25 Sep
2002
25 Sep
'02
10:17 a.m.
From: "Peter C. Wallace"
<pcw(a)mesanet.com>
On Wed, 25 Sep 2002, Patrick Rigney wrote:
> > Better still, if you are testing the cards one at a time with an
external
> > supply, tack on a 5V (set to 5.5 or so)
crowbar on the output side of
the
> > local 5V regulator -- no cut traces, only
tacking on 2 wires
> > temporarily...
> >
> > Peter Wallace
>
> Peter, yeah, this seems like an easy approach. Question: could a simple
> 6.2V Zener be installed reverse-biased from the 5V regulator output to
> ground? Seems like you'd want inline fuses between the unregulated
supply
and the
backplace as well for this kind of test...
Patrick
You would have to have a monster zener to be safe, probably 50 W or
so. Those arent that common. A crowbar circuit stolen from another supply home made one would be better than a zener.
Instead of a crowbar (that usually use SCRs), You could also use a
5.6v or so shunt regulator made with a big (20A) power transistor and a 5V
10W zener...
Peter Wallace
Hi
I think you are all a little paranoid. Regulators rarely
fail in storage. If it is blown, it most likely blew while
the power was on and any damage has already long since been done.
That being said, there is a failure that can happen in
storage that is related to the regulation. Many regulators,
especially the TO-3 types need the screw holding them to
the board to have a vary good connection. If this is
bad, the voltage will go unregulated and it will over
voltage. You should remove and check under these screws
to look for corrosion and clean the surfaces. I recommend
a star washer between the PCB and nut/washer. This should
be checked on any electronics that has been stored.
Often this is a good point to restore some fresh
heatsink compound as well. The old dry stuff is less
effective.
The most common failure is the old electrolytics and
the tantalums. I've seen flames from a blown tantalum.
If you connect a voltmeter to the output leads of any
regulators and watch the voltage as you bring the line up
with a variac ( adjustable auto-transformer ), you should
be able to catch any over voltage before it does any harm.
( note: don't use a variac on a switcher supply! )
Later
Dwight