The Direct Answer: Is Your Relay Dead or Just Misapplied?

A faulty relay typically fails in one of two distinct ways: an open coil (preventing the magnetic pull) or welded/pitted contacts (preventing the switch from opening or closing cleanly). To check a relay, you must perform a dead test to measure coil resistance and a live test to verify contact voltage drop under load. If the coil reads infinite resistance, the relay is dead. If the contacts show a voltage drop greater than 0.1V under load, the contacts are degraded and the relay must be replaced.

Relays are not solid-state; they are mechanical wear items. Every time contacts open under load, an arc forms, slowly vaporizing the contact material. Before you throw a relay in the trash, however, you need to verify whether it actually failed, or if it was simply misapplied for the specific load type it was switching.

Decoding Relay Ratings: Which Column Governs Your Load?

The most common reason a 'new' relay fails prematurely is misreading the datasheet. A relay rated for '10A' is almost never 10A across all load types. The governing rating column changes entirely based on the physics of the load you are switching.

Load Type Governing Rating Column Typical Derating Factor Required Contact Material
Resistive (Heaters, Incandescent) Resistive Rating (e.g., 10A @ 250VAC) 100% (No derating) Silver Nickel (AgNi)
Inductive (Solenoids, Contactors) Inductive / L/R Rating 30% to 50% of resistive rating Silver Tin Oxide (AgSnO2)
Motor (Compressors, Fans) Motor / HP Rating (LRA/FLA) Derate to 1/6th of resistive rating Silver Tin Oxide (AgSnO2)
Selection Decision Path by Load Type:
  • If switching a heating element: Look at the standard AC/DC resistive column. A 10A relay handles a 10A heater.
  • If switching a solenoid valve: Look at the inductive column. A 10A relay might only handle 3A of inductive current due to the magnetic field collapse arc.
  • If switching a compressor motor: Look exclusively at the Motor or Horsepower (HP) rating. Motors draw Locked Rotor Amps (LRA) which can be 600% of Full Load Amps (FLA) for a fraction of a second. A '10A' relay will weld its contacts shut on the first startup of a 10A motor. You need a relay with a 1/2 HP or 1 HP specific rating.

Coil vs. Contact Wiring: The Two Circuits of a Relay

A relay provides galvanic isolation between two separate circuits: the low-power control circuit (coil) and the high-power load circuit (contacts). Mixing these up or failing to protect the control side is a primary cause of 'faulty' relay diagnoses when the actual fault lies in the driving circuitry.

The Coil Side (Pins 85/86 or A1/A2)

The coil is simply an inductor. When you apply the rated voltage (e.g., 12VDC or 120VAC), it generates a magnetic field that pulls the armature.

DC Coil Flyback Protection: If you are driving a DC coil with a transistor (like a 2N2222) or a microcontroller GPIO via an optocoupler, you must install a flyback diode (e.g., 1N4007) in reverse bias across the coil pins. When the coil de-energizes, the collapsing magnetic field generates a high-voltage reverse spike (inductive kickback) that will instantly destroy your driving transistor. For AC coils, use an RC snubber network or a Metal Oxide Varistor (MOV) instead of a diode.

The Contact Side (Pins 30/87/87a or COM/NO/NC)

The contacts carry the load. Pin 30 (COM) is your line input, Pin 87 (NO - Normally Open) goes to your load, and Pin 87a (NC - Normally Closed) is used if you need the circuit active when the relay is off. Always wire the load to the NO contact unless your specific safety logic demands a fail-closed state. Note that a relay is a switching device, not a protective device; you must still install upstream overcurrent protection (fuses or breakers) sized to the wire ampacity, not the relay contact rating.

How to Check a Faulty Relay: Dead and Live Testing Steps

To accurately diagnose the component, isolate it from the circuit. Testing a relay while it is still plugged into a complex harness often leads to misdiagnosing a wiring fault as a relay fault.

Step 1: The Dead Test (Coil Integrity)

  1. De-energize and Remove: Disconnect all power and pull the relay from its socket or desolder it from the PCB.
  2. Set Multimeter to Ohms (Ω): Place your probes across the coil pins (A1/A2 or 85/86).
  3. Read the Resistance:
    • 50Ω to 1000Ω: Normal. (A 12VDC relay typically reads 150-400Ω; a 120VAC relay reads 2kΩ-10kΩ).
    • Infinite (OL): The coil wire is broken internally. The relay is dead.
    • Zero or near-zero (0.1Ω): The coil is shorted. The relay is dead and likely took out your driving transistor with it.

Step 2: The Live Test (Contact Voltage Drop)

A contact resistance test with a multimeter's continuity beep is useless for power relays. A multimeter outputs milliamps; a power relay carries amps. You must test under actual load to find pitted contacts.

  1. Power the Coil: Apply the exact rated coil voltage from a known-good bench power supply. You should hear a distinct, sharp click.
  2. Apply Load to Contacts: Wire a known load (e.g., a 12V 50W halogen bulb drawing ~4A) through the COM and NO pins using a separate power source.
  3. Measure Voltage Drop: Set your multimeter to DC or AC Volts. Place the probes directly on the relay's COM and NO terminals while the load is running.
  4. Calculate Degradation: A healthy relay will show a voltage drop of less than 0.05V. If you read 0.8V across the contacts while passing 10A, the contacts are dissipating 8 Watts of heat ($P = I \times V_{drop}$). This internal heating will melt the relay housing. The contacts are heavily pitted or carbon-fouled, and the relay must be replaced.

Repair vs. Replace: The Decision Tree & Concrete Picks

Electromechanical relays are generally considered non-repairable consumables. While open-frame contactors can sometimes be serviced, sealed PCB and DIN-rail relays cannot be safely opened without compromising their dielectric strength and environmental seals. Use the decision matrix below to determine your next step.

Symptom / Test Result Diagnosis Action Required
Coil reads Infinite (OL) or 0Ω Coil burnout or internal short Replace. Check driving circuit for voltage spikes.
Coil clicks, but load receives 0V Welded contacts (stuck open) or broken armature Replace. Inspect load for short circuits.
Voltage drop across contacts > 0.1V under load Severe contact pitting / carbon buildup Replace. Do not attempt to sand or file contacts.
High contact resistance on low-current signal (< 100mA) Oxidation on contact surface (dry circuit failure) Repair (Conditional). Spray with DeoxIT D5 and cycle 20 times. If it fails again, replace with gold-flashed contacts.

The Final Verdict: What to Buy

Stop guessing and standardize your inventory. If your relay has failed and you need a reliable, globally available replacement that handles the derating curves discussed above, use these concrete picks as your default baseline:

  • For PCB / Electronics Projects (up to 16A): Buy the Omron G2R-1-E 12VDC (Part# G2R-1-E-DC12). It features AgSnO2 contacts, meaning it safely handles inductive and motor loads without premature welding, and has a massive installed base with verified datasheets.
  • For Industrial DIN Rail / Control Panels (up to 8A per pole): Buy the Finder 40.52 12VDC (Part# 40.52.9.012.0000). It is a DPDT (Double Pole Double Throw) relay with a built-in mechanical test button and LED indicator, making future live-testing trivial without a multimeter.

For deeper reading on relay contact materials and arc suppression, refer to the All About Circuits relay guide and the NFPA National Electrical Code for proper upstream overcurrent protection sizing when wiring motor loads.