How to Test Current Transformers

Designed to change or “transform” electrical energy, transformers reduce voltage between two or more AC sources by routing the current through wire coils. Found in many small appliances, transformers are most often used in low-voltage DC (direct current) devices, such as LED lights, laptop computers and model train sets. If a device connected to AC power through a transformer stops working, the transformer can easily be tested with a standard multimeter.

Multimeter

Instructions

- 1
  Test the electrical outlet supplying power to the transformer. Set a multimeter to the 120-volt AC setting. Place the black probe of the meter in either of the outlet’s vertical slots and insert the meter’s red probe into the receptacle’s other vertical slot. Confirm the meter reads 120 volts.
- 2
  Find the transformer’s output voltage rating printed on the transformer or the device the transformer powers. Set the meter to the DC voltage setting that matches the output of the transformer. If the transformer has a voltage rating between meter settings, such as 14 volts, set the meter to the next higher setting.
- 3
  Locate the terminals on the output side of the transformer – the side opposite from where the AC cord enters the transformer. Touch the meter’s red probe to the transformer’s positive terminal, indicated with a red screw or a “+” sign. Touch the meter’s black probe to the transformer’s negative (-) or black terminal. For devices like laptop computers, which have pin-style connectors that plug into the device, insert the red meter probe into the center of the pin and touch the back probe to the outside of the pin’s metal casing.
- 4
  Check the meter. If the transformer is functioning, the meter reading will match the output voltage listed on the transformer.

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