Overheating 317 reg...

Hey guys,

I just made myself another filament power supply with a 317 regulator. Problem is that it goes into thermal shutdown after about a minute when it feeds a 6N1P heater. The circuit is the standard circuit in the datasheet with protection diodes and the regulator is sitting on a heatsink that looks like this:





I'm using one of those little pads that replaces mica/grease, which must be doing its job, 'cause the heatsink gets HOT!!!

So how much heatsinking do these things need to deliver the full 1.5A on the datasheet? The plastic enclosure I'm using comes with a flat aluminum bottom that's about 7" x 5". Would that be enough if I bolted the regulator to it instead?

Thanks!

Peace,
Al.

Al, how big is the voltage drop between input and output of the 317?
Interesting: http://sound.westhost.com/heatsinks.htm

alo
i`m no expert,but when i was doing my g9,i had heat problems,which lead to thermal shut down:as soon i connected vr to the chassis,no more problems.
another solution,is to do a L shaped aluminium metal piece,and connect vr to that piece,that in turn gets connected to the chassis.
hope i made myself understod.
best regards
pedro

You guys know this already, but I just wanted to mention "out loud" for the benefit of anyone who doesn't, that while it's a good idea for the LM317's mounting tab to be thermally coupled to the chassis, it should be electrically insulated from it. This is easily done with the usual insulators, nonductive shoulder washers, etc.

By the way, my own experience has been that while the "greaseless" insulators are convenient, you seem to get better thermal coupling with mica insulators and yucky silicon/zinc oxide grease. Just don't wear your favorite shirt when you're working with that stuff; I speak from experience!

alo
didn`t mention that,but what newyorkdave said,it is a must.
sorry
best regards
pedro

> how much heatsinking do these things need to deliver the full 1.5A

Depends on the voltage drop.

You's all so sharp!

I got the wrong transformer (22V secondary instead of 12V) and my little heatsink had to dissipate about 20W of heat!

I'll bolt the regulator to the chassis and report back.

Peace,
Al.