For my CNC project i originally intended using my converted PC power supply for powering the stepper motor drivers. However when testing the CNC mill I soon found that applying higher voltage significantly raised maximal speed and torque in my system. Therefore I needed to make a power supply with higher voltage output. As my stepper drivers have a maximal voltage rating of 35V I aimed at 32V output. And as the current rating pr driver is 2,5A I wanted to be able to supply at least 7,5A.
Selecting the transformer
Rectifying and filtering an AC voltage gives a DC voltage of appx the value of the amplitude (max voltage) of the AC voltage. However the voltage rating of transformers are usually given as the RMS value, where RMS of a sine wave (as the mains) is given by amplitude/√2.
The image below shows a transformer with 30V output rating, measured by an (RMS) multimeter.
The next image shows the actual AC voltage on an oscilloscope. Scale is 20V/div meaning the spacing in the grid is 20V. Here we see a peak-to-peak voltage of almost 90V, or in other words, an amplitude of just below 45V.
Adding a rectifier and a filtering/charge capacitor, we see that the resulting DC voltage is 42,5V. This corresponds well with the amplitude voltage of appx ( less than) 45V, and the voltage reduction of 0,7V trough the rectifier diodes.
As we want our DC voltage to be 32V, the amplitude of the AC voltage should also be 32V, and this gives a RMS voltage of 32V/√2 ≈ 32V/1.4 = 23V. Hence we should use a 23V 7,5A transformer.
The price of a 23V 7,5A (172,5 VA) transformer is very high. Typically €70-80 or more. However I was able to find a batch of 30+30V 300VA transformers at approximately half that price. The problem of course is that 30V RMS gives appx 42V after rectification. And the stepper drivers can only handle 35V as mentioned. Making a voltage regulator capable of regulating the 42V down to 32V is also a challenge, because at 7,5A the power dissipated in the regulator would be (42V-32V)*7,5A = 75W. and this would require a very large heatsink to ensure the regulator does not burn up.
I therefore decided to reduce the output voltage from the transformer by removing some of the windings. This is actually not that hard. Just follow these simple steps:
Remove the outer insulating layer, a long strip of plastic wound around the transformer windings. If you have the patience to unwind completely this strip, it can be re-used after you are finished with the modification. I am however not of the most patient type, so I just cut it away using a small paper knife.
Then remove the layer of tape around the circumference of the toroidal transformer, and the tape holding the wire ends in place. Now the windings are completely uncovered, and the unwinding can begin. Always keep a couple of fingers on the remaining windings to ensure they do not unwind more than desired. Remove one winding at a time. After removing 3-4 windings, measure the new voltage to determine the aproximate voltage reduction per winding removed. Then you can calculate aproximately how many more windings you need to remove. Make regular voltage measurements while removing the last few windings to ensure you do not remove too many.
When you have the correct voltage, cut the length of the wires, and use some strong tape to secure the loose winding ends to the toroidal core, as shown in the images below.
If you did fully unwind the outer plastic strip without cutting it, you can now use it to rewind the outer insulation layer. If you did destroy this plastic strip, as I did, an alternative can be to cover the transformer in PlastiDip® or similar rubber compound. If you are using PlastiDip notice that the solvent in the PlastiDip might loosen your tape holding the winding ends, so care must be taken so these does not displace during dipping. Place the transformer in a tray slightly larger than the transformer, pour in the PlastiDip, and slowly lift the transformer out of the goo.
After the first coat of PlastiDip I appliead a second coat using a small brush, to ensure everything was properly coated, applying an extra amount to the points where the windings end and the connection wires comes out from the transformer.
When the coating has solidified the transformer is ready. Note however that the wires are coated in quite tough insulating laquer, and this must be removed from the wire ends to be able to solder the wires. The easiest way is to remove this laquer by sanding the wire ends using sand paper (abrasive paper).