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Name: Jan
Status: Student
Grade: 9-12
Location: NV
Country: United States
Date: March 2007

I was opening an alarm clock the other day to examine a transformer. I notice that it has 2 wires on one side but 4 wire on the other side. I look at a diagram of a transformer and saw that it should have only 2 wires on each side. What are the other 2 extra wire for and how do you test which one of the 4 wire is negative and positive for the output voltage?

Jan -

There are a two possibilities - your secondary may have taps or you may have two secondaries. If you check for continuity between the four secondary leads you can tell which ones are connected. If you have continuity between A and B and between C and D... but not between A and C... you have two separate secondaries. They will be A-B and C-D.

More likely, you will have two taps in the center of the secondary. If so, you will have continuity between all of the secondary leads.

Once you know which of the above you have, you can input a known a/c voltage into the primary and measure the output from your secondary or secondaries. The ratio in the voltages will be the ratio between the turns of the transformer.

If you find that you have a tapped secondary, the voltages from A to B plus B to C plus C to D will equal the voltage from A to D

Coming from an alarm clock is it likely that you have a step-down transformer - the voltage output is lower than the voltage input. Yet, you would want to be careful when testing the voltages. A step-up transformer can could put out a voltage high enough to be harmful. Use caution.

By the way, there is no positive and negative, because you are dealing with alternating current. A transformer depends on the change in current flow to operate.

Larry Krengel

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