"Uddo Graaf" bravely wrote to "All" (14 May 04 12:01:12)
--- on the heady topic of "How do shorted electrolyic capacitors measure up?"

UG From: "Uddo Graaf"

UG I'm trying to repair a TV which blows the fuse every time I turn it
UG on. I suspect one of the capacitors is shorted but I've taken out a
UG couple of big electrolytic capacitors and one of them seems to measure
UG OK, although it's rated voltage is lower than the suspected peak
UG voltage.

Peak surge turn-on voltages may slightly exceed capacitor rated
voltages by 10% or even more and this is relatively normal for a brief
period of time. If the overvoltage condition persists then I'd start
to worry.


UG The other one I can't measure with a cap meter because it
UG exceeds the maximum capacity the meter can measure. But I was

Use a constant current and time it with a stopwatch using C = t x I/V.
Just measure the time it takes a fixed current I to charge from one
voltage level to another say a difference of 10 volts or even just 1.
If you have an accurate stopwatch and a keen eye you may get 10%
precision or better, BON. I often eyeball large caps this way. Usually
I only really need a ballpark value since it's not like I'm using
these for precise timing purposes or accurate filter design anyways.
Let's be pragmatic here.


UG But I was
UG wondering: would a shorted electrolytic actually show an abnormal
UG capacitance value? Would it show up on a resistance test?

It may show a very small DC resistance on the ohms scale but this may
be due to the very large capacitance value. You need to charge it with
a small fixed voltage (an AA cell?) and see if it loses the charge
rapidly. If it holds that charge then try the rated voltage and see if
it holds that too (this test may be dangerous above 50 volts so take
the usual HV precautions).


UG There are also a couple of transformer protection capacitors in the
UG thing as well, but they're pretty small (15nF) and I have my doubts
UG that they could carrry more than 2A needed to blow out the fuse. Am I
UG right in this regard?
UG Thanks in advance.

The 15nF could be a dead short but I think it would typically explode
if that were so. They are so easy to test it doesn't require further
comment.

A rule of thumb is that when fuses blow this means something is
draining a lot of power and usually it is a higher power handling
device such as a rectifier or power transistor, etc. For example the
HOT section is often vulnerable to power line surges, like turning on
and off the tv in rapid succession many times as a toddler may want to
experiment with.

Beware of lethal high voltages inside the cover of a tv.

Asimov
******

.... Now touch these wires to your tongue!