[time-nuts] HP 5328 PSU nightmare... Or stupid engineer, you decide...

Hal Murray hmurray at megapathdsl.net
Thu Sep 10 04:50:04 UTC 2009

> If you don't have one, you can wire a light bulb in series with the
> power cord. Use 40,60,100 watt - whatever you need. 

Neat.  Thanks.  That trick wasn't on my list.

I think you can get lower wattage bulbs (at 120V).  I think I've seen 25W, 
but I'm not sure.  Old style tungsten night-lights are/were 6W.

> The Variac is preferred because you may find that everything looks
> good until you hit X volts. The light bulb obviously isn't that
> selective.

I'd expect a light bulb to work well.  If the DUT needs X volts to 
demonstrate the problem, it will act as a high resistance at low voltage and 
low resistance at high voltage.  Light bulbs are (very) non-linear in the 
other direction.  If you connect the two in series, I'd expect it to be 
stable at a balance point with some current, hopefully enough to debug things 
without burning anything out.

Here is another variation ...  I'm assuming you are chasing something like a 
short on the power rail, probably a dead bypass cap.  Things get more 
complicated if it's only sick rather than a solidly short.

If you have localized the problem to one board, power that board from a power 
supply with a current limit knob.  You may want to solder some wires onto the 
board rather than using the normal connector.

Crank the current up until you get enough voltage drop to be interesting but 
nothing is smoking.  Poke around with a volt meter.  If you have traces 
rather than a ground plane, a few amps and a reasonable meter will localize 
things.  Even with a ground plane, it will get you pretty close.

I think I remember dunking one board in a pan of Freon(?).  It was some 
liquid that bubbled at the shorted cap.  That was ~30 years ago.  Maybe I'm 
dreaming and confusing things with another story.  It does seem like a good 
approach.  Are there any good Freon like chemicals available/legal these days?

These are my opinions, not necessarily my employer's.  I hate spam.

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