Small laminated-core transformers for low-voltage control circuits
Generic photo: Retired electrician · CC0

24 V control transformer (HVAC)

The control transformer steps line voltage down to about 24 V AC for the thermostat, board and contactor coil. Measure primary and secondary under power; a blown low-voltage fuse means a short in the 24 V wiring, not a bad transformer.

Cross-checked ~20 min moderateBrands: All

Before you start

  • Live-voltage tests: use a CAT III-rated meter, insulated probes and one hand; keep the other hand away from the chassis
  • The primary side is line voltage
  • Replace only with the same VA rating or higher and the same primary voltage tap

Diagnostic steps

Unlock all 4 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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24 V control transformer (HVAC) · OnCall Tech