By Blake Flournoy, updated March 24, 2022

Transformers are the backbone of modern power distribution, stepping voltage up or down to match the needs of homes, businesses, and industry. When a transformer behaves oddly, an ohmmeter can quickly reveal whether its windings are healthy.

TL;DR

Measure the resistance of a transformer’s windings with an ohmmeter. First disconnect the unit, set the meter to its lowest range, and touch the red and black leads to the opposite ends of the winding. If the reading deviates significantly from the manufacturer’s specification, replace the transformer.

Why an Ohmmeter Works for Transformers

An ohmmeter reads electrical resistance, or impedance, between two points. In a transformer, the windings form a coil of wire whose resistance is fixed by the number of turns, wire gauge, and core material. By checking this resistance, you can detect shorted turns, open circuits, or excessive leakage.

Preparation Checklist

• Unplug the transformer and ensure it is isolated from any power source.
• Place the ohmmeter on its lowest range (e.g., 200 Ω or 2 kΩ depending on expected value).
• Short the meter’s leads together to verify a zero reading. If not zero, adjust the meter’s calibration knob.
• Remove any plastic or insulation from the conductors you will test.

Step‑by‑Step Test

1. Connect the red probe to one end of the primary winding and the black probe to the other end.
2. Read the displayed value. Compare it with the winding resistance listed on the transformer’s data sheet or enclosure.
3. Repeat the measurement three times to rule out transient errors.
4. If the reading is within ±10 % of the specified value, the transformer is likely healthy. A reading that is too low (shorted turns) or too high (open turns) indicates a fault that warrants replacement.

Always double‑check that the transformer is fully isolated before taking any measurement to avoid electric shock or damage to the meter.