Relay
A low current coil switching a higher current load, and the relay types you meet in HVAC controls.
Safety
What it is
A relay uses a small electromagnet to move a set of contacts. A control circuit energizes the coil, the contacts change position, and a separate circuit is switched. This lets a 24 volt thermostat circuit control line voltage loads safely.
| Type | Coil | Typical use |
|---|---|---|
| General purpose relay | 24 volts alternating current | Switching a blower, a damper motor or an accessory |
| Fan relay | 24 volts | Energizing the indoor blower on a fan call |
| Potential relay | Senses start winding voltage | Removing a start capacitor from the circuit |
| Current relay | Senses run winding current | Start assist on small hermetic compressors |
| Solid state relay | Low voltage signal | Staging electric heat or controlling without moving contacts |
Why it matters
Relays are the point where the control circuit meets the load. When something does not run, the relay tells you which side of the problem you are on. Coil energized and contacts not passing power means the relay failed. Coil not energized means the fault is upstream in the control circuit.
Contact arrangements
- Normally open contacts are open with the coil de-energized and close when energized.
- Normally closed contacts are closed at rest and open when energized.
- A single pole double throw relay has a common terminal that moves between a normally open and a normally closed terminal.
- Contacts are rated for a specific current and load type. Using a relay below its rating leads to welding and pitting.
How to diagnose it
Measure voltage across the coil during a call
Rated voltage present means the control side is doing its job
Listen and feel for the click when the coil energizes
No movement with rated coil voltage means an open or seized coil
Measure voltage across the closed contacts
Near zero is healthy. Significant voltage across closed contacts means resistance and burning
Measure coil resistance with power off
Open reads infinite, shorted reads very low and will blow the transformer fuse
Check whether contacts stay closed with the coil de-energized
Welded contacts leave the load running continuously
Tools involved
- Multimeter
- Wiring diagram
- Clamp ammeter to confirm load current
Symptoms
Practice & Mastery
- Relay Troubleshooting Scenarios2
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