Re: Power distribution panel problem

[Peter Schoch <pschoch@NAC.NET>]

I have had my main panel power supply do a similar thing, on two
occasions.  The first time, the meter in the panel was bad.  I checked
the output voltage at the benches and it was fine at 20 V, but the panel
meter read only 6 no matter what I did.  Replacing the meter was fairly
easy, and not too expensive, I went to our local (commercial) electrical
supply store.

The next time something similar happened I first thought it was just
another bad meter.  However, when I checked at the bench, it wasn't -- I
was only getting 3V.  Since this was our 120 V variable line, I was just
a bit trepidatious about tackling it alone.  So I asked for the
electrician to come have a look (which only took 3 weeks).  Turns out,
the coils in the variable output knob were burned out in a section; so,
it wouldn't go any higher.  It took him 6 weeks to get the part!

I guess the bottom line for my two cents is that you shouldn't always
trust the meter on the power supply.  Check that first, then you might
want to check on the variable resistance control to make sure it is
actually cycling.

Whatever you do, do it carefully!  I have a set of melted probes to show
my students how dangerous this can be!  (The probes got melted while we
were checking and fixing the second problem.  The electrician insisted
on using my equipment, because it was better!)

Peter Schoch

"Lemmerhirt, Fred" wrote:
> Our physics lab has a central panel that distributes regulated DC to the lab
> benches.  It was built for us by Hampden Engineering in about 1972 and has
> worked perfectly until today.  Now suddenly the output of one of the 15-volt
> regulated power supplies will only go up to about half a volt when the
> variac is turned up all the way (with no load), as indicated by its own
> meter.
>
> Before starting to pursue this problem, I'm wondering if any of you have had
> experience troubleshooting or repairing such devices and could offer any
> suggestions.
> ______________________________________
> Fred Lemmerhirt
> Waubonsee Community College
> Sugar Grove, Illinois
> flemmerhirt@mail.wcc.cc.il.us <mailto:flemmerhirt@mail.wcc.cc.il.us>
> http://chat.wcc.cc.il.us/~flemmerh/physics.html
> <http://chat.wcc.cc.il.us/~flemmerh/physics.html>