Replacing failed powersupplies on qbus PDP-11s
mouse at Rodents-Montreal.ORG
Sun Feb 1 22:43:17 CST 2009
> In the normal small SMPSUs, the chopper drive is not synchronised ot
> the mains frequency , so there's no way of the supply knowing waht
> the mains input will actually be when the chopper turns on.
Yes...but (assuming the chopper frequency is high compared to the mains
frequency) there will be a period near the top of the mains AC cycle
when there is a chopper-frequency component to the draw. If the filter
caps are decently large, it will be tiny, but it will be present -
well, possibly excepting the case where the filter caps are large
enough that the time current is drawn for is less than one chopper
cycle wide, in which case I'm not sure how fair it is to say that. In
the presence of resonance it doesn't take much, though; the saving
grace is probably that the resonance has to be both very close to exact
and very low loss for the small boost each peak to be enough to produce
an oscillation that grows in the long term - and neither of those is
likely to be true.
> [1[ There heve been supplies using SCRs triggered at the right point
> in the mains cycle. They're not common,
Not in computers, maybe. But - well, incandescent lamp dimmers.
Variable-speed power hand drills.
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