I figured out the cause: some inverted diodes from VCC to GND (had to be
from GND to VCC).
Both LM338 and 7805 share a radiator, but I had to isolate it on the
7805's side, as I figured out that pins no. 2 wheren't the same at the 2
ICs. Anyway, th maximum current is around 500mA through the 7805.

Lucian
-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Herbert Graf
Sent: 14 mai 2004 16:47
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Cascading Regulators

> 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

----------------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

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