How to Test a Step Down Transformer

Written by richard asmus
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How to Test a Step Down Transformer
Use a multimeter to check your step-down transformer. (The Readout image by Cinneman from

A step-down transformer changes a voltage from one level to a lower level. The input voltage goes to a coil of wires called a primary, which induces a voltage into another coil of wires called a secondary. The amount of voltage induced into the secondary depends on the number of turns of wire in the coils of the primary compared to the secondary. A ratio of 10:1 turns will step normal household voltage from 120 down to 12 volts. You need to know how to use a voltmeter and work safely around electrical circuits to test a step-down transformer.

Skill level:

Things you need

  • Specification sheet for your step-down transformer
  • Multimeter with an AC voltmeter scale

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  1. 1

    Find out the specified output voltage of your step-down transformer from its specification sheet.

  2. 2

    Apply power to your step-down transformer.

  3. 3

    Set up your multimeter to read AC volts at a scale above the voltage you expect to read.

  4. 4

    Touch the two leads of your voltmeter to the output terminals of your transformer. If the transformer is OK, the voltmeter should read the same voltage as in the step-down transformer's specifications.

Tips and warnings

  • Don't confuse a step-down transformer with a charger. Although a charger may have a step-down transformer inside, the output will be DC and you will have to check it with the DC scale of your voltmeter.
  • Never attempt to read the output of a power company transformer by climbing a telegraph pole or opening a power company transformer cabinet. If you have questions or problems with your power, call your power company.

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