If you are asking what is a relay switch, the direct answer is: it is an electromechanical component that uses a low-power electrical signal (the coil) to generate a magnetic field, which physically moves metal contacts to switch a separate, higher-power circuit on or off. This galvanic isolation allows a fragile 3.3V ESP32 GPIO pin or a 12V automotive sensor to safely control a 120V AC mains motor or a 30A DC headlight circuit without the high current ever touching the control side.
Unlike solid-state relays (SSRs) which use semiconductors like TRIACs or MOSFETs, electromechanical relays (EMRs) rely on physical movement. This gives them extremely low contact resistance (often under 50 milliohms) and zero off-state leakage current, but introduces mechanical wear, contact bounce, and arcing. Choosing the right relay requires understanding the strict separation between the coil side (control) and the contact side (load).
Anatomy and Rating Table: Coil vs. Contact Side
A relay is essentially two distinct circuits sharing a magnetic core. The coil side is an inductor (a spool of fine copper wire). When voltage is applied, current flows and creates a magnetic field. The contact side consists of a movable armature and fixed terminals (Common, Normally Open, Normally Closed) made of conductive alloys like silver tin oxide (AgSnO2) designed to extinguish arcs.
The most common mistake beginners make is looking only at the "10A" printed on the relay casing. That number is usually the resistive contact rating, which drops drastically for inductive or motor loads. Below is a spec-sheet-table comparing four common relays across different applications to show how coil resistance and breaking capacities vary.
| Manufacturer & Model | Application | Coil Voltage & Resistance | Contact Config | Resistive Rating (AC/DC) | Inductive / Motor Breaking Capacity |
|---|---|---|---|---|---|
| Omron G2R-1-E (12VDC) | PCB / General Purpose | 12V DC / 275 Ω | SPDT | 16A @ 250VAC / 13A @ 30VDC | 6A @ 250VAC (cos φ=0.4) |
| Finder 55.34.9.012 | Industrial DIN / Plug-in | 12V DC / 190 Ω | 4PDT | 7A per pole @ 250VAC | 3A per pole @ 250VAC (AC-15) |
| Bosch 0332014150 | Automotive (Mini ISO) | 12V DC / 80 Ω | SPDT (Changeover) | 30A @ 14VDC (Resistive) | 20A @ 14VDC (Inductive) |
| Schneider 8501KPR12 | Heavy Duty Motor Control | 120V AC / 1700 Ω | DPDT | 10A Pilot Duty | 1/3 HP @ 120VAC (Motor FLA) |
Note: Data sourced from manufacturer datasheets. Always verify specific part number suffixes for exact coil and contact materials.
Selecting a Relay by Load Type (Decision Path)
When sizing a relay, you must identify your load type first. The governing rating column changes entirely depending on whether the load is resistive, inductive, or a motor. Inductive loads store energy in magnetic fields and release it as high-voltage arcs when the contacts open. Motors draw massive inrush currents (Locked Rotor Amps) when starting.
| Load Type | Inrush & Arcing Profile | Governing Rating Column | Derating Factor (vs Resistive) | Common Examples |
|---|---|---|---|---|
| Resistive | No inrush. Minimal arcing on open. | Standard Contact Rating (e.g., 10A 250VAC) | 1.0x (Use full rated capacity) | Heaters, incandescent bulbs, power resistors |
| Inductive | No major inrush. Severe arcing on open due to collapsing magnetic field. | Inductive Breaking Capacity (cos φ ≤ 0.4) or AC-15 rating | 0.3x to 0.5x (Derate by 50-70%) | Solenoids, transformers, contactor coils, relays |
| Motor | Massive inrush (5-8x FLA). Moderate arcing on open. | Motor Load Rating (FLA / LRA / Horsepower) | 0.15x to 0.25x (Derate by 75-85%) | Compressors, HVAC fans, pumps, power tools |
| Capacitive | Massive inrush (acts like a short circuit). No arcing on open. | Capacitive Inrush Rating (often specified in peak amps) | Highly variable; use pre-charge or NTC thermistor | Switching power supplies, LED drivers, capacitor banks |
If you attempt to switch a 10A compressor motor using a relay rated for "10A Resistive," the inrush current will quickly pit and weld the contacts shut, causing the relay to fail in the closed position—a critical safety hazard. For motor loads, always look for relays specifically rated in Horsepower (HP) or with explicit Full Load Amp (FLA) and Locked Rotor Amp (LRA) specifications.
Wiring the Coil and Contact Side Safely
Wiring a relay requires treating the coil and contacts as entirely separate circuits. In automotive and standard DC relays, the coil terminals are typically labeled 85 and 86. The contacts are labeled 30 (Common), 87 (Normally Open), and 87a (Normally Closed). In industrial DIN-rail relays, the coil is usually A1 and A2, and contacts are numbered (e.g., 11/12/14 for the first pole).
When wiring a DC coil (like a 12V automotive relay driven by an ESP32, Arduino, or transistor), you must install a flyback diode (e.g., 1N4007) in reverse bias across the coil terminals (cathode to positive, anode to negative). When the coil is de-energized, the collapsing magnetic field generates a reverse voltage spike ($V = -L \frac{di}{dt}$) that can exceed 100V. Without a diode to recirculate this current, the spike will instantly destroy your driving MOSFET, optocoupler, or microcontroller GPIO. AC coils do not require DC flyback diodes, but may use RC snubber networks to suppress AC arcing.
Coil Side Wiring Steps:
- Connect your control voltage positive to terminal 86 (or A1).
- Connect terminal 85 (or A2) to the collector of your switching NPN transistor or the drain of your N-channel MOSFET. The emitter/source goes to ground.
- Solder or crimp a 1N4007 diode directly across terminals 85 and 86, with the silver stripe (cathode) facing terminal 86.
Contact Side Wiring Steps:
- For a standard switch-on activation, wire your load's positive supply to terminal 30 (Common).
- Wire the load's input to terminal 87 (Normally Open).
- Ensure the load's ground is connected back to the power supply ground. The relay only switches the "hot" or positive leg.
Testing, Troubleshooting, and Replacement
Relays are consumable electromechanical parts. The mechanical spring fatigues, and the contacts erode from arcing. Knowing how to test them and when to replace them saves hours of diagnostic time. For authoritative troubleshooting methodologies, resources like Macromatic's technical guides and All About Circuits provide excellent baseline procedures.
How to Test a Relay Dead (Bench Test)
Remove the relay from the circuit. Set your multimeter to the Ohms (Ω) range.
- Coil Resistance: Measure across 85 and 86. A 12V DC automotive relay should read between 60Ω and 120Ω. A 120V AC industrial relay might read 1,500Ω to 3,000Ω. If it reads infinite (open) or near zero (shorted), the coil is dead.
- Contact Continuity: Measure across 30 and 87a (NC). It should read less than 1Ω. Measure 30 and 87 (NO); it should read infinite (OL).
- Active Click Test: Apply the rated coil voltage (e.g., 12V DC) directly to 85 and 86 using a bench supply. You should hear a distinct, sharp click. Re-measure continuity: 30 to 87 should now be < 1Ω, and 30 to 87a should be OL.
How to Test a Relay Live (In-Circuit)
Safety Note: Only perform live testing on low-voltage DC circuits or properly enclosed AC mains circuits using rated CAT III/IV probes.
- Coil Voltage Drop: With the circuit commanded "ON", measure DC/AC voltage across A1/A2 or 85/86. It must be within 85% to 110% of the nominal coil voltage. If voltage is present but the relay doesn't pull in, the coil is internally open or the armature is mechanically jammed.
- Contact Voltage Drop (The ultimate health test): With the relay energized and the load running, measure the millivolt (mV) drop across the closed contacts (terminal 30 to 87). A healthy relay will drop less than 50mV. If you read > 150mV, the contacts are pitted, carbonized, or suffering from spring fatigue. This resistance generates heat ($I^2R$) and will eventually melt the relay housing.
When to Repair vs. Replace
In 95% of hobbyist, automotive, and PCB applications, you must replace the relay. Standard Omron, Finder, and Bosch relays are sealed or semi-sealed units. Attempting to file down pitted AgSnO2 contacts with sandpaper destroys the protective oxide layer and alters the contact gap, leading to rapid failure and potential welding.
Repair is only viable when:
- You are working with a large, expensive industrial contactor (e.g., a 100A Eaton or Schneider contactor) where the manufacturer sells modular, replaceable contact blocks or arc chutes.
- The failure is external to the relay mechanics, such as a cracked solder joint on the PCB footprint or a corroded spade terminal on an automotive harness.
If a relay fails in the welded closed position (the load stays on even when coil power is removed), this is a catastrophic failure mode often caused by switching an underrated inductive load. Replace the relay immediately, and upgrade to a model with a higher inductive breaking capacity or add an external RC snubber across the load to suppress the arc.
