[sdiy] Regulator / PS question

[Eric Brombaugh]

You're right - the TO-220 tabs aren't at the same potential. Just look 
for the datasheets at the manufacturers websites. Eg:

http://www.national.com/search/search.cgi/main?keywords=lm7905
http://www.national.com/search/search.cgi/main?keywords=lm7805

On the 78xx, the tab is GND, on the 79xx the tab is the input. Use 
appropriate insulating hardware on the 79xx parts.

Eric

Tim Parkhurst wrote:
> Hi All,
>
> I'm working on some documents, and I'm trying to find a little info
> about using 78xx and 79xx voltage regulators. I seem to recall finding
> out at some point that you can't attach a 78xx and a 79xx to a common
> heat sink, because the heat sink tabs on the TO-220 packages are NOT
> at the same potential. HOWEVER, I can't find anything to confirm/deny
> this. I seem to recall having to use insulating hardware (a plastic
> sleeve) AND MAKING SURE there was a coat of silicone grease between
> the regs and the heat sink when attaching a 79xx to the same heat sink
> as a 78xx regulator (silicone heat sink goo is thermally conductive
> but NOT electrically conductive). A mica insulator would do the same
> trick I guess. If this is true, I would think that the same cautions
> would apply when attaching a regulator and/or heat sink to a synth
> cabinet!
>
> Am I halucinating here? Am I thinking of some other regulator family?
> Does anybody have documentation showing which pin attaches to the heat
> sink tab on the 78xx and 79xx? Why do birds suddenly appear every time
> you are near? Are we there yet?
>
>
> Tim (the caffeine is kicking in now) Servo
> --
> "Imagination is more important than knowledge." - Albert Einstein
>
>
>