[sdiy] Regulator / PS question

[Gene Stopp]

For the three-terminals-inline package (TO-220 I think) I always remember
the pinouts as follows, left to right, with the terminals facing down, the
mounting tab on top, and the writing towards you:

78xx = IN/GROUND/OUT

79xx = GROUND/IN/OUT

The middle terminal is connected to the mounting tab. So for positive
regulators, the mounting tab is ground, but for negative, it's the raw
negative DC. Not good to have those touch. In the past I was a little wary
about having raw DC that close to ground, separated by a thin piece of mica
and a thin flange on a washer, so my usual tactic was to use separate
heatsinks with no electrical connection to anything.

- Gene
 



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