>I have a 7805, 220 style case running my PIC board.  I am using a total of
>0.700 amps.  Do I need to heat sink the 220 case/ 5v regulator??????
>(This is a 3.0 amp device!)

You didn't provide enough info--the unregulated supply voltage (under load)
is really important.  I'll do the math assuming the best case scenario.

Ok, according to my datasheet (National), the 7805 needs 7.5Vdc to maintain
line regulation.  That's probably cutting it close, but since you didn't
tell us the loaded unregulated supply voltage, we'll have to guess.  That
makes 2.5V across the device, so it's got to dissipate 1.75W.

Now according to my datasheet (you've got the datasheet, right?), Junction
to ambient thermal resistance is 54 degrees C/watt.  That means the temp
rise will be (54*1.75) above ambient, or about 95 degrees C.  Max junction
temperature (again, this is all in the datasheet) is 150 degrees C, so
you'd be ok if the ambient temperature is always below 55 degrees C.

This is right on the edge, but I wouldn't run a chip this hot without a
good reason.

This is all covered in the book "Art of Electronics" by Horowitz and Hill,
which should be a required document for all companies that design or
produce electronic devices.


newell