On Wed, May 24, 2006 at 08:20:59AM -0400, Spehro Pefhany wrote:
> At 07:35 AM 5/24/2006 -0400, you wrote:
> 
> >I assume in the above instance the output of my wallwart would go from,
> >say, 10V to 60V which the linear reg could probably handle briefly.
> >Sounds about right?
> 
> No, the transformer in linear wall-warts will saturate and the output
> voltage will never get that high. The primary current will be very high,
> through thin wire, and the primary may open up if there is enough energy,
> but the circuit should be okay if the output voltage is only 10V and a
> typical 40V regulator is used.

Interesting, I've seen references to "1000V 1minute" tests that wall
warts have to pass to be UL listed. I always thought it was purely a
function of don't burst into flames... Now I see why the 1 minute part
of the test is important!

> 50/60Hz transformers are actually very effective surge protectors in
> themselves if the circuitry on the other side is designed a bit
> conservatively.

Could a simple 1:1 isolation transformer be engineered to provide this
effect?

> It will probably fry every SMPS that's turned on at the time, but again
> not much should get through. Could be expensive with things like large
> flat-panel monitors, high end printers, and plasma TVs though.

The tiny little transformer in a SMPS should also saturate too right? Of
course, as you say, lots of supply-side electronics are there to be
fried...

> Last time I heard of such a thing happening, the power company picked
> up the tab for repairing the damaged stuff, at least for 'consumers'.

In highschool my electronics teacher told us a story once of how in the
late 60's the electricity to his rural neighboorhood in England was
sagging. This caused a lot of motors, especially as this was farm
country, to use more current and burn out. Of course, buracracies being
what they are the local power company wouldn't admit fault, and every
time a third party tested it, they would say their equipment was out of
calibration. So one day he managed to get some equipment requition forms
(he had worked for the company before) and tricked them into installing
a perfectly good calibrated long duration voltage meter on the pole
outside his house.  Just before the standard weekly log rotate would
happen, he climbed up the pole himself and retrieved the whole thing,
including the calibration documents...

The power company repaired everyones motors for free... 

-- 
pete@petertodd.ca http://www.petertodd.ca
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