Using Old Transformers

By Jon Tirone

(Also known as John Fisher)

There are times when I needed a power transformer with a certain voltage that was not a stock item or if so, it was very expensive.

There is one thing good about not having a lot of money. It forces you to learn new things and to be a lot more innovative.

Transformers can be a tedious thing to tackle and in many cases it's not worth the trouble to make them when you can just go out and buy them. But I have found in a few cases some practicality in partially rewinding transformers that I have just found in the junk. The bigger the transformer that you need to wind the greater there is an advantage to rolling your own. Why? The bigger the transformer the more costly it is to buy and the bigger the transformer , the easier it is to wind yourself. I have even at times rewound secondary windings on large transformers without any machinery because the wire was so thick and there were few windings.


Therefore I have found it practical to wind the secondary coils of already existing transformers in order to get the exact voltage I wanted. In most cases it is only the secondary windings that need to be changed as there is rarely any need to change the primary windings because you are just going to plug it in the wall anyway. Fortunately, the secondary windings are usually on the outside of the primary windings. Doing this requires very little mathematical skill because the existing transformer has already been calculated so you need to just adjust the already existing known values for the secondary windings.

Here is a basic example:

Let's say you have an existing transformer with a primary of 115volts and the secondary is 12 volts and you want to make the secondary to be 24 volts for some other application. So logically speaking you know if you rewind the secondary with wire half the thickness (square surface area, not diameter) and the wire is twice as long, you would have a transformer with an output of 24 volts. It is inportant to note that a wire that is twice as thick is not twice the diameter but twice the mass of the wire. (See the chart for details of the surface area.)

In order to know the exact amount of turns for a specific voltage you can unwind the transformer counting the secondary coil windings. Say the transformer is 12 volts and when you unwound it you counted 50 turns. Simply divide the volts into the number of turns to find out how many turns per volt the transformer is. In this case, 50 divided by 12 = 4.166 turns. So now you know that there is 4.166 turns per volt. Let's say that in this case you want the transformer to be 18 volts, just multiply 18 X 4.166 = 74.988 turns. So about 75 turns will give you 18 volts. Every transformer is different depending on what the ratio of turns of the primary windings are compared to the secondary windings.

Taking Transformers Apart

For me , the hardest part is taking the laminations apart. If the laminations are not welded together they are probably bolted together. After removing the bolts I usually tap the laminations with a hammer to loosten them a little as they are probably very tight and sometimes are stuck together with some sort of varnish. If you can manage to pull out the first couple of laminations the rest gets a lot easier. I usally try to pull them out with a pair of large needle nose pliers and the first lamination or so usually gets bent up a bit. If anyone has any better ideas for this please let me know.


What about wire thickness?

We know that the higher that the voltage is, the thinner the wire for the windings will be.

Say the secondary wire is 2 millimeters thick for the 12 volt transformer. (I like using "metric" measurements for this as it's a lot easier to calculate.) Looking at the chart that I have provided, you will see that a wire that's about 2 mm thick is good for about 9.5 amps. To make an 18 volt transformer, divide 12 into 18 which is: 1.5. Now divide 9.5amps by 1.5 and you get 6.333 amperes. Now look at the chart and you will see that for 6.333 amperes the wire should be roughly 1.6 mm thick to make the transformer for 18 volts.

How to Measure the Thickness of the Wire without a Micrometer

Since this is a poor man's project, we run into the situation where we don't have a micrometer to measure the thickness of the wire. There is a simple way to measure the thickness of copper enameled wire or at least it's accurate enough for this application.

Get a pencil and mark a 1 centimeter section on it. Now wind an unknown peace of wire on it until you fill up the 1 centimeter space. Say for example, if 10 turns equaled 1 centimeter or 10 millimeters you can automatically assume that the wire is 1mm thick.

Another basic example is that if you were to count 20 turns in a 1 centimeter width you would know that the wire is .5mm thick.

The supplied chart will help you find the thickness of the wire if it is from #14 to #44 wire. It is in the purple row that says "Number of turns per cm"

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