A relay is an electrically operated switch where a low-power control circuit (the coil) magnetically actuates a high-power load circuit (the contacts). When DC or AC current flows through the coil, it generates a magnetic field that pulls an iron armature, physically moving the contacts to open or close the load path. This provides galvanic isolation between your fragile microcontroller (like an ESP32 or Arduino) and a 120V/240V AC mains load or a high-current DC motor. But knowing the basic physics is only half the battle; selecting the right relay requires understanding how its spec sheet maps to real-world electrical loads.
The Spec Sheet Decoded: Which Rating Column Governs Your Load?
When you look at a relay datasheet, you will see multiple current ratings. The most critical rule in relay selection is this: the lowest applicable rating for your specific load type always governs. Never use the maximum resistive AC rating to switch an inductive or motor load. The table below compares three industry-standard 10A-16A PCB and DIN-rail relays to illustrate how contact material and load type drastically alter the safe switching capacity.
| Relay Model | Coil Voltage | Contact Form | Resistive Rating (AC) | Motor / Inductive Rating | Contact Material |
|---|---|---|---|---|---|
| Omron G2R-2 (PCB) | 12V DC | DPDT | 5A @ 250VAC | 2A @ 250VAC (cos φ=0.4) | AgSnO2 (Silver Tin Oxide) |
| Finder 40.52 (DIN/PCB) | 24V DC | DPDT | 8A @ 250VAC | 3A @ 250VAC (AC15) | AgNi (Silver Nickel) |
| TE OJE (PCB) | 5V DC | SPST-NO | 10A @ 125VAC | 1/3 HP @ 120VAC | AgCdO (Silver Cadmium Oxide) |
| Panasonic JW2SN (PCB) | 24V DC | DPDT | 8A @ 250VAC | 1/4 HP @ 125VAC | AgSnO2 (Silver Tin Oxide) |
Notice the massive drop-off between the resistive rating and the inductive/motor rating. A relay rated for 10A resistive might only safely switch a 2A inductive load. This is because inductive loads (like solenoids and transformers) store energy in magnetic fields, creating high-voltage arcs when the contacts open. According to the Finder Relay Application Guide, AC15 (inductive) ratings are typically 30% to 40% of the AC1 (resistive) ratings for the exact same hardware.
Wiring the Two Halves: Coil Drive and Contact Switching
A relay is essentially two separate circuits sharing a magnetic bridge. Wiring them correctly requires treating the coil side and the contact side as entirely independent systems.
The Coil Side (Control Circuit)
The coil is an inductor. On a standard DIN relay, the coil terminals are labeled A1 (positive/hot) and A2 (negative/neutral). On PCB relays, they are typically pins 2 and 7. You drive the coil using a low-side NPN transistor (like a 2N2222) or an N-channel MOSFET, controlled by your microcontroller's GPIO pin.
When you de-energize a DC relay coil, the collapsing magnetic field induces a massive reverse voltage spike (inductive kickback) that will instantly destroy your driving transistor or fry your ESP32 GPIO. You must wire a flyback diode (e.g., 1N4007 or 1N4148) in reverse bias across the coil terminals (cathode to A1/Positive, anode to A2/Negative). For AC coils, use an RC snubber network or a bidirectional TVS diode instead of a standard diode.
The Contact Side (Load Circuit)
The contacts are labeled COM (Common), NO (Normally Open), and NC (Normally Closed).
- COM: The moving armature. This is where your load's line voltage (Hot) connects.
- NO: Connects to COM only when the coil is energized. Use this for loads that should default to OFF (e.g., heaters, lights).
- NC: Connects to COM when the coil is de-energized. Use this for fail-safe circuits (e.g., emergency stop interlocks, alarms).
Always wire the mains 'Hot' or DC positive to the COM terminal, and switch the load from the NO/NC terminal. This ensures the load is completely de-energized when the relay is open, preventing shock hazards if the load wiring faults to ground.
Load Selection Decision Path: Resistive, Inductive, and Motor
Choosing the right relay means calculating the worst-case inrush current, not just the steady-state running current. Use the decision matrix below to determine how to derate your relay based on the load physics. For deeper theory on contact arcing and bounce, All About Circuits provides an excellent breakdown of the electromechanical physics at play.
| Load Type | Examples | Inrush Multiplier | Selection Rule & Derating | Arc Suppression Needed? |
|---|---|---|---|---|
| Resistive | Heaters, incandescent bulbs, toasters | 1x to 1.2x (Cold filament) | Use the standard AC1 resistive rating. Size relay at 125% of steady-state load. | Rarely (unless high voltage DC) |
| Inductive | Solenoids, contactor coils, transformers | 1x (Inrush is steady, but break current arcs heavily) | Derate to 30% of resistive rating. Use the AC15 column. Add an RC snubber across the load. | Yes (Snubber or Varistor across load) |
| Motor | HVAC fans, compressors, pumps, conveyors | 6x to 8x LRA (Locked Rotor Amps) | Must use the specific HP (Horsepower) or LRA rating on the datasheet. Never use the resistive Amp rating. | Yes (Snubber across contacts) |
| Capacitive | LED drivers, SMPS, capacitor banks | 20x to 50x (Charging inrush) | Derate heavily (often 1/10th of resistive rating). Use relays specifically rated for TV-5 or tungsten loads. | Sometimes (NTC thermistor on load side) |
Pro-Tip for LED Drivers: Modern LED power supplies have massive input capacitors. When a relay closes, those capacitors look like a dead short for the first few milliseconds. A 10A relay switching a 2A LED driver will weld its contacts shut on the first click due to the 40A+ inrush spike. Always look for a 'TV-5' or 'Tungsten' rating if switching switched-mode power supplies, or place an NTC thermistor in series with the load.
Bench Testing and the Repair-vs-Replace Verdict
Relays are mechanical wear items. The contacts physically slam together millions of times, eventually pitting, oxidizing, or welding. Here is how to diagnose a suspect relay on the bench and decide its fate.
Testing Dead (Power Removed)
- Test the Coil: Set your multimeter to Ohms (Ω). Place probes across A1 and A2 (or pins 2 and 7). A healthy 12V DC coil typically reads between 150Ω and 400Ω. A 24V DC coil will read higher (400Ω - 1000Ω). If it reads OL (Open Line), the internal copper winding is broken. If it reads near 0Ω, it is shorted.
- Test the Contacts: Set the meter to Continuity or low-Ohms. Measure between COM and NC. It should read less than 0.1Ω. Measure between COM and NO; it should read OL. If the NO contact reads a low resistance while de-energized, the contacts are welded shut.
Testing Live (Energized)
Safety Note: Only perform live testing if you are trained in mains voltage safety and using properly rated CAT III/IV meter probes.
- Apply the nominal coil voltage. You should hear a distinct, sharp 'click'. A dull buzz indicates a failing coil, low drive voltage, or AC hum from a missing shading ring.
- With the load connected and running, measure the AC voltage drop across the COM and NO terminals. A healthy relay will drop less than 0.2V. If you measure 2V to 5V dropping across the closed contacts, the internal silver is heavily pitted or carbon-fouled, generating dangerous heat.
When to Repair vs. Replace
For sealed PCB and small DIN relays (under 30A), always replace. They are not serviceable. Attempting to pry open a sealed relay to clean contacts destroys the housing and compromises the dielectric isolation.
For large industrial contactors and open-frame heavy-duty relays, you can sometimes repair them by replacing the coil assembly or the contact cartridges. However, never use sandpaper or a file to clean pitted silver contacts. As noted in Macromatic's technical guides, filing removes the silver oxide layer and exposes the softer base metal (like copper or cadmium), which will rapidly oxidize, increase resistance, and cause the contact to overheat and weld on the very next operation. If the contacts are pitted, replace the contact block or the entire unit.
