I have found a rather extensive source of used portable drills that people have discarded because the batteries gave out and new batteries for obsolete drills are too expensive. I would like to use these units for several projects around the shop. The problem is, their operating voltages vary from 3.5 to 24 volts (DC). I would like to construct a DC power supply with an adjustable output voltage. At a garage sale recently I obtained a Staco, Inc. brand “Variable autotransformer”, (120v input, 0-140v output, 3.5kva, 25 a.), which appears to be unused. Because of its size and weight, it appears to be overkill, but I would like to use it for my power supply source. Can anyone help me with construction assistance for this project ?? I’m not totally “electrically challenged”, but have not idea about circuits and specific components for this project. I believe most of these drill motors would draw, at the most, about 5amps at their respective voltage. I researched the archives for this type of device, but all of the power supply designs seem to be very low current outputs for electronic uses.

 

First, I advice you to use a two windings step down transformer which offers isolation from the mains voltage. Auto transformers don't offer isolation and if a short occurs between their windings the whole 120V will appear at your motors and may be lethal.
To make an adjustable power supply you can use this linear 10Amp variable voltage regulator:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C3,P1243,D1059

 

Thanks for the quick response, mik3, and the warning about using the autotransformer. I guess I'm more electrically challenged than I thought. The url that you referenced appears to be for a regulator, with a low DC voltage input. With just an AC transformer as a starting point, I need help with necessary diagram/components to get to the DC low voltage output with that kind of current.
Thanks, Art

 

The hard part of this is the rectification and filtering of the AC voltage. To achieve 5A at 24V you will need a 120V/24V 140VA transformer, a 10A bridge rectifier which can handle large transient currents due to the charging of the filtering capacitor when you first turn the power supply on and a 15mF (approximately) 50V capacitor.
Another better option is to use a switched mode power supply which will be more efficient.