Hal Murray wrote:
[variac]
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.
Yes, and if your junk box goes back far enough, you may find some 120V
Christmas tree bulbs! Not sure what their wattage is. Basically, use
the smallest 120V (or 240V - depending on where you live) incandescent
light bulb that gives you reasonable current / voltage in your sick
circuit. You can also use lower voltage bulbs on the secondary side of
a power transformer if that works better.
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.
True, but with the light bulb you'd miss the fact that everything looks
good below X volts. That could be a significant piece of info.
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've got a Jupiter GPS receiver that suddenly decided to start sucking
silly amounts of current. Never was able to find the short on that.
I've got a better meter now, maybe I'll put it back on my list of things
to fix. Sigh.
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?
Freon was nice because it was non-conductive, evaporated cleanly, and
wasn't flammable. I'm drawing a blank on what would be a good substitute.
Ed
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