How to Use an Ohmmeter to Test an AC Transformer

Transformers are electrical devices used to transfer electrical energy between two or more circuits. Commonly used to decrease the voltage of electricity generated at power plants to the lower-voltage currents capable of powering home appliances, lighting and similar systems, transformers use electromagnetic induction and are critical to power distribution and consumption. In the event that your transformer appears to be malfunctioning, you can easily test its operations through the use of an ohmmeter.

TL;DR (Too Long; Didn't Read)

The resistance of an alternating current (AC) transformer is held within the wires wound around its core. Transformers experience power loss due to load resistance, which you can test with an ohmmeter by touching the meter's red and black pins to opposite ends of the transformer's wiring. Just be certain to disconnect the transformer from the circuit before testing to avoid the risk of serious injury. If the ohmmeter's reading is significantly different than the resistance listed on the transformer's data sheet, it should be removed and replaced immediately.

Ohmmeters and Transformers

Ohmmeters are used to test the electrical resistance (sometimes referred to as the impedance) present in a device or circuit, measured in ohms. In the case of a transformer, which uses alternating current (AC) to increase or decrease the voltage of the electrical energy running through it, this resistance is held within the coiled wires wound around its core.

Test Preparations

To test a transformer, however, you will need to disconnect it from the circuit before doing anything else. This will prevent inaccurate readings and ensure your own safety. Put your ohmmeter on its lowest scale and after removing the plastic sheaths from the conductors, touch its leads together to verify that it is ready for testing. If the reading is zero, you can continue. If it is not zero, adjust the variable knob to make the ohmmeter read zero before proceeding.

Simple Testing

To test your transformer, simply touch the red and black pins of your ohmmeter to the opposite ends of the transformer's wiring. Read the display and compare the resistance on your ohmmeter to the resistance stated on the transformer's data sheet. This is sometimes listed on the transformer's enclosure. If there is a dramatic difference between the reading and the listed resistance, it is likely that the transformer is faulty and should be removed and replaced as soon as possible. Check three times before drawing a conclusion, as your ohmmeter may not be perfectly accurate.

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