Please, help with a dc power supply

Thread Starter


Joined Feb 11, 2010
This is crazy to me. I have built a dc power supply with a total 25.2 V transformer and the typical lm317, 220ohm, two caps, and a 5000k Pot. The problem is when i turn the pot down around 13V or less, the pot starts sparking and i actually burned up a lm317 at one point.
Now what is crazy to me is that it is wired correctly. The lm317 is not grounded other than tied to the pot like it should be. The 220ohms goes from one leg of the lm317 to the tie on tab and goes to the last Cap from there.
Im probably rambling so i'll stop at that , but does ANYONE have a clue as to what could cause this.
Any info. would be greatly appreciated,


Joined May 18, 2010
if you use LM317 uoy have to good qualty heatsink and your case it,s look like som risistor wrong noumber if you stell facing problem tell me your requer i well give you circuit diagram


Joined Jul 17, 2007
You say your input transformer is 25.2v.

After rectification, you should have about 25.2*1.414-1.5v on your filter cap, or about 34v.

If you then try to set your LM317 to output 13v, the LM317 will have 34v-13v=21v dropped across it, and your load will have 13v across it.
The regulator will be dissipating in excess of 62% of the power in the circuit, and your load the remainder.

If your load is 1A, your LM317 will be dissipating 21 Watts of power. The TO220 package has a thermal resistance of 4°C per Watt, so 4*21=84°C over ambient temperature, even if you have a PERFECT heat sink. The regulator will shut down at around 125°C.

If you have NO heat sink, then the inside of the LM317 would resemble what you see when you gaze into the funnel of an active volcano. :eek:

John P

Joined Oct 14, 2008
I always think of an LM317 as the most bomb-proof component in existence. Try to overheat it--even with a large voltage drop and too much current with no heat sink--and it just shuts itself down. If someone has burned one out, my guess is that he's put more voltage on it than it can stand, or it's just plain not wired properly.

Thread Starter


Joined Feb 11, 2010
Thanks all, sorry its taken me so long to get back here and read the interesting inputs. I can not for the life of me figure out how to post the schematic on this site. I'm going to build this thing one more time. You've given me some ideas, especially sgtwookie. I did actually have the lm317 smoking at one time, but i believe that was when i 'accidently' had it grounded......ooppps. yes, i have replaced that one.
one more question though, please, if its possible to answer without my specs.
The specs call for a 2watt 5k pot. Can i use a 1/2watt pot instead. I ask because i can get them cheaper and easier and i have seen at least 2 other specs, the same as mine that did not specify any wattage at all.
Thanks again and hopefully i will be able to give someone a good laugh in a couple of days as too why this thing didn't work for me.