How To Test A 24 Volt Transformer

A 24-volt transformer is typically used as a control device to turn a higher voltage, generally 120 VAC, into a lower working voltage for switches or relays, such as in a furnace thermostat transformer. If you are noticing bad furnace transformer symptoms, such as regular blowouts, run through a basic testing procedure to ensure your 24-volt transformer is operating correctly.

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Before you do any testing, remove all electrical power from the electrical appliance or device. Never carry out testing with an outside source of electrical power going to the device. Always test in a safe, dry place that is away from children and animals.

You should only test a transformer if you are comfortable working with electrical equipment and are familiar with a multimeter, which is a device that measures electric current, voltage and resistance over several ranges of value.

There are two sides to the 24-volt transformer: A high side and a secondary – or low-voltage – side. The high side is the line-voltage of the transformer and the electrical connection to the feeding voltage, generally a 120-VAC power. The secondary or low-voltage side is the power that is transformed into 24 volts. There is no direct electrical connection between the high side and the low side areas of a transformer used for a 24-volt application.

Use a screwdriver to remove any electrical covers that may conceal the transformer. If necessary, check your manufacturer's instructions to ensure you access the transformer correctly.

Use a multimeter to test your transformer. Switch the multimeter to "ohms" and insert the red and black test leads into the meter. The red lead goes into the slot that reads "ohms" and the black lead goes into the common opening. Touch the two leads together. The ohmmeter should read zero ohms or a closed circuit.

Place the leads of the ohmmeter to the leads on the high side of the transformer. The meter should be identifying a resistance reading. This reading will vary by the type of transformer used in the circuit. If the meter indicates an open circuit or infinite resistance, the transformer is bad on the high side and must be replaced.

Follow the same procedure on the low side connections. The meter should give the same resistance reading results in ohms for the low side. If the meter shows an infinite or wide open reading, the low side is broken internally and the transformer must be replaced.