How Does an Electrical Relay Work? A Bench-Tested Guide to Coil, Contacts, and Load Selection

An electrical relay works by using a low-power electromagnet (the coil) to mechanically close or open high-power contacts, providing galvanic isolation between your control circuit and your load. When current flows through the coil, it generates a magnetic field that pulls an armature, physically moving the contacts to complete or break the high-current path. This allows a fragile 3.3V microcontroller GPIO or a low-current thermostat to safely switch a 120V AC compressor or a 12V 40A automotive fuel pump without the two circuits ever sharing a direct electrical connection.

The Core Mechanism: Coil vs. Contact Side Wiring

To wire a relay correctly, you must treat it as two completely separate components sharing the same plastic housing: the control side (coil) and the load side (contacts). Standard industrial and automotive relays use the DIN 72552 numbering system to keep these sides distinct.

The Coil Side (Control Circuit)

• Pin 85 & Pin 86: These are the coil terminals. Polarity generally does not matter for standard DC coils unless the relay has an internal suppression diode or LED indicator, in which case the datasheet will mark 85 as positive and 86 as negative.

The Contact Side (Load Circuit)

• Pin 30 (Common / COM): The moving contact. This is where your load's power source (Line or Battery +) typically connects.
• Pin 87 (Normally Open / NO): The contact that connects to Pin 30 only when the coil is energized. Use this for standard "turn on when triggered" applications.
• Pin 87a (Normally Closed / NC): The contact that connects to Pin 30 when the coil is de-energized. Used for fail-safe circuits or alarm loops.

Decoding the Datasheet: Which Rating Column Governs Your Load?

The most common mistake makers and junior technicians make is looking only at the bold "10A" or "30A" printed on the relay cover. That number is almost always the resistive rating. If you use that same relay to switch an inductive or motor load, the contacts will pit, arc, and weld shut at a fraction of the rated current. For a deeper dive into electromagnetic theory and contact physics, refer to the All About Circuits relay chapter or Electronics Tutorials.

Load-Specific Selection Decision Path

Use this decision tree to determine which rating column governs your specific application and how to size the relay.

Bench Testing: How to Verify a Relay Dead and Live

Before throwing away a suspected bad relay or installing a new one into a panel, verify it on the bench. You need a digital multimeter (DMM) and a matched DC power supply.

1. Dead Testing (Coil & Contacts)

1. Test the Coil: Set your DMM to Ohms (Ω). Place probes across pins 85 and 86. A healthy 12V DC coil typically reads between 100Ω and 200Ω. A 24V DC coil will read roughly four times higher (400Ω - 800Ω). If the meter reads "OL" (Open Loop), the internal coil wire is broken; bin the relay.
2. Test the Contacts (NC): Set the DMM to Continuity (the diode/beep symbol). Place probes on Pin 30 and Pin 87a. The meter should beep (reading < 1Ω). Place probes on Pin 30 and Pin 87; it should read "OL".

2. Live Testing (Under Power)

1. Energize the Coil: Apply the exact rated voltage (e.g., 12.0V DC) to pins 85 and 86. You should hear a distinct, sharp mechanical "click."
2. Verify Contact Closure: With the coil still energized, check continuity between Pin 30 and Pin 87. It should now beep.
3. The Voltage Drop Test (Crucial): Continuity mode is not enough to detect pitted contacts. Wire a known load (e.g., a 5A 12V halogen bulb) through pins 30 and 87. While the circuit is live, set your DMM to DC Millivolts (mV) and place the probes directly on the metal tabs of Pin 30 and Pin 87. A healthy relay will show a voltage drop of less than 50mV. If you read 200mV or higher, the internal contacts are carbonized or pitted, creating a dangerous high-resistance point that will melt the housing under continuous load.

Repair vs. Replace: When to Bin the Component

A common question on the bench is whether to repair a relay or throw it away. The answer depends entirely on the physical form factor and the cost of the component.

• PCB Relays & Standard "ICE Cube" Relays (e.g., Omron LY2, Schneider RXM): ALWAYS REPLACE. These cost between $3 and $15. Never attempt to file or sand the contacts to remove pitting. The contacts are plated with a microscopic layer of silver or gold; filing removes this plating, exposing the base metal, which will oxidize and fail within days. If the coil is burned or contacts are welded, bin it.
• Heavy Industrial Contactors (e.g., Eaton C25, Siemens Sirius): REPAIR (Conditionally). If a $150+ 3-phase contactor fails, diagnose the specific failure. If the coil is burnt out but the main power contacts and arc chutes are clean, you can buy a replacement coil assembly for $30 and swap it. However, if the main copper contacts are deeply pitted or welded, replace the entire contactor. Do not file heavy contacts either.

The Default Pick: Concrete Part Recommendations

Stop scrolling through distributor catalogs trying to find the "perfect" relay for general-purpose switching. Use this decision matrix to make a concrete selection based on your exact build scenario.