A relay is an electrically operated switch that uses a low-power control signal to isolate and switch a high-power load. At its core, it provides galvanic isolation between your fragile control circuit (like an Arduino GPIO pin or a PLC output) and your heavy load circuit (like a 240V AC compressor or a 12V DC winch). When you need to ask 'what does a relay do' in a practical sense, the answer is simple: it acts as a heavy-duty remote-controlled gatekeeper, allowing a 5V, 20mA signal to safely command a 240V, 30A load without the two circuits ever physically touching.
The Core Mechanics: Coil Side vs. Contact Side
The most critical concept to grasp is that a relay consists of two entirely separate circuits housed in one package: the coil side (control) and the contact side (load).
When you apply the rated voltage to the coil pins, current flows through thousands of turns of fine copper wire wrapped around an iron core. This creates an electromagnetic field that pulls a spring-loaded steel armature. The armature's movement physically pushes the contact side's metal blades together (or apart), completing the load circuit. When you remove power from the coil, the magnetic field collapses, and the spring snaps the contacts back to their resting state.
Because these two sides are separated by air and plastic, a fault on the load side (like a dead short) generally will not travel back into your microcontroller, provided the relay's dielectric insulation holds.
| Manufacturer / Model | Coil Voltage | Coil Resistance | Contact Config | Max AC Resistive Rating | AC Breaking Capacity |
|---|---|---|---|---|---|
| Omron G2R-1-E | 12V DC | 120 Ω | SPDT | 16A @ 250VAC | 16A |
| Finder 55.34.9 | 24V DC | 1150 Ω | 4PDT | 7A @ 250VAC | 28A (total) |
| TE Connectivity T92S11D22-12 | 12V DC | 120 Ω | DPST-NO | 30A @ 277VAC | 30A |
| Song Chuan 833HM-1C-C | 5V DC | 70 Ω | SPDT | 20A @ 250VAC | 20A |
Decoding Relay Ratings: Which Column Governs Your Load?
The biggest mistake makers and junior technicians make is looking at the 'Max AC Rating' (e.g., 16A 250VAC) and assuming the relay can handle any 16-amp load. That number almost always applies strictly to resistive loads. If you are switching motors or transformers, that top-line number is physically meaningless and potentially dangerous.
To select the right relay, you must follow a decision path based on your specific load type. The governing rating column changes depending on the physics of the load you are switching.
| Load Type | Examples | Governing Rating Column | Derating / Selection Rule |
|---|---|---|---|
| Resistive | Heaters, incandescent bulbs, dummy loads | Nominal AC/DC Current (e.g., 16A 250VAC) | Use nominal rating. Note: Cold tungsten filaments have a 10x-15x inrush current; derate by 50% for incandescent. |
| Inductive | Solenoids, contactor coils, transformers | AC Inductive Rating (often marked 'cos φ = 0.4') | Derate nominal resistive rating by 30% to 50%. Inductive loads generate massive voltage spikes when broken. |
| Motor | Compressors, pumps, fans, conveyors | HP (Horsepower) or FLA/LRA (Full Load / Locked Rotor Amps) | Never use resistive ratings. A 1/2 HP motor draws ~4A running (FLA) but up to 25A starting (LRA). The relay must be explicitly rated for the LRA. |
| DC Loads | 12V/24V winches, LED strips, automotive accessories | DC Breaking Capacity (e.g., 10A 30VDC) | DC ratings are drastically lower than AC. AC arcs self-extinguish at the zero-crossing; DC arcs sustain and melt contacts. |
A relay's 'breaking capacity' refers to its ability to interrupt normal operating current without the contacts welding shut. It is not a substitute for overcurrent protection. A 30A relay cannot safely interrupt a 2,000A short-circuit fault. You must always install a properly rated fuse or circuit breaker upstream of the relay. For motor loads, ensure your breaker has the correct magnetic trip curve (e.g., a Type D or motor-rated breaker) so it tolerates the motor's brief startup inrush without nuisance tripping, while still clearing a true dead short before the relay is destroyed. For more on protective device coordination, refer to the All About Circuits relay guide and local electrical codes.
Wiring, Flyback Protection, and Field Testing
Getting the right relay is only half the battle; wiring it correctly and verifying its health in the field is where practical experience separates the pros from the hobbyists.
Coil Wiring and the Mandatory DC Flyback Diode
When you wire the contact side, follow standard gauge and torque practices for your load current. But the coil side requires specific attention, especially on DC circuits. When you de-energize a DC coil, the collapsing magnetic field induces a massive reverse voltage spike (back-EMF) that can easily exceed 100V. If your relay is driven by a bipolar transistor, MOSFET, or microcontroller GPIO, this spike will instantly punch through the silicon and destroy your driver.
The Fix: You must wire a flyback diode (a standard 1N4007 is perfect for most 12V/24V coils) in reverse parallel across the coil pins. Connect the diode's cathode (the striped end) to the positive coil pin, and the anode to the negative/ground pin. Under normal operation, the diode blocks current. When the coil is switched off, the back-EMF forward-biases the diode, safely circulating the spike back into the coil until the energy dissipates as heat. AC coils do not require flyback diodes, though an RC snubber across the contacts may be needed to suppress arcing on inductive AC loads.
How to Test a Relay: Dead and Live
When troubleshooting a suspected faulty relay, follow this exact sequence to avoid chasing ghosts in your control logic.
1. Dead Testing (Power Off & Isolated)
- Coil Continuity: Set your multimeter to Ohms. Measure across the coil pins (usually A1/A2 or 13/14). You should read the exact coil resistance listed on the datasheet (e.g., ~120Ω for a 12V G2R). If it reads infinite (OL), the internal copper wire is snapped. If it reads near 0Ω, the coil is shorted.
- Contact Continuity: Measure Common (COM) to Normally Open (NO). It must read infinite (OL). Measure COM to Normally Closed (NC). It must read less than 0.5Ω. If the NC contact reads high resistance, the contacts are oxidized or pitted.
2. Live Testing (Power On & Under Load)
- Coil Voltage: Back-probe the coil pins while the circuit is commanded 'ON'. The voltage must be within ±10% of the nominal rating. If you read 9V on a 12V coil, the relay will chatter and the contacts will bounce, causing rapid arcing. This usually means your drive transistor is undersized or your ground trace has excessive resistance.
- Contact Voltage Drop: This is the ultimate test of contact health. With the relay energized and the load running, measure the AC or DC voltage directly across the COM and NO pins. A healthy relay will drop less than 0.1V. If you measure a 1.5V drop across a 15A load, the contacts are heavily pitted and dissipating 22.5 watts of heat inside the relay housing. Replace it immediately.
When to Repair vs. Replace
In modern electrical practice, electromechanical relays are treated as consumable, non-serviceable components. If a relay fails a live voltage-drop test, exhibits welded contacts, or smells of ozone and burnt phenolic resin, replace it.
A common and dangerous mistake is attempting to 'repair' pitted contacts by filing them smooth with a jeweler's file. Relay contacts are not solid silver; they are a base metal plated with a specific silver-tin oxide or silver-nickel alloy designed to resist welding and arc erosion. Filing them strips this crucial plating, alters the precise contact gap, and ruins the spring tension. A filed relay might work for three cycles before the contacts weld permanently shut in the 'ON' position, creating a severe fire or runaway machinery hazard. For a deeper dive into contact metallurgy and arc suppression, the Electronics Tutorials relay section provides excellent schematic breakdowns. Always swap a failed unit with an exact OEM match or a verified cross-reference that matches both the coil voltage and the specific load-type breaking capacity.
