How to Measure Secondaries on a Power Transformer

Measure secondary outputs with a voltmeter.

Any power transformer changes one level of AC (alternating current) voltage to another, either by stepping it up (increasing it) or stepping it down (decreasing it). The input to the transformer, or primary, induces voltage into the output or secondary. A simple transformer has only one secondary, but complex transformers may have several, with voltage outputs both higher and lower than the input. To measure secondaries, you need the ability to safely operate an AC voltmeter, the ability to read the specifications to find out what voltage you should read and the ability to find the secondary output measurement points.

Things You'll Need

• Multimeter with AC voltage scale
• Technical specifications for the transformer

Instructions

1. 

   Measure each secondary output separately.

   Read the specifications for the transformer to know what output voltage(s) to read on the secondary.

2. 

   Never attempt to measure a power company transformer.

   Adjust your multimeter to read voltage AC voltage at the scale appropriate for whichever voltage you want to read first.

3. Touch the leads of the voltmeter to the two terminals for the specific output you want to read. Read the voltmeter and record the results.

4. Touch the leads of the voltmeter to the remaining terminals for the specific outputs you want to read. Read the voltmeter and record the results.

Tips & Warnings

• Secondary voltages will vary with input voltages. If a secondary measures bad, check the input on the primary.

• Check your specifications for voltage tolerances, usually expressed as plus or minus a certain amount or percentage.

• Some transformers put out dangerously high voltages on the secondary. Be extremely careful measuring these outputs.

• Never try to measure the secondary of a transformer to which you do not have authorized access. Never open a power company transformer box, enter a substation or climb a utility pole to try to measure a secondary.

References

• GlobalSpec: Power Transformer Specifications
• Triplett: Model 9045 Instruction Manual

Resources

• Elliott Sound Productions: Transformers - The Basics