uCurrent accuracy - what am I missing?

[int2str]

I'll assume that I'm doing something wrong, but I can't really figure out my uCurrent doesn't seem to be accurate.

I have a DMM Check Plus which has a 1mA current source built in.

The certificate that comes with it states it to be 1.0000mA.
If I measure it directly with a Fluke 189, I get 999.9uA.
Measured with a Fluke 8840A or an Extech EX330, I get 1.00mA.
So overall the reference seems to be good.

When I run it through my uCurrent Gold, I get these readings:
Fluke 8840A: 0.98680V
Fluke 189: 986.5mV
Extech EX330: 0.986V

I've changed the batter to be sure, but the reading hasn't changed.

What am I missing?

 

[Martin.M]

please change the instruments to read mA, not volts.

greetings
Martin

[boffin]

I'll assume that I'm doing something wrong, but I can't really figure out my uCurrent doesn't seem to be accurate.

I have a DMM Check Plus which has a 1mA current source built in.

...
What am I missing?

Did you get one from the batch with the bum reference resistors?   
 

Strikes me that was a long time ago though

 

[borjam]

please change the instruments to read mA, not volts.

µCurrent is a current sensor, and it's read in volts, not amperes

[Fungus]

please change the instruments to read mA, not volts.

The entire point of a uCurrent is that you see a reading in volts, not amps.

[Martin.M]

thank you for correction, I don`t have one.

greetings
Martin

[LA7SJA]

This readings are with the uCurrent Gold in the 1mV/uA setting, what du you read in the 1mV/mA setting? If you measure the resistance on the "input" in 1mV/nA, 1mV/uA and 1mV/mA  settings (Preferably with kelvin probe and four-wire measurement) what do you read?

Johan-Fredrik

[Fungus]

I'll assume that I'm doing something wrong, but I can't really figure out my uCurrent doesn't seem to be accurate.

I have a DMM Check Plus which has a 1mA current source built in.

The certificate that comes with it states it to be 1.0000mA.
If I measure it directly with a Fluke 189, I get 999.9uA.
Measured with a Fluke 8840A or an Extech EX330, I get 1.00mA.
So overall the reference seems to be good.

When I run it through my uCurrent Gold, I get these readings:
Fluke 8840A: 0.98680V
Fluke 189: 986.5mV
Extech EX330: 0.986V

I've changed the batter to be sure, but the reading hasn't changed.

What am I missing?

 

[Fungus]

You've got a lot of multimeters: Try measuring the current with a meter and uCurrent simultaneously.

If the results are very different then maybe you got a bad uCurrent.

[int2str]

If you measure the resistance on the "input" in 1mV/nA, 1mV/uA and 1mV/mA  settings (Preferably with kelvin probe and four-wire measurement) what do you read?

Hmmm....

1mV/nA = 0.008 Ohm
1mV/Ua = 9.875 Ohm
1mV/mA = 1.36106 kOhm

[3db]

please change the instruments to read mA, not volts.

greetings
Martin

I thought the uCurrent output was a voltage.

[LA7SJA]

If you measure the resistance on the "input" in 1mV/nA, 1mV/uA and 1mV/mA  settings (Preferably with kelvin probe and four-wire measurement) what do you read?

Hmmm....

1mV/nA = 0.008 Ohm
1mV/Ua = 9.875 Ohm
1mV/mA = 1.36106 kOhm

I get about the same as the schematic.

10 mOhm @ 1mV/mA (0.01Ohm)
10 Ohm    @ 1mV/uA
10 KOhm  @ 1mV/nA

Are the error the same on 1mV/mA and 1mV/uA range? What voltage do you read on the 1mV/mA, 1mV/uA and 1mV/nA range with shorted input terminals?

Johan-Fredrik

[int2str]

The schematic I can find seems to be for the original model.
On the uCurrent Gold, which resistors on the back do I need to check?

R2 - 10kOhm
R9 - 10Ohm

What's the last one, is it R1 (the big one?)?

[LA7SJA]

Here is the schematic: http://www.eevblog.com/files/uCurrentRev5schematic.pdf

R1 is 10 milli Ohm (0.01Ohm)
R2 is 10 Kilo Ohm
R9 is 10 Ohm

It would be great if you answer this question:

What voltage do you read on the 1mV/mA, 1mV/uA and 1mV/nA range with shorted input terminals?

Johan-Fredrik

[int2str]

Shorted:
1mV/nA = -0.237mV (fluctuating a bit)
1mV/uA = -0.080mV (also fluctuating a bit)
1mV/mA = -0.080mv (about the same as above)

[LA7SJA]

This readings in post #1 are with the uCurrent Gold in the 1mV/uA setting?
If so what du you read in the 1mV/mA setting?

1mA thru  a 10Ohm resistor =  10mV times 100 should read 1.000V
1mA thru your r9 measuring 9.875Ohm =  9,87500mV times 100 will read 0.98750V - 0.000080V = 9.87492V

So the culprit is the r9 resistor that has a wrong value of 9.875Ohm insted of 10Ohm

Johan-Fredrik

[guenthert]

Shorted:
1mV/nA = -0.237mV (fluctuating a bit)
1mV/uA = -0.080mV (also fluctuating a bit)
1mV/mA = -0.080mv (about the same as above)

Do you happen to have an oscilloscope you could connect to the output of the ucurrent?  Or at least measure the A/C component of the output using one of your DMM?  I had here some issues with EMI and adding bypass capacitors to the input of the ucurrent helped.

Edit:
Uh, I just (now sufficiently caffeinated) see that you shorted the inputs.  Nevermind then. 

[Skimask]

Hi.
I watched 3 YouTube videos on 3 different pieces of test equipment.
I am now an expert.
Thanks for playing.

Some people will NEVER get it and don't know they'll why they'll never get it.

[int2str]

Hi.
I watched 3 YouTube videos on 3 different pieces of test equipment.
I am now an expert.
Thanks for playing.

Some people will NEVER get it and don't know they'll why they'll never get it.

Just whatever you do, don't accidentally be helpful or anything.

[LA7SJA]

Some people will NEVER get it and don't know they'll why they'll never get it.

I must be one of them, that sentence makes no sense to me (but I have no formal English education).

[int2str]

So, does it seem that my resistors are broken / out-of-spec?

[wraper]

Shit happens, any device that is sold in quantity will have some small amount being defective. If other ranges are accurate, then most likely something wrong with shunt resistor.

[Skimask]

Some people will NEVER get it and don't know they'll why they'll never get it.

I must be one of them, that sentence makes no sense to me (but I have no formal English education).

Ya, that's what I get when I don't correct the autocorrect which should've corrected the correction that I didn't correct.

[SaintGimp]

1mA thru  a 10Ohm resistor =  10mV times 100 should read 1.000V
1mA thru your r9 measuring 9.875Ohm =  9,87500mV times 100 will read 0.98750V - 0.000080V = 9.87492V

So the culprit is the r9 resistor that has a wrong value of 9.875Ohm insted of 10Ohm

This is really interesting because I also have a uCurrent Gold (SN 00796) and just tonight got a DMMCheck Plus.  Just like the original poster I found that my uCurrent was reporting the 1mA current source on the DMMCheck as 0.9725mA (i.e. 0.9725V on the uA range).  Measuring the current source with my UT61E directly shows exactly 1.000mA.

I measured the resistance of R9 alone and it's 9.74R, similar to what the O/P saw.  That's just not right.  So either R9 was bad from the start (and I couldn't tell until I got the tools to check it accurately), or it has somehow become bad since I received it, or maybe the surprisingly heavy coating of flux gunk around the shunt resistor area is throwing off the resistance.  I'll try cleaning the board and see if that helps.

[EEVblog]

I just tested one from the latest batch and it's bang-on. 1.000V output for 1.000mA input. Resistor is 10ohms as expected of course.