Re: gEDA-user: Dropping a few volts

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Sat, Oct 13, 2007 at 08:08:39AM -0400, Bob Paddock wrote:
> On Friday 12 October 2007 11:48:12 am John Doty wrote:
> > On Oct 12, 2007, at 7:20 AM, Randall Nortman wrote:
> > > So I have a 42VDC supply that I want to feed into a linear regulator.
> > > But almost all linear regulators want 40V absolute maximum, and the
> > > ones that have higher maximums are not reliably stocked anywhere.
> >
> > What's wrong with an LM317HV?
> 
> They have a tendency to explode in my experience with them.
> The TI TL783 does not.

I came across the TL783 in my first-pass search, and was initially
excited, until I checked distribution and found that stocking of this
part is spotty, especially in the package I'd be interested in
(TO-263).

Anyway, I decided that I am not going to worry about output shorts.
My 42V calculation is a worst-case estimate, including a fair margin
for Murphy's Law.  That's only 2V above the rated max.  On top of
that, an output short is highly unlikely in my situation -- the
closest thing likely to happen is a 150 Ohm load, which would limit
current to 160mA, well within the regulator's limits.  Unfortunately,
that makes the load -- an 0805 resistor -- dissipate almost 4W, which
is of course going to do bad things to it.  If it fails short, I'm in
trouble, but then I'm still assuming that my worst-case voltage is
present at the same time that somebody sticks a wire in the wrong
hole.  I'm not flying planes here -- I'll take that risk.  (Famous
last words)

  Post-mortem analysis: Operator error.  Device operated outside of
  specified bounds.  Recommend immediate apologies to designer.

  P.S.  Too bad about burning that house down -- stuff happens, right?

-- 
Randall


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