Crazy Transformer Idea (Part II)
By John Fisher
I was building a simple amplifier as a reference monitor for my studio. I had a hard time finding a suitable power transformer locally. I needed something around +35 volts to ground and -35 volts to ground. Dreading the thought of having one specially made or even making one myself, I got this crazing idea.
I just so happened that to have a large amount of identical 12 volt transformers on hand that were with some discarded 12 volt lights for a living room. They were all identically rating at 50 watts. I thought that if I could string them together in series I could come close to the voltage that I needed for the amplifier. As a matter of fact, this could be done in a variety of situations for future projects depending on what transformers are on hand. One important thing is that they need to be identical in power and voltage rating or else one or more could unduly draw more current then the others.
Above is a simple example of what you can do with 2 identical 12 volt transformers to make a 24 volt transformer with the added bonus of a center tap. If each transformer is rated at 50 watts, you should end up with a 100 watt, 24 volt transformer. The primary windings that go to the wall current are in parallel and the secondary windings are in series.
Here is an example of putting 4 transformers together to get an even larger voltage. I used this idea to make a power supply for a 100 watt amp that I built.
Important note: It is imperative that the transformers are strung together in phase. If not, they will cancel each other out and yield a much lower voltage then predicted and will eventually over heat. A simple way to determine this is to wire up 2 at a time and see if they work correctly. If the voltage is much lower then desired, then simply reverse one of the secondary windings and it should correct the problem. Leave the primary windings as is in parallel. If you do this test fairly quickly you won’t damage the transformers.
As in the example above, when you have the 2 pairs working correctly, you can then hook their secondary windings in series to give you 4 transformers with the secondary windings in series and the primaries in parallel. As with the 2 transformers, If you don't get the correct voltage with the 4 transformers, (which would be much lower) reverse one of the connections of the pairs in relation to the other set of transformers and this will correct the problem.
Above is a picture of my idea strictly in the experimental stage. I have the power supply hooked up to my home made stereo 100 watt amplifiers and it works just fine. Now all I have to do is get it built in a box.
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