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M Philbrook

In article , says...

Hi all,

Well, having established that the mains filter had fused and having cut
it out of circuit, I was advised to reconnect the supply to the scope and
whack up to full volts from the mains supply. I did this through the
variac I'd recently acquired and that seems to have blown it, I'm sorry
to say. Not the variac so much as my false assumption that a variac is
simply a jumbo-sized rheostat that when turned up to maximum could only
output no more than its input. Big mistake! I reckon I applied about
10-12% more than the smps's design voltage of 230V to it and now
something else has gone phut. No smoke, but a distinct sizzling noise
that lasted about 5 seconds before petering out.
So the question now is, what type of component is least likely to be able
to tolerate this marginal over-voltage?
thanks,
cd.

Many switch mode supplies do not like a slow ramp up on the input
voltage. Some cheap designs require it to have voltage sitting there
for proper inrush sequencing to take place for the feed back to get
started.

Some of these cheap supplies will actually hold the switching mosfet
in a DC state and over head the inductor, power tranny or both.

Most variacs have only a little more than input side, I don't think
that little over percentage caused the problem.

Jamie