wallwarts and regulation..

[Scott Gravenhorst]

On Fri, 6 Mar 1998 19:11:00 -0000, "Tony Allgood"
<oakley at enterprise.net> wrote:

>Terry said...
>
>And for heaven's sake don't hook a pot up to an LM7805 and think you have
>an adjustable regulator. It will regulate under load but not with respect
>to the unregulated DC. I've tried it on the bench; it doesn't work.
>
>I have tried this with 78L05 and it works OK. Now there are different
>manufacturers of the 7805 chip and it could be that different designs push
>more current down the ground pin than some of the others. This will lead to
>changes in the output pd. Having said that, you might as well get a 317 and
>the two resistors, one cap and two diodes... more elegant somehow. Horses
>for courses as my mother used to say.

More likely, it is a lucky combination of well matched load resistance
and input voltage.  Any current not conducted by the load will be
conducted through the ground pin in an effort to keep the output
voltage at spec.  Essentially, the output circuit of these regulators
consists of two transistors that form a voltage divider.  Too high an
input voltage will cause the lower transistor to conduct excessive
current through the ground pin.  Even if the input voltage is
*perfect*, anything in series with this pin will have to be able to
dissipate the power and will result in heat in both the lower
transistor and the resistance in series with the ground pin.  If the
load gets too light, the resistance in series with the ground pin may
cause the lower transistor to saturate.  This could allow the output
voltage to rise above spec. and could result in a catastrophic
destruction of the load and/or regulator.

I'll say it again:  Why risk disaster for a couple of dollars and a
little work?  These components are really pretty inexpensive.  Also,
if you ever add something to the circuitry, you'll need to modify or
replace the power supply to compensate for the increased load.  

LM317 is a good bet.  There is a circuit for an adjustable regulator
in the app notes.

I'm not casting "asparagus" at your design, just trying to save you
wierd symptoms and unexplained blowouts in the future.  Imaging being
on stage in front of talent scouts when your equipment takes a dump.
Remember that the power supply is one of the most important parts of
the design of any electronic device.  If your supply goes wierd, so
will the circuit powered by it.

-- Scott Gravenhorst