Variable Output Power Supply Troubles

Thread Starter


Joined Dec 11, 2006
I'm building the following variable output DC power supply for a school project.

I've checked all my capacitor & resistor values but my output LOADED is between 6V & 24V. And even with a linear pot I see a sharp jump in output voltage between 6V & 18V. Several other students have looked at my circuit and they beleive it is contected correctly but who knows for sure.

Any ideas on what to check?

Thanks for any help!


Joined Apr 20, 2004

No good guesses without poking a meter at the setup. The circuit on the hobby page looks a bit over-the-top for elaboration, though. Have you gone to National Semiconductor and got the spec sheet for an LM317? I've used 'em before - without the protection diodes and fixed resistor paralleling the adjustment pot (which may be the problem). Look up how the manufacturer suggests using his device and try it that way. As I rcall, you only need to lead DC to pin 1. Place 240 ohms from 3 to 2, and run 2 to ground through a 5K pot for the full range of adjustment.


Joined Apr 26, 2005
I would recommend you set the potentiometer to 50% of the value, connect load capable of drawing one half ampere at the output voltage you calculated, and begin troubleshooting from there.

You can measure the Output voltage and output current, the adj voltage, and the LM317 input voltage.

Report back on what you recorded for those measurements or how sucessful you were on the repair.