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0-30V/10A Adjustable power supply - Part 1 (Transformer modification)

While cleaning out the shack in february 2007, I came across a metal cabinet with a heatsink on the back. After opening it up, it turned out to be a half-finished bench power supply that I had started some years ago and never finished. I do have a number of PSUs in the shack, but they all had their specific purpose and I really wanted a universal supply. Time to finish this project.

The transformer
The transformer I had kept in the box was an Amplimo one, 2x30V @ 5A per secondary winding. I hooked up the transformer to the mains and measured the secondary voltages : 33.2V per winding which I felt was a little much. As I was going to use a full-wave (or bridge) rectifier, 1.4V of the AC was going to be lost in the diodes of the rectifier. See the image below that illustrates this :


When adding a capacitor for smoothing, you have to keep in mind that smoothing significantly increases the average DC voltage to almost the peak value (1.4 * RMS value) which in case of my transformer would be (33.2 - 1.4) * 1.4 = 44.52V !



I had planned using the good old 723 regulator chip which can handle a maximum of 37V at it's input. Also, most of my needs would be in the 12V range, and having this high of a voltage before the regulator circuit means a lot of heat will get dissipated in the heatsink at some stage. I had a choice : Run to the store and get a new transformer, or modify the one I had laying around. Of course I went for the modification :).

Transformer modification
The Amplimo transformer has 2 secondary windings which need to be put in parallel (for this power supply anyway) to double up on maximum current. You have to be carefull to to put the windings in parallel though. This image shows the correct way of doing it - good thing the wires are color coded - and the end result :



I started by removing the insulation tape. Find the beginning and carefully remove this layer turn by turn. As you progress, it will get harder as the length of tape you have to pull through the torroid will get longer. After a few minutes, you should be looking at the bare copper wires forming the secondary windings. A carefull inspection indicated that the Yellow and Grey coded wires are at the end of each secondary winding. I started with the gray wire and unwound about 5 turns and measured the AC voltage again to give me an indication of how many turned needed to be removed to get to the AC voltage I wanted - about 23V which after bridge rectifier and smoothing would give me (23-1.4) * 1.4 = 30.24V. Having established the number of turns per volt, I carried on unwinding. For this transformer, getting to the required voltage meant removing about a full layer of copper wire from the torroid. I measured the voltage again at this stage : 22.75V - not bad. Next was the yellow coded wire. I had to remove the same amount of copper and started unwinding. Getting close to the required number of turns to be removed, I measured again, removed a turn or 2 and measured again. This way, I wanted to get the other secondary winding as close to the first as I could as they would end up in parallel. When done, the first secondary measured 22.75V, the other 22.76V. Close enough for my needs. Having secured the ends with clear tape, I started the tedious process of wrapping the whole torroid in insulation tape again. The end result is shown above.

Next - Part 2, mains section

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