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Multimeter shows incorrect reading after replacing fuse

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vidra:
Hi everyone,

My name is Kristof and I'm new on the forum. I was wondering if any of you could shed light on my problem and suggest a course of action.

I recently managed to blow the miliamp range fuse on my Aneng AN870 multimeter. I promptly bought a replacement fuse with the same specs (250V, 250mA, quick blow), only they didn't have ceramic ones so I bought a box of glass fuses instead. Sure enough, when I replaced the fuse, the miliamp range started working again, but the reading was off. I read the current consumption of my joule thief and it was 44mA using my other multimeter and I got the same reading when I stuck the probe into the 20A socket of the AN870. However, when I stuck the probe jack into the miliamp socket, the meter measured 27 mA - so the 20A socket on the same multimeter measures fine, but the miliamp socket does not.

Could it be that the miliamp range needs to be recalibrated? Is it possible that the two ranges have separate calibrations? Or might it be that I blew something else apart from the fuse?

Thanks in advance,
Kristof

StillTrying:
"using my other multimeter"

Try measuring some DC currents with both meters in series to see if they match.

capt bullshot:

--- Quote from: vidra on April 11, 2019, 10:23:48 am ---Could it be that the miliamp range needs to be recalibrated? Is it possible that the two ranges have separate calibrations? Or might it be that I blew something else apart from the fuse?

--- End quote ---

From your description, I'd guess you've damaged the current measurement shunt for that range. You'll have to locate that component, replace it (and maybe also some protection diodes / and or blown traces) and re-adjust the multimeter.Usually the fuse protects you from an exploding multimeter, but isn't able to protect the components within the multimeter. This depends on the overload conditions that made the fuse blow.

ArthurDent:
The meter may be just fine. If you are measuring current from your low voltage source then you have the weak battery in series with the test leads and wiring, in series with the fuse, in series with the shunt, and in series with the joule thief circuit. If the new fuse (that you say is different) has a higher resistance than the original fuse, it might have enough added resistance to limit the current you can draw in your circuit. It is the total resistance in the circuit that is limiting the current and that resistance is much less when you switch to the high current range so that reads correctly, or at least closer. If you were to put your two meters in series (as suggested above) my guess is they will read the same and show less current than just one would in the joule thief circuit.

I would try to use a bench supply with a 5 to 10 volt output and use a proper value resistor in series with the multimeter and check the current. My guess is it will read o.k..

Read this explanation of 'burden voltage'   http://zone.ni.com/reference/en-XX/help/370384V-01/dmm/burden_voltage/

vidra:
Thanks very much for your replies. I will indeed try to put two meters in series. However, I fear capt bullshot may be right because when I stick my probe lead into the 20A socket, the meter measures the correct current, but not in the mA/uA socket. When I opened the meter, I didn't notice any visible signs of damage. The current I blew the fuse with was about 500mA at 12V.

Anyway, I'll open up the meter when I get back from work and try to troubleshoot the problem.

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