Ceramic fuse 3ab ?

mzeitlin3348

mzeitlin3348

See it and Believe
When I went to use my radio shack digimeter (voltmeter) to test some circuits I was working on - the device did not seem to work. It came on - I saw numbers in the LED display - but it would not register anything. I tested it with simple new AA battery and nothing - no reading - just random numbers around 1-5 milivolts. I was puzzled - I never had a voltmeter go bad - and I have had this one for years.

So I opened it up and in the back were fuses I am not familiar with ... two - apparently ceramic fuses "3ab - 500mA 250V". There were two of them and one of them tested open (using another voltmeter). I replaced it with the spare and the voltmeter started working again like new.

Why use a ceramic fuse in a voltmeter? I'm puzzled why a simple glass fuse with a thin wire is not sufficient.

Just curious ...
 
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The ceramic fuses are filled with silica (or some other substance, I think it's silica) and withstand an initial higher inrush than standard fuses do.
 
And, perhaps, the filling helps to extinguish any arc that may try to form as the fuse burns out, thus, improving (decreasing) the circuit interruption time.
 
whoaru99 said:
And, perhaps, the filling helps to extinguish any arc that may try to form as the fuse burns out, thus, improving (decreasing) the circuit interruption time.
Click to expand...

+1 The sand is what makes them fast acting. The next time you see the blown AGC (glass) fuses in an amplifier that has failed, you will notice it's black due to the arcing when the fuse tried to open. The sand in the ABC snuffs out the arc and allows it to clear faster.

If anybody want to learn about fuses try a search on time/current curves for a fuse.
 
FYI, the same construction exists in larger non-glass fuses. They are filled with quartz sand. However, unlike glass fuses, they are not watertight. Once a standard cartridge fuse gets wet, it will corrode the fuse link inside, and can also fail to open completely, as the moisture inside will conduct electricity around an open link. Bottom line, if you've got some old fuses you're hoarding, throw 'em out if they gotten damp or wet.
 
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