Alphabet transformers

August 3, 2016
 by Paul McGowan

Now that we understand the benefits of transformers, isolation and getting the voltage where equipment wants it, let’s take a moment to discuss the different types.

The two main styles of transformer construction are the traditional brick-looking affair called an EI, and the donut-like toroid.

Today we’ll cover the classic EI type. Here’s a picture of what the finished EI frame transformer looks like:

EI transformer manufacturer

You’ve no doubt seen many of these in equipment. They are low cost, efficient, sound good, easy to make.

The name “EI” refers to the way the iron laminations are shaped. Thin sheets of steel are cut into two types of shapes, a capital E and a capital I. Here’s a picture:


We need two coils of wire for our transformer: one for the incoming AC, the second for the outgoing. These coils of wire are wound around the center of the E. The number of times the first coil is wrapped around the center of the E, relative to the second coil, determines the output voltage. The amount and size of the steel laminations, plus the thickness of the wire in the coil, determine how many Watts the transformer can handle.

Before the outer window dressings are applied, here’s what the transformer looks like.


This transformer has more wires coming out of the bottom because it also has “taps” which means added wires at different points in the coil, so multiple voltages can be had.

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8 comments on “Alphabet transformers”

  1. Interesting post with very descriptive images . Thank you.

    If only indicated, what is the difference between the sound of EI transformers when compared to the toroids?

    It is not often see in tube amplifiers , toroidal transformers, particularly at the output, (with the exception of BAT). I do not know why

  2. Paul, this is really good stuff, I have enjoyed relearning these basic principles and understanding better what goes on “under the hood”. Keep it coming.

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