µCurrent Gold and simple circuit

[dddd]

Hi,

I have a simple circuit : 1V power supply and 2 x 125? resistor in series. I expect that when I connect the µCurrent Gold between my power supply and the resistors in this circuit that the voltage output will be 4mV (which is equivalent to 4mA).


Instead I get 3.8 mV when I connect the + of power supply to the + of the current input, and I get -4mV when I connect the + of the power supply to the - of the current input.

So I have a couple of questions:


Which is the correct way to connect the current meter and power supply? Is it + to + or + to -?
Why do I see 3.8 mV and not 4mV?

Thanks.

[EEVblog]

For starters you are using the uCurrent right down at it's minimum point on the scale. You can't expect accuracy to be maintained that low.
Second, I presume you are using a 3.5 digit meter to last the measurement on the hundreds of mV range, in which case you are once again at lower scale of the range, and you are dealing with LSD errors. The meter you are using will have a +/-count reading accuracy that is more than the 2 least significant digit error you are talking about.

Basically, you are measuring down in the noise.

And we haven't even started to talk about resistor or supply accuracy yet.

[dddd]

Thanks for the quick reply, very helpful.

I had thought that because the µCurrent can measure down to µA and nA level that it would be able to handle small mA values without any issue. I now understand that this is not the case, which makes sense to me and is fair enough.

And indeed, looking at the manual of my multimeter I see that the its accuracy for DC mV is 1.0 % + 10(!).

[mbless]

I had thought that because the µCurrent can measure down to µA and nA level that it would be able to handle small mA values without any issue. I now understand that this is not the case, which makes sense to me and is fair enough.

It has separate current sense resistors for the mA, µA and nA ranges; it's not the same continuous circuit measuring all 3 ranges. They will all have the same issues for measurements at the low end of their respective scales.

And Dave's right that it is most likely your DMM. I don't have that much error measuring down to 1mA with the Brymen BM235. (Of course I switch to µA when it's within range.)

[Gyro]

I had thought that because the µCurrent can measure down to µA and nA level that it would be able to handle small mA values without any issue. I now understand that this is not the case, which makes sense to me and is fair enough.

Your problem is that it tries to cover 9 decades with only 3 current sense resistors (1mV/mA, 1mV/uA, 1mV/nA) so resolution at the bottom end of each range is poor. Your best option to get more resolution on low mA would be to source your own close tolerance shunt resistor and strap it across the input terminals.

You would need its value to be slightly high to offset the shunting effect of the internal sense resistor still being in circuit (set it to the lowest range so that the internal resistor is a high as possible - 10k I think).

With appropriate external resistors you can get 10mV/mA, 100mV/mA etc.


P.S. If you're not interested in measuring nA level currents then you could simply remove the 10k current sense resistor for the 1mV/nA range and dedicate that range to external sense resistors only. This would simplify external resistor values (in fact you could also use an external 10k resistor resistor to retain nA capability).