An output relay is an electromechanical isolation switch used to translate a low-power logic signal from a PLC, Arduino, or HVAC control board into a high-power switching action. While solid-state relays (SSRs) handle high-cycle silent switching, the electromechanical output relay remains the bench and jobsite standard for its low contact voltage drop, multi-pole flexibility, and inherent galvanic isolation.
Choosing the right output relay requires looking past the bold '10A' printed on the plastic casing. The actual switching capacity depends entirely on the load type, the contact material (usually Silver Tin Oxide or Silver Nickel), and whether you are switching AC or DC. Below is the practical framework for selecting, wiring, and diagnosing these components.
Decoding the Output Relay Rating Table
Relay manufacturers publish multiple rating columns because a contact that can safely break 10A of resistive heater current will weld itself shut if asked to break 10A of inductive solenoid current. When reviewing datasheets like the Omron G2R series, pay attention to the utilization categories (IEC) or specific load types (UL).
| Part Number | Coil Voltage | Resistive (AC-1) | Inductive (AC-15) | Motor (120VAC HP) | Breaking Capacity |
|---|---|---|---|---|---|
| Omron G2R-1-E DC24 | 24V DC | 16A @ 250VAC | 5A @ 250VAC | 1/2 HP | 4000 VA (AC) |
| Finder 40.52.9.024 | 24V DC | 8A @ 250VAC | 3A @ 250VAC | 1/4 HP | 2000 VA (AC) |
| Schneider RSB2A080BD | 24V DC | 12A @ 250VAC | 4A @ 250VAC | 1/3 HP | 3000 VA (AC) |
Key Takeaway: The 'Resistive' column is the absolute maximum thermal limit. The 'Inductive' and 'Motor' columns represent the magnetic blowout limits—how much current the contacts can safely interrupt without sustaining an arc that melts the silver alloy.
Wiring the Coil and Contact Sides (and DC Flyback Protection)
An output relay has two completely isolated circuits: the coil (control) and the contacts (load). Mixing these up or ignoring the coil's inductive nature is the most common cause of destroyed PLC output transistors.
The Coil Side (A1 and A2)
The coil is an electromagnet. Terminals are typically labeled A1 (positive/hot) and A2 (negative/neutral). A standard 24VDC relay coil (like the Omron G2R) has a resistance of roughly 1100Ω, drawing about 22mA. This is well within the 500mA sourcing limit of most PLC transistor outputs.
The Contact Side (COM, NO, NC)
Load wiring connects to the switch contacts. Standard IEC numbering uses 11 for Common (COM), 14 for Normally Open (NO), and 12 for Normally Closed (NC). Always wire the line/hot source to the COM terminal and the load to the NO terminal. This ensures that when the relay is de-energized, the load terminal is dead, making downstream troubleshooting safer.
Selection Decision Path by Load Type
Unlike branch circuit protection where fuses and breakers are sometimes swapped by amateurs without considering time-current trip curves, relay contacts have strict, unforgiving thermal and magnetic limits. You must derate the relay based on the exact physics of the load.
| Load Type | Characteristics | Derating Factor | Selection Rule & Example |
|---|---|---|---|
| Resistive (Heaters, Incandescent) | No inrush current, no phase shift. Current is steady. | 100% of Nominal | Use the AC-1 rating. A 10A relay can switch a 10A (2400W) heater. |
| Inductive (Solenoids, Contactors, Transformers) | High inrush, severe arcing on break due to stored magnetic energy. | 30% to 40% of Nominal | Use the AC-15 rating. A 10A relay should only switch a ~3A solenoid coil. |
| Motor (Compressors, Fans, Pumps) | Locked Rotor Amps (LRA) can be 6x to 8x the Full Load Amps (FLA). | 15% to 20% of Nominal | Use the HP or Motor rating. A 10A relay is only safe for a ~1.5A to 2A motor FLA. |
| Lamp/Ballast (LED Drivers, Fluorescent) | Massive capacitive inrush current (up to 100x steady state for micro-seconds). | 20% of Nominal | Look for 'Tungsten' or 'Ballast' ratings, or use a zero-cross SSR instead. |
Reference: For deeper physics on contact arcing and material migration, see the Electronics Tutorials relay guide.
FAQ: Output Relay Diagnostics and Selection
Which output relay rating column governs my specific load?
The governing column is dictated by the load's inrush and breaking characteristics, not just its steady-state running current. If you are switching a 120VAC water heater (pure resistive), the AC-1 (Resistive) column governs. If you are switching a 24VAC irrigation solenoid valve, the AC-15 (Inductive) column governs, meaning you must derate the relay's capacity by roughly 60-70%. If you are switching a 120VAC fridge compressor, the Motor (HP/FLA) column governs. Never use the resistive rating for inductive or motor loads; the resulting arc will pit the contacts and cause them to weld shut in a closed position, creating a severe fire hazard.
How do I test an output relay dead and live with a multimeter?
Dead Testing (Power Removed & Locked Out):
- Coil Continuity: Set your multimeter to Ohms (Ω). Place probes on A1 and A2. A healthy 24VDC coil will read between 800Ω and 1200Ω. An 'OL' (open loop) reading means the internal copper wire is broken; the relay is dead.
- Contact Resistance: Set the meter to continuity or low Ohms. Place probes on COM (11) and NO (14). It should read 'OL'. Press the relay's manual test button (or apply a 9V battery briefly to the coil). The meter should drop to less than 0.5Ω. If it reads higher than 1Ω, the contacts are pitted or carbon-fouled.
Live Testing (Energized & Under Load - Use Extreme Caution):
- Coil Voltage: Set meter to AC or DC Volts. Measure across A1 and A2 while the PLC commands the output ON. It must read within 85% to 110% of the nominal coil voltage. A 24VDC coil dropping to 18VDC indicates a failing PLC output channel or excessive voltage drop in the control wiring.
- Contact Voltage Drop: With the load running, measure the voltage directly across the COM and NO terminals. A healthy relay will drop less than 50mV (0.05V). If you read >200mV under load, the contacts are degrading and generating excess heat.
When should I repair an output relay versus replace it entirely?
For 99% of standard PCB-mount or DIN-rail output relays (like the Omron G2R, Finder 40 series, or Schneider RSB), the answer is always replace, never repair. These relays are sealed or riveted to prevent dust and moisture ingress. Attempting to pry the plastic shell off to 'file down' pitted contacts destroys the silver tin oxide (AgSnO2) plating. Filing exposes the base brass or copper, which will oxidize rapidly and cause high-resistance heating on the very next cycle.
The only exception is heavy-duty industrial contactors (e.g., Square D Definite Purpose or Siemens Sirius 3RT contactors used as the load for your output relay). Contactors are designed with modular, replaceable contact blocks and coils. If a contactor coil burns out or a contact block melts, you can buy a $15 replacement coil or contact kit rather than replacing the entire $150 assembly. But for the output relay itself, treat it as a consumable component. Keep a $5 spare in the panel and swap it when the voltage drop test fails.
