HELP! How hot should a voltage regulator get?

[macumbista]

I've got a Benjolin that is supposed to be running +/-9V, and I've got an 18V AC adapter running to a Vellman regulator board (LM337 and LM317 pair), giving out 15V DC which goes to the Benjolin's 9V regulators. Total current is supposed to be about 50mA. Both the regulators on the Vellman and the Benjolin are getting quite warm! Everything runs fine, however. So how hot is too hot? I'm a bit worried about this one...

[JovianPyx]

The rule of thumb I use (heh pretty much literally) is that you should be able to pinch the regulator between thumb and finger and hold it for 10 seconds without so much pain that you _must_ remove it. If you can't do that, it's too hot.

Check the specs on the regulators for the optimum input DC voltage. Many regulators will get hot if the input voltage is too high even if the output current is small. I know from the datasheets the LM78xx regulators like 2.5 volts above the output voltage as optimum. Max input voltage is rated much higher - meaning that it won't self destruct, but it's not advised to run it that way. Read your regulator datasheets to find that optimum input voltage and design your raw DC supply to give that voltage or close too it.

[macumbista]

Thanks for your thoughts! The regulators aren't untouchable hot, but that much heat in a box makes me nervous. I put some heat sinks in already, spent some time reading about power dissipation online (so many things out there you never thought of before!!!) and I'll look into downsizing the AC power supply when I have time to get back to the project.

[elmegil]

My understanding is that regulators need to have about 3V overhead to generate a good signal (e.g. for a 12V regulator you'd want minimum 15V coming in from the bridge), but anything over that is going to get dissipated as heat. Going from 15V to 9V seems likely to be one of the causes of your heat, but not necessarily one to be concerned about as long as it can escape the enclosure and you have heatsinks in place.

[JovianPyx]

yeah, one of the things manufacturers might do is to design the power supply so that it's load drags the input down into the "goldilocks zone". So be carefull when modifying those things where you add current draw to an existing power supply. You might make it non-functional if the design was really close.

[Osal]

[bubzy]

ive had them hot enough to melt solder on before,
just sayin....
if they get this hot, then they are too hot.