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Posted by Scott Willing on August 8, 2005, 10:32 pm
>I have a 12 volt power supply that converts 110AC to 12V DC (actually 13.5.)
>I haven't used it for some time.
>I was using it to check some wiring and find it is now putting out 21 VDC.
>???
>ds
You haven't given us much to go on. I presume though that this is
probably a linear power supply, maybe 3 or 5A output? This would
basically consist of a transformer, rectifier/filter capacitor and
some kind of regulator. The latter is probably a simple discrete
circuit consisting of a few components. One of them will likely be a
transistor in a metal can mounted on a heatsink.
If you haven't used this thing in a loooong time but it was working
the last time you put it away, the first thing I would suspect would
be any electrolytic capacitors, including the big one that filters the
output of the transformer/rectifier combination. The most likely
semiconductors to fail would be the rectifier diodes (2 or 4 discrete
diodes, or one square bridge rectifier perhaps) and the transistor.
But then... that might be all the semiconductors there are.
Do you measure any AC voltage on the output? A small AC voltage (aka
ripple) riding on the high DC would indicate a failed pass transistor,
but if the AC voltage is significant, the filter cap has probably
dried out.
Of course if this is a switchmode supply you can forget most of this,
but most switcher failures result in loss of output rather than
overvoltage.
-=s
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