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Lesson 10 – Transformers

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Lesson 10 – Transformers

As I introduce new terms, I have included a link to Wikipedia. Read ahead a little, and if you still need help you can click on any orange word below for more information than you probably want. 🙂


What is a Transformer?

A Transformer is a device that “transfers” voltage in one circuit to another circuit, and at the same time may also change the voltage to either a higher or a lower level, thus “transforming” it. It consists of two or more windings of wire, that are wound close to each other, sometimes around or inside of an iron core.

In the previous lesson, we talked about inductors. Well, a transformer, is basically two or more inductors, wound together on the same core. One of the windings is called the primary, and the other is the secondary. There can be several secondarys, depending on the design. Sometimes the secondary winding has a connection in the middle in addition to the ends of the winding, called a center-tap.

Due to magnetic qualities, the current in the “primary” winding transfers current to the “secondary” winding. Depending on how many turns of wire there is in each winding, the voltage at the secondary winding can be higher, lower, or the same as the primary winding.

The transformer shown above is wound on an iron core. It has a heavy coil wound on top of a thinner coil of wire. If the thin coil of wire were the primary, then the voltage at the heavier secondary winding would have less voltage than that of the primary. It all depends on the ratio of primary turns of wire to secondary turns.

For example, if that transformer had 400 turns of wire in the thinner primary winding and only 40 turns of the heavier wire in the secondary winding, it would have a 10 to 1 ratio. So if the primary were hooked to 120 volts AC, the secondary, because of the 10 to 1 ratio, would have 12 volts AC as an output. Note that if the current flowing in the primary circuit were 1 Amp, the secondary current, in an imaginary 100% effecient transformer, would be 10 Amps due to the 10 to 1 ratio.

The schematic symbols above show, from left to right, a step up transformer, a step down transformer, a center tapped transformer, and a transformer with one primary and two secondarys. The last one is an “air core” transformer, meaning there is no iron as in the other four. Iron core transformers are usually used in lower frequency circuits, like power or audio. Air core transformers, sometimes called inductors, even though they have 4 terminals insteaad of just 2, are used in higher frequency devices.

Transformers are usually rated with their voltages, like Primary 120VAC – Secondary 12VAC. They usually have a current rating as well for the secondary, like 8 Amps. Transformers do not work with Direct Current because we need the voltage changing to produce the magnetic “flux” required for the secondary to work.


Test your knowledge if you feel like it with a little test. No cheating now!

Q1: True or False….Primary and Secondary windings always have the same number of turns.

Q2: Transformers work with _______________ current.

Q3: Iron core transformers are used for _______________ frequencies.


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