> Hello,
>
> I might be disturbing you with a very simple question, but I ran out of
> solutions for this problem.
> I have designed a power supply which gives 13.8V, with the adjustable
> LM338, and tied to the output, the input of an LM7805, in order to
> obtain 5V also (I need both 13.8V and 5V). Is it ok to do this ? Because
> when I power the board, the 7805 gets very hot quickly and I had to pull
> it off (there is no power consumption as I haven't planted all the IC's
> yet).
> I am wondering what could be the cause of this, as all the connections
> are ok.
> Please give me a suggestion. Thank you !

        Three things I can think of off hand:

        1. You have something wired wrong. Triple check (or get someone else check)
your wiring. If you've got no load you shouldn't have a noticeable heating
of those components).

        2. A rare one, but one I have seen: did you follow the datasheet on input
and output caps for both regulators? I've seen a case where if you don't put
the output cap on a 7805 it can sometimes start oscillating, drawing a huge
amount of current.

        3. You have a short somewhere, relates to number one, but since wiring is
the likely cause of the problem it's good to make this one a separate point!

        It could be a bad component, but I doubt that very much.

        On top of all this, how much current do you plan to draw on the 5V rail?
You're 7805 will very likely need a heatsink unless it's supplying a very
small current. TTYL

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