Variable Power Supply

I was working on a low voltage cutoff circuit to protect my new LiPo battery from getting drained down too far. So naturally, I needed to simulate the voltage of the battery draining from Vmax of 8.4 volts down to Vmin of 6.0 volts.

BUT....I don't have a benchtop power supply to test my circuit. So I was messing around with a voltage divider, but I decided that if I want to do this right then I need a variable voltage supply that is a bit more sophisitcated.

I could buy a power supply for less than $100 like this one at However, I figure I would learn more and maybe save some money if I built one myself. That last bit about saving some money is questionable after I consider adding a microcontroller to monitor current sensing and and drive an LED screen then put the whole thing in a decent enclosure.

First I'm building this basic power supply around an ST Semiconductor's, LM317T voltage regulator. Check out the datasheet here. From the datasheet I'm building this.


Aug 22 - I was able to build the circuit above and sourcing Vi from two nine volt batteries in series. From this 18V source I was able to use the regulator to smoothly deliver power from 13 volts to 2 volts by adjusting the 5K poteniometer. This should be good enough to continue testing for the low voltage cutoff circuit now that I have a variable voltage power source.

Aug 28 - I used my breadboard based power supply for about a week, and decided to go ahead and solder it up and put it into a little enclosure. I also ordered a better voltage regulator than the LM317 that can handle up to 3A, and I bought a transformer so I could run the project off line power without having to worry about batteries. The components used in this little project are:

I like the Radio Shack black boxes because they are a little pliable and I can work them pretty easily with the dremel tool. On this project I cut an openining in the top of the project box to mount the potentiometer and will cut a hole in the bottom to run the line in from the transformer.

The regulator works as designed! I've got some photos to put up and show the process a little more when I get some time.

Some follow on improvements for this basic 2-12V power supply might be: