To check if an electromechanical relay is good, first isolate it from all power sources. Set your multimeter to the resistance (Ohms) setting and measure across the coil pins; a healthy DC coil typically reads between 50Ω and 1000Ω, while an open reading (OL) or a dead short (near 0Ω) indicates a burnt coil. Next, check for continuity across the Common (COM) and Normally Closed (NC) contact terminals. When you apply the rated DC or AC voltage to the coil, the relay should audibly click, and continuity should instantly shift from the NC to the Normally Open (NO) terminal. If the contacts show high resistance when closed, or the coil fails to pull in the armature, the relay is dead.
Decoding Relay Ratings: Which Column Governs Your Load?
A common bench mistake is assuming a '10A relay' can switch 10A of any load. Electromechanical relays like the ubiquitous Omron G2R series or Finder 55.34 are rated differently depending on the physics of the load they are switching. Inductive and motor loads generate massive voltage spikes and inrush currents that can weld contacts shut or cause destructive arcing.
| Parameter | Resistive Load (Heaters, Incandescent) | Inductive Load (Solenoids, Contactors) | Motor Load (Compressors, Fans) |
|---|---|---|---|
| Nominal Contact Rating | 10A | 5A (cos φ = 0.4) | 3A to 4A (LRA dependent) |
| Breaking Capacity | 2500VA | 1250VA | 750VA |
| Inrush / Surge Current | 10A (Steady state) | 15A (Transient break spike) | 30A+ (Locked Rotor Amps) |
| Expected Electrical Life | 100,000 operations | 50,000 operations | 10,000 to 20,000 operations |
Selection Decision Path by Load Type
- Resistive (Heaters, LED drivers): Use the primary nominal contact rating column. A 10A relay handles 10A safely.
- Inductive (Solenoids, valves, transformer primaries): Use the inductive column (usually 50% derated). The breaking capacity governs here; when contacts open, the collapsing magnetic field creates an arc. If the voltage exceeds the DC break rating, the arc will sustain and melt the contacts.
- Motor (Pumps, compressors): Use the motor column. You must size the relay to handle the Locked Rotor Amps (LRA) inrush, which is typically 6x to 8x the running current. If your motor runs at 2A, the LRA might be 14A, requiring a relay rated for at least 15A inrush or a dedicated motor contactor.
Coil vs. Contact Side Wiring and the DC Flyback Rule
Relays have two completely isolated circuits: the low-power control side (coil) and the high-power load side (contacts).
Coil Wiring (A1/A2 or +/-): The coil is simply an electromagnet. For AC coils, polarity does not matter. For DC coils, observing polarity is sometimes required if the relay includes an internal status LED or built-in suppression diode, but for standard raw DC coils, either direction will generate a magnetic field.
Contact Wiring (COM, NO, NC): The Common (COM) terminal is your feed. Normally Open (NO) powers the load only when the coil is energized. Normally Closed (NC) powers the load when the relay is at rest. Always wire the load to the switched side, and the power source to the COM terminal, to ensure the internal flex braids handle the current correctly.
Step-by-Step Testing: Dead and Live Procedures
To definitively diagnose a relay, you need to test it both de-energized (dead) and under power (live). For deeper component theory and application notes, refer to resources like SparkFun's Relay Tutorial or All About Circuits.
1. The Dead Test (Multimeter Required)
- Coil Resistance: Set the multimeter to Ohms (200Ω or 2kΩ range). Measure across the coil pins. A 12V DC relay typically reads 150Ω to 400Ω. A 24V DC relay reads 600Ω to 1200Ω. A 120V AC coil reads 2kΩ to 10kΩ. If it reads OL (open), the coil wire is broken. If it reads 0.5Ω, the coil is shorted.
- Resting Contacts: Switch to continuity mode. Place probes on COM and NC. It should beep (near 0Ω). Place probes on COM and NO. It should read OL.
- Insulation Resistance (Optional but recommended for mains): Use a megohmmeter to check resistance between the coil pins and the contact pins. It should read >100MΩ. If it reads low, internal arcing has carbonized the plastic barrier, creating a lethal shock hazard.
2. The Live Test (Voltage Drop Measurement)
A relay might click perfectly but still fail under load due to pitted, carbonized contacts. To test this:
- Energize the coil with its rated voltage (e.g., apply 12VDC to a 12V relay).
- Connect a known load (like a 12V 50W halogen bulb drawing ~4A) through the COM and NO contacts.
- Set your multimeter to DC Volts. Place the probes directly across the COM and NO terminals while the load is running.
- The Verdict: A healthy relay will show a voltage drop of less than 0.1V. If you read 0.5V or higher across the closed contacts, the internal silver-alloy contacts are pitted or oxidized, creating a bottleneck that will eventually melt the relay housing.
When to Repair vs. Replace
For PCB, automotive, and standard plug-in ice-cube relays (typically under $15), always replace. The contacts are sealed inside a plastic shell and cannot be serviced; attempting to file them down removes the protective silver-nickel plating and accelerates failure.
For heavy-duty industrial contactors (e.g., Schneider Electric TeSys D or Eaton XT series costing $100+), you can sometimes replace just the contact blocks or arc chutes if the coil tests good. However, if the main contacts show severe pitting or the voltage drop test fails, a full unit swap is the only reliable fix to prevent phase-loss in three-phase motor applications.
Frequently Asked Questions
How do you check if a solid state relay (SSR) is good?
Unlike electromechanical relays, SSRs use TRIACs or MOSFETs and will not show simple continuity on the output side when tested with a multimeter, due to internal snubber circuits and semiconductor junction characteristics. To test an SSR, you must wire it in a live circuit with an actual load (like a light bulb). Apply the control voltage (e.g., 3-32VDC) to the input terminals. If the load turns on, and the voltage drop across the output terminals is within the datasheet's specified on-state voltage (usually 1V to 1.5V for AC SSRs), the SSR is good. If the output remains open or the SSR gets excessively hot without a load, it has failed short or open.
How can I tell if a relay is bad without a multimeter?
If you lack a meter, rely on sensory diagnostics. First, swap the suspect relay with an identical, known-good relay from an adjacent circuit (e.g., swapping the horn relay with the fog light relay in an automotive fuse box). If the fault moves with the relay, the relay is bad. Second, feel the relay housing while it is supposedly energized; a relay with a shorted coil or high-resistance contacts will often become noticeably hot to the touch. Finally, listen closely: a healthy relay produces a sharp, single 'click'. A buzzing, chattering, or weak click indicates insufficient coil voltage, a failing driver transistor, or a mechanically binding armature.
Why does my relay click but the circuit doesn't work?
A click confirms that the coil is energized and the armature is physically moving, but it does not guarantee electrical continuity. The most common cause is carbon buildup or severe pitting on the contact faces, which acts as an insulator and blocks current flow. Another failure mode, especially in high-vibration environments like automotive or industrial panels, is a broken internal copper flex braid. The braid connects the moving armature to the external terminal pin; if it snaps from metal fatigue, the armature will move and click, but no current will reach the NO terminal. A live voltage drop test across the contacts will instantly confirm this failure.
