Ok, the problem might be somewhere else, so I will accurately describe
my setup.
I have a machine-made board, with a power supply on it and a
microcontroller. The power supply contains an LM338 and an LM7805. Out
of the LM338 comes 13.6V.
Between the ADJ and VOUT pins of the LM338 I have a 220R resistor, and
between ADJ and ground, an 2.2K resistor and a 5K potentiometer. Still,
I cannot obtain 13.8V out of the regulator. But I am pretty satisfied
with the 13.6V...
As regards the 5V part, the regulator heats up very quickly. I have
double-checked and the wiring is correct. But I've tried to program ICSP
the microcontroller (without any other piece on board) and the voltage
between VCC and GND dropped at 0.85V (I had 5V from the programmer). So
I should assume a short somewhere ? I have one 100nF capacitor between
VCC and GND, could this be the cause of the voltage drop ?
The pinout of the 7805 is IN-GND-OUT, when looking from it's front,
isn't it ? (at least that's what I've found on the datasheets). It's an
LM7805CV (if this matters).

Lucian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jinx
Sent: 14 mai 2004 15:50
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Cascading Regulators

> Is it ok to do this ?

Yes

> 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)

Something is wrong. There is power consumption, possibly a short
or pins swapped. Have you double-checked the 7805 pin-out and
then checked it again ? How about the voltage out from the 7805 ?
Sure there's less than 30V on the input to the 7805 ? Looked for
solder bridges or a problem with the wiringtracks ? It'll be something
simple

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