Voltage & Current..

From: Kevin Handy <kth_at_srv.net>
Date: Mon Jan 26 15:28:09 2004

Pete Turnbull wrote:

>Well, there might be a way to deal with the problem. Most of the poor
>regulation is down to the following. Most small DC wall warts consist
>of a transformer feeding a bridge rectifier with a moderate
>electrolytic capacitor across it. Under low or zero load, the
>capacitor charges to the peak voltage. As you apply more and more
>load, you get more and more ripple, and the average voltage goes down
>(in fact, because there is resistance between the transformer and
>capacitor, so does the peak). However, you could analyse the ripple
>and work out what the input was doing. At moderate loads, the
>capacitor will charge rapidly to something near the peak voltage on
>each half-cycle, and discharge relatively slowly; at high loads, you'll
>get a more symmetric ripple. If you want to know in detail, you could
>sample it, but my guess is if you're just looking for brownouts all
>you'd need to do is compare the amount of ripple to the average voltage
>and apply some rule of thumb. Or compare the slope of the rise to the
>slope of the fall. Of course when the ripple goes away completely
>you've got no mains input at all :-)
I've been following this thread, but I'm betting that the original
author plugged an AC adapter where it wanted DC, or vice versa.
That will often cause a "blue-smoke" level event.

Some wall-worts are seriously badly designed, but usually if the
voltage rating is right and you are not using a excessively overrated
supply (1kA supply used to power a 600ma load?) it's usually does
not cause the magic-smoke to escape.
Received on Mon Jan 26 2004 - 15:28:09 GMT

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