# Re: Checking power supply fuse

Terry wrote:
On Fri, 03 Aug 2007 08:50:25 -0400, Paul <nospam@xxxxxxxxxx> wrote:

Terry wrote:

500k is still a complete circuit. A good fuse will always give you a
something?

http://www.pavouk.org/hw/en_atxps.html

If you get a reading of infinity, either one or both of the bleeder
resistors is open, or the fuse is open. Infinity is not conclusive
for fuse state. If the supply was damaged by lightning, there may
be no "logic" to what blows, so you cannot really say that bleeders
never fail.

And, in any case, to read 500K, you need "high power ohms". To get
a reading, two diodes in the bridge rectifier have to be forward
biased. If your meter uses low power ohms, you might get a reading
of infinity, even when the fuse is good. So, you need to know
something about your meter. By using two meters, setting one to volts,
and the other to ohms, you can investigate what kind of open circuit
test voltage is being used.

Paul

Looking at the schematic you provided, the first thing in the circuit
is a fuse. If you get continuity, then 100% the fuse is good.

Like you say, you can get infinity for more than one reason. Infinity
does not necessarily mean the fuse is bad, but continuity does always
mean the fuse is good.

My statement still seems true to me.

I don't understand why the state of the fuse is important to you.

If you want unambiguous results, the best way is to get as close as
possible to the source. The results of a "distant" measurement, are
only as good as your understanding of the physical construction of the
thing. For example, if there was carbon scoring on the unit somewhere,
maybe you could get your 500K reading that way. The only unambiguous
reading to me, is to take fuse in hand and measure it. That eliminates
a lot of other (obscure) possibilities. If I was to rely on such a
remote measurement, I'd at least want to visually examine the area
in question, to see if there are other ways to interpret the results.

In your case, I would treat a non-infinite reading as an "indication"
that the fuse could be good, but not a guarantee. For example, if
I smelled a burning smell coming from the unit, I'd have to open
it up and see what kind of a mess was in there. Say that something
exploded, and blew conductive material around...

Generally, when I'm using a multimeter, I'd debugging something. Like
a detective, I'm gathering evidence. I never trust one single piece
of evidence by itself. If several pieces of evidence point in a
particular direction, then there may be a theory to explain all the
observations.

To give you another silly example, take an ordinary filament lightbulb.
Occasionally, one end of the filament comes loose, and flops around
inside the bulb. The filament is still welded to one leg of the bulb,
but the other end is loose. Now, I could measure the light bulb, and
get a zero ohm reading, I could tap the bulb lightly on the table,
and get an infinite reading. Should I have concluded, from the first
measurement, that the bulb was good ? In your view, the bulb would be
good. Yet, if we take the two readings together, we have a body of
evidence. A possible explanation, is one end of the filament is loose.
A third measurement would be visual inspection of the bulb, for signs
of a loose filament.

Paul
.

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