-24 is nearing the -25 limit, which is probably why it's getting hot - but -12
is still probably a bit more than it needs. Can you drop it down to 7 for
testing purposes? As long as the rest of your circuit is right, and your sure
it's drawing 280mA... there doesn't seem to be an obvious reason why it would
heat up.

Quoting fred jones <boattow@HOTMAIL.COM>:

> Hi all,
> I thought I'd ask this question here as my pool of friends here can't seem
> to offer any help.  I have a circuit where I am feeding -24V into a 7905
> linear regulator.  It gets very hot, can't keep your finger on it.  It is on
> a pretty hefty heatsink.  I took the output leg loose and measured the
> current draw its supplying and it shows to be 280ma.  I would think that it
> should not have any problem with this and shouldn't be getting hot.  I
> thought maybe it was getting so hot because there was such a large voltage
> feeding it so I then took the circuit off of the 24v supply and fed it with
> 12V and again it still gets too hot to touch.  I had a .1uf cap on the input
> and output.  I looked at the data sheet and from that I tried changing them
> out to a 2.2uf on the input and a 1uf on the output.  At the same time, I
> chaged out the regulator to one of a different brand.  Still the same
> results.
>
> My questions would be should it be getting that hot?  I thought that it
> shouldn't at only 280ma.  If not, can you point me toward any problems I
> might be having that could cause this?  Suggestions of things to look at
> would be greatly appreciated.
> Thanks,
> FJ
>
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