3 May
2005
3 May
'05
9:24 a.m.
>>>> "der" == der Mouse <mouse
at rodents.montreal.qc.ca> writes:
> As usual during initial power tests - pulled all
cards, and began
> to power-up through variac and series light bulb - Bulb glows
> brightly as power comes up, suggesting system is drawing far more
> power than it should, [...]
> Can anyone explain to me what is going on, and why
the unloaded
> power supply is drawing so much on it's input?
der> One possibility comes to mind immediately.
der> Current flowing != power being drawn. A serial resistance will
der> show you, approximately, current drawn. But if the load is
der> heavily reactive, as may well be the case here, it's entirely
der> possible for the current drawn to be all out of proportion to
der> the power consumed.
der> An ideal capacitor or inductor, for example, presents a purely
der> reactive load and consumes no power even though it draws
der> current.
That's what I figure, too. With a dual trace scope, or an X/Y scope,
you can test this easily -- set one channel to measure the current
(voltage across a *small* series resistor) and the other voltage.
Trigger on one, and observe the two traces. Or feed them to X and Y
and see how close the Lissajous figure is to a circle.
paul