Multimeter shows incorrect reading after replacing fuse

[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]

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?

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.

[David Hess]

The fuse should be able to protect the low current shunt but it needs to be checked.  You might try running a known current through the bad range and measuring the voltage drop across the shunt which should correspond to the reading and input current.

I had an older meter fail in the way you describe and what happened was the switch which controlled the high current range got welded closed.

[capt bullshot]

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.

I agree, at  500mA I'd expect the shunt and protective circuitry to survive without noticeable influence on the accuracy. But one never knows until it's all checked. Things can be different if there was a paralleled bulk capacitor that could provide a significant surge current.

[vidra]

OK, so here's what I've been able to establish so far:

- There's no visible damage anywhere on the meter. I inspected both sides of the PCB and everything seems pristine.
- When I connect the "faulty" multimeter in series with my "working" one, they both show the incorrect current of 27mA (instead of 44mA)
- The ceramic fuse is a weird-@ss size (29mmx6mm) and the glass replacement is 32mmx6mm. I don't know if this affects the reading. Perhaps the additional length introduces added resistance in series. Could this be the reason for the faulty reading and if so, what can I do about it?

Another question for all of you who are far more experienced than me: I've tried searching everywhere for an identical fuse but to no avail. Does anyone know where I could get hold of 29mmx6mm ceramic fast-blowing fuses @250v/250mA?

I really appreciate all your help!

[ArthurDent]

vidra- "..they both show the incorrect current of 27mA (instead of 44mA)"

That should read that "both show the correct current." Reread my previous post. As I said, it is the the added resistance of the new different fuse that is the problem. Read the link about burden voltage referenced in my post to see why this is happening.

[vidra]

vidra- "..they both show the incorrect current of 27mA (instead of 44mA)"

That should read that "both show the correct current." Reread my previous post. As I said, it is the the added resistance of the new different fuse that is the problem. Read the link about burden voltage referenced in my post to see why this is happening.

Thanks, I will. The reason I called it the "incorrect" current was because the joule thief draws 44mA as measured independently on 2 other meters.

[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]

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.

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. 

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.