How to Identify and Wire the Pins of a Relay: Pinouts, Ratings, and Load Selection

When you hold a standard electromechanical relay, the pinout can look like a jumble of numbers and letters. The direct answer for industrial relays is that the pins are strictly divided into two galvanically isolated circuits: the coil pins (designated A1 and A2) and the contact pins (designated by two-digit numbers like 11, 12, and 14). For legacy 8-pin flat relays (like the Omron MY2N), the coil is on pins 7 and 8, while the contacts map to 1, 3, 4 and 2, 6, 5. Getting these wrong means either frying your microcontroller's GPIO or failing to switch your mains load. This guide breaks down the exact pin mappings, the rating columns that actually matter for your specific load, and how to test the component on the bench.

Decoding the Pins of a Relay: Coil vs. Contact Side

The fundamental purpose of a relay is to provide galvanic isolation between a low-power control circuit and a high-power load circuit. To do this, the physical pins are split into two completely separate internal systems.

The Coil Side (Control Circuit)

The coil pins (A1 and A2 on IEC-standard relays) connect to an internal copper winding wrapped around an iron core. When you apply the nominal voltage across these pins, the winding generates a magnetic field that pulls the movable armature.

• DC Coils: Rely purely on the resistance of the wire to limit current. They are generally non-polarized unless they contain a built-in LED indicator or flyback diode, in which case A1 is positive and A2 is negative.
• AC Coils: Include a copper 'shading ring' on the core to prevent the armature from chattering at 120Hz (twice the 60Hz line frequency) as the AC sine wave crosses zero.

The Contact Side (Load Circuit)

The contact pins route the actual load current. Under the IEC 60947 standard, the numbering follows a strict logic:

• Common (COM): Ends in 1 (e.g., 11, 21). This is the moving blade.
• Normally Closed (NC): Ends in 2 (e.g., 12, 22). Connected to COM when the coil is de-energized.
• Normally Open (NO): Ends in 4 (e.g., 14, 24). Connects to COM only when the coil is energized.

Load Types and Breaking Capacity: Which Rating Governs?

A common and dangerous mistake is looking at the 'Resistive' rating on the relay's cover (e.g., '10A 250VAC') and assuming it can safely switch a 10A motor. It cannot. The rating column that governs your application depends entirely on the physics of the load you are switching.

When you open a relay contact under load, the current attempts to keep flowing, creating an electrical arc. Resistive loads (like heaters) have minimal inrush current and the arc is relatively easy to extinguish. Inductive and motor loads, however, store energy in magnetic fields. When the contacts open, that collapsing magnetic field induces a massive voltage spike that sustains a destructive arc across the pins.

If you are switching a 5A compressor motor, you must look at the AC-3 Motor rating, not the AC-1 resistive rating. A relay rated for 10A resistive might only be rated for 2A for AC-3 motor loads. For a deeper understanding of how contact materials handle these arcs, Finder's guide on relay contact materials details why AgSnO2 resists the welding and material transfer that destroys AgNi contacts under high inrush conditions.

Testing, Flyback Protection, and When to Replace

The Mandatory DC Flyback Diode

If you are driving a DC coil relay with a transistor (like a 2N2222) or a microcontroller GPIO via an optocoupler, you must install a flyback diode (e.g., 1N4007) across the coil pins. Wire the diode in reverse bias: the cathode (striped end) to the positive A1 pin, and the anode to the negative A2 pin. When the coil is de-energized, the collapsing magnetic field generates a reverse voltage spike (back-EMF) that can easily exceed 100V, instantly destroying your driving transistor. The diode provides a safe path for this inductive kickback to dissipate.

How to Test Dead (Bench Testing)

1. Coil Resistance: Set your multimeter to Ohms. Measure across A1 and A2. A 24VDC relay should read between 600Ω and 2000Ω depending on the model. If it reads 'OL' (Open Loop), the internal coil wire is broken; the relay is dead.
2. Contact Continuity: Measure across COM (11) and NC (12). It should read less than 1Ω. Measure across COM (11) and NO (14). It should read 'OL'.
3. Energize Test: Apply the nominal DC voltage to A1/A2 using a bench power supply. You should hear a distinct 'click'. Re-measure the contacts: COM-NC should now be 'OL', and COM-NO should be less than 1Ω.

How to Test Live (In-Circuit Diagnostics)

When troubleshooting a live panel, measure the voltage directly across the coil pins (A1 to A2) while the circuit is commanded 'ON'. The voltage must be within 85% to 110% of the nominal rating. A 24VDC relay will reliably pull in at 20.4V, but if your voltage drops to 18V due to a long wire run, the relay will chatter or fail to close.

Next, measure the voltage drop across the closed contacts (e.g., from pin 11 to pin 14) while the load is running. A healthy relay will show a voltage drop of less than 50mV. If you read 200mV or higher, the internal contacts are pitted, oxidized, or carbon-fouled, creating a localized heat source that will eventually melt the relay housing.

When to Repair vs. Replace

Electromechanical relays are strictly replace-only components. A common amateur mistake is to open the relay casing and use sandpaper or a file to clean pitted contacts. This removes the microscopic silver-alloy plating, alters the precise contact gap, and creates metal dust that will cause a short circuit on the very next operation.

Replace the relay immediately if:

• The coil reads open or smells of burnt varnish.
• The contacts are welded shut (COM and NO read 0Ω even when the coil is unpowered).
• The live voltage drop across closed contacts exceeds 250mV.
• The plastic housing shows any signs of thermal warping or brown scorch marks near the pins.