Problem with assembled uCurrent GOLD

[vijayenthiran]

I have assembled uCurrent GOLD on my own since it was out of stock in EEVBlog store a few weeks back.

I am testing it with 316K? 1% resistor with 3.3V supply expecting 10.44mV reading across the uCurrent. But I am getting 7.9mV (with a little bit of fluctuation). DMM is showing 10.4uA when measured.

Measurement with uCurrent GOLD:



Measurement with DMM:



Here's the picture of my assembled uCurrent GOLD.


I checked for shorts. I did a visual comparison with other [1][2] uCurrent GOLD. The only difference I could find is that R6 and R7 resistor code are shown as 010 (1?) on those pictures whereas it is 104 (100K?) in mine (which is correct as per original BOM).

Please suggest some troubleshooting method.

Here's my BOM and I have used few equivalent parts instead of the one mentioned in the  original BOM.

[vijayenthiran]

Today I was debugging again. When I measure the resistors R6 and R7 on the uCurrent board, it is showing 88.8K for both the resistors. When I measured that resistor externally, I get 100K. Any idea why this is happening?

I am using this resistor (ERA3AEB104V)

[vijayenthiran]

Found the issue. I was using cheap 50cm Crocodile Clips Cable which had a resistance of 1.2 Ohm. My bad. When I connected wires and multimeter probe directly to the uCurrent binding post, it worked like charm.

Culprits:


The Cable which I was using measured 1.2Ohm. I was using 4 of them 


This worked.

[claytonedgeuk]

Thanks for posting that!  Something to be more mindful of when you're working with low currents!

[spacejunkie]

I wonder why a resistance of 1.2 ohm was a problem while measuring voltage. Voltmeter is a high impedance device and the uCurrent output is driven by an opamp so eve 120 ohm resistance should not cause that much error.

[Tomsik]

I've noticed thin wire between -IN terminal and sensing resistor...
Does somebody know what is the reason? Or is it issue?

[SilverSolder]

Maybe the thin link acts as a crude fuse, in case of gross overload?

[SilverSolder]

Found the issue. I was using cheap 50cm Crocodile Clips Cable which had a resistance of 1.2 Ohm. My bad. When I connected wires and multimeter probe directly to the uCurrent binding post, it worked like charm.

Nice work - looks good.