uCurrent not working on uA

[robomaniac]

Hi guys,

I have gen 1 uCurrent. Batterie is now at 2.951V (I was 3.12V, I just forgot it ON this evening  ) I set switch to mA and I do measure the proper mA. I can easily compare it with my Fluke multimeter and I get same reading. I can only go down to 1mA with my fluke.

the project I am working on is nA. So I set my switch to nA and it's only show 1.542V, I plug and remove LEDs and nothing change, I still read 1.542V I unplug the same components when testing in mA.
Now I remember the last time I used this (years ago) and that 1.5V does bring back memories.



I had a deadline for a digikey order so I quickly ordered u1 and u2. That did not solve the problem.



now I took the time to check the schematic.

I measure resistance of the current input and I measure
mA = 0.3 Ohm
uA = 10.4 Ohm
nA = 9.99k Ohm
My fluke wire measure 0.2 Ohm


R1 = 0.2 Ohm
R2 = 10.00K Ohm
R8 = 10.5 Ohm

R6 = 99.9k Ohm
R7 = 99.7k Ohm
R3 = 75.1k Ohm
R11 =23.97k Ohm

I initially thought the Switch (SW1) was broken or something. I did not use this product a lot. But the fact that the current input changes when I change the switch.

Kean mention to check virtual ground
https://www.eevblog.com/forum/testgear/ucurrent-blown-by-overcurrent/msg1776830/#msg1776830
>>Also check the virtual ground (negative output jack) is at 1.5V with respect to the battery negative terminal (half battery voltage).
1.435V



I am looking for suggestion, next time I will order the entire BOM

thanks

[robomaniac]

I will order and replace all the components. Also the switches because that is the only mechanical components of the kit.

[Kean]

Sounds like a problem with SW1 based on your measurements.
Or you are just overloading it in nA mode, as a 1.5V output is effectively max range.  Or do you get that in the nA range even with nothing connected at the input?

[robomaniac]

Sounds like a problem with SW1 based on your measurements.
Or you are just overloading it in nA mode, as a 1.5V output is effectively max range.  Or do you get that in the nA range even with nothing connected at the input?

right not I have a 5mA load and get 1.5V

Or do you get that in the nA range even with nothing connected at the input?

no I read 0V


btw, I change all the components on the PCB (except voltage monitor components) still have same issue. So I am lost.



is the max 1000µA? then that might explain some stuff. That's 1mA
 

[Kean]

Yeah, the full scale specs are +/-1250 mA/uA/nA depending on the range - i.e 1.25A, 1.25mA, 1.25uA.
From memory that is allowing for the 3V battery to drop by 10% or so - you can get a bit more with a brand new battery.
Or you can use a different battery config, e.g. 3xAA to get a bigger range - just don't exceed the specs of the opamps.