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Adjustable Bench Power Supply

UPDATE: 22 September 2005

I fried this power supply circuit working on my HBridge. Bought an variable output regulated current limiting power supply. Oh well, it was fun while it lasted.

An adjustable bench power supply

This project was an exercise in identifying useful parts from old electronics junk. I did purchase the LED readout, the potentiometer, the terminal strip, and some of the components on the regulator board.


Part Source
LM317 Regulator Glitchbuster
200V Meter readout coldfusionX store on eBay
6 pos Terminal Strip, Knob, Potentiometer Austin Electronics
5V Power Supply Old Inkjet Printer
Transformer Old Dot Matrix Printer
Big 470uf caps, 3A Rectifier diodes Dead PC power supply
3 pos Terminal strips, breadboard Local Radio Shack
Plastic Case Old Photocopier case modified with Table Saw.


  • Input: 120VAC
  • Outputs: 5V DC fixed, 32V DC fixed, Adjustable 0V-30V DC.
  • UL Listing... just kidding.


Someone suggested that if I wanted to make robots cheaply to get some old printers and take them apart for the parts inside. So far I haven't made any robots from these parts, but I have been able to recycle the parts for my own projects. For example, I have been using a power supply out of a printer to supply 5V power. I've wanted to put it in a case and add an adjustable output for testing motors and other circuits other than 5V logic.

I learned quite a bit about electronics in the course of the past six months or so. I put together a simple adjustible power supply circuit based on the LM317 datasheet (look it up on Digikey). I used a 10K ohm resistor from the output to the Adjust pin and a 100K ohm potentiometer to adjust the output.

Another important thing I learned that you can make really neat sparks and ruin the leads on your multimeter by trying to read current across ground to 120VAC. You will notice that there is some strategically placed packing tape over the back of the unit to guard against accidental fireworks.

The LED voltage meter was a $10 item I found on eBay. ColdfusionX has some interesting stuff, but be warned, none of his items come with usable instructions.

Future plans

Here are some things I'd like to do in the future to enhance the power supply:

  • Make sure there is a large high value resistor to slowly drain the charge out of the big caps in the power supply when the switch is turned off.
  • Add LEDs to indicate when the power is on.
  • Add an ammeter to show the current draw.
  • Add LED to indicate when the current gets too large.
  • Build a nicer case. Maybe ask me step-father to build me something out of wood.
  • Build a switching power supply instead of the LM317 regulator.
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