15A heater fine. |
| Inductive (AC-15) | 5x to 10x Rated | Inductive Amps / VA Breaking Capacity | Solenoids, contactor coils. A 16A resistive relay is only rated for ~2A to 3A inductive. |
| Motor (AC-3) | 6x to 8x (LRA) | HP Rating / FLA & LRA Specs | HVAC compressors, pumps. Must use a relay explicitly rated for motor FLA (Full Load Amps) and LRA (Locked Rotor Amps). |
| Lamp (Tungsten) | 10x to 15x Rated | Tungsten / Ballast Rating | Incandescent lighting arrays. Cold filament resistance is extremely low. |
Which rating column governs this load? Always look for the specific utilization category (AC-1, AC-3, AC-15) or the explicit HP/Motor rating on the datasheet. If your load is inductive or a motor, the general "Resistive Amps" column is irrelevant and using it will result in welded contacts.
Repair vs. Replace: Diagnosing Relay Failure
When a relay fails, the decision to repair or replace comes down to the physical form factor and the voltage class of the equipment.
- When to Replace (99% of cases): Standard PCB-mount relays, automotive blade relays, and plug-in ice-cube relays (like the Finder 40 series or Omron MY2) are sealed, disposable components costing between $3 and $15. Never attempt to pry open the plastic housing to file down pitted contacts or rewind a burnt coil. Doing so compromises the arc chamber and creates a severe fire hazard.
- When to Repair: The only time you repair a "relay" is when you are actually working with a heavy-duty industrial contactor (e.g., Allen-Bradley 100-C series). These larger electromechanical devices are designed with modular parts. If the coil burns out, you can unbolt and replace just the coil module. If the main power contacts are pitted, you can swap out the contact cartridge. You still do not file or sand the contacts; you replace the modular block.
Frequently Asked Questions
How to check a relay switch without a multimeter?
If you lack a multimeter, you can perform a functional "jump" test on an automotive-style relay. Use a fused jumper wire to apply 12V directly to the coil pins (85 and 86). You should hear a distinct, sharp click. Next, use another jumper to bridge the battery positive to pin 30, and connect a test light from pin 87 to ground. When the coil is energized, the test light should illuminate brightly. If it doesn't click, the coil is dead. If it clicks but the light doesn't turn on, the internal contacts are burnt. Note: This method does not detect high-resistance pitted contacts that a multimeter voltage-drop test would catch.
How to check a relay switch on a PCB without desoldering?
Testing a relay in-circuit is notoriously unreliable because parallel circuit paths will give you false continuity readings on the coil and contacts. To test it accurately, you must desolder at least one leg of the coil and one leg of the contact side to isolate it from the board's traces. If you hear it clicking but the load isn't powering, and the board traces look intact, the relay contacts are likely fouled, and replacing the component is the only fix.
How to check a solid state relay switch?
Solid State Relays (SSRs) contain no moving parts and use an internal optocoupler and TRIAC or MOSFET. You cannot test them with a simple continuity check. Set your multimeter to the Diode Test mode. Across the DC control input terminals, you should read a forward voltage drop of about 1.0V to 1.5V (the internal LED). Across the AC load terminals, it should read OL in both directions when off. The only definitive way to test an SSR is to apply the control voltage and measure the AC voltage across the output terminals under a live load; a failed SSR will often fail "closed" and pass voltage even with no control signal.
How to check if a relay switch is welded shut?
A relay is welded shut when the high heat of an arc melts the contact metal, fusing the NO and COM terminals together. To check for this, remove all power from the circuit. Set your multimeter to continuity mode and place the probes on the COM and NO terminals. With the relay completely de-energized and the coil disconnected, this circuit should read OL (Open Loop). If your multimeter beeps and reads near 0.0Ω, the contacts are physically fused together. The relay must be replaced, and you must investigate why the load exceeded the relay's breaking capacity.
