Electronics > Beginners

Multimeter shows incorrect reading after replacing fuse

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Kleinstein:
The new fuse might have a slightly higher resistance. The joule thief circuit is not a good test for 2 reasons:
1) the voltage is low and the burden can thus make a difference (already noted by ArthurDent)
2) the joule thief can have quite some AC component superimposed. Especially cheap meters might show odd values with too much AC or current spikes on top.

Getting the same 27 mA from both meters is indicating a working meter.

If the 32x6 mm fuse fits, one might stick to this more common size. Usually the ceramic case fuses are a little better protected against a blast. However with such a cheap meter it is likely not a good idea to use it with mains voltage anyway, or only with some extra care (e.g. not hold the meter in the hand, make double sure not to test the fuse and have a fire extinguisher at hand).  At least it's a "30" x6 mm fuse and not a 20x5.

vidra:

--- Quote from: Kleinstein on April 13, 2019, 08:17:31 pm ---
Getting the same 27 mA from both meters is indicating a working meter.

If the 32x6 mm fuse fits, one might stick to this more common size. Usually the ceramic case fuses are a little better protected against a blast. However with such a cheap meter it is likely not a good idea to use it with mains voltage anyway, or only with some extra care (e.g. not hold the meter in the hand, make double sure not to test the fuse and have a fire extinguisher at hand).  At least it's a "30" x6 mm fuse and not a 20x5.

--- End quote ---

Whew! That's a relief. The fuse fits, but I'll have to make some slight modifications to the fuse holder with a pair of needlenose pliers to make the fit more snug, and as some of you suggested, I'll try it out with a 9V battery and a purely resistive load.

vidra:
Ok, so here's the epilogue of my troubleshooting:

It turns out that glass fuses have a much higher resistance than ceramic ones. The glass fuse measured in at 12Ω, whereas the healthy ceramic fuse was 0.1Ω. Quite a difference!

So I popped the high current fuse into the low current socket just to get a reading and it gave the same result as my other multimeter.  :-DMM

I had to search a bit, but I ended up using my first-time-buyer credit on Ebay to order a single fuse for £1.6 (the original price was £5 higher) and I ordered 10 500mA fuses from Aliexpress for $0.72 (it was the lowest amp rating I could find for the 6mmx30mm format).

Thanks everyone for your help :)

ArthurDent:
Glad it was an easy (and fairly inexpensive) solution to the problem. Thanks for posting what the final solution was, feedback is important to all who try to help.

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