uCurrent always provides low, negative voltage - Page 1
Forum section: electronics / testgear

author: chrille_dk
author_rank: Newbie
timestamp: September 02, 2016, 07:30:39 pm
content: I just bought a uCurrent Gold and it's not giving the results I would expect, so hope someone can help me find out if I received a defect unit, or if I'm doing something wrong. Initially I connect "voltage out +" to DC + on my DMM and "voltage output -" to common ground. Install a new battery, set range to 1mv/mA and turn on the uCurrent with no connections to the current connectors. The DMM shows -8 mv to -10 mv. If I touch one of the input terminal it reads between -25 and -30 mv. Shortening the inputs results in -28 mV.Now I connect a small microcontroller project with the ucurrent in series on the VCC side. It's a board the runs for a few seconds and go back to sleep for some time. The DMM still shows approx -20 to -30 mV - both while the processor is running and while it's sleeping. Switching the two inputs (polarity) doesn't seem to change anything. Changing the DMM from mV to V results in the same behavior. I verified with a 1.5V battery that the DMM works.Swiching from 1mV/mA range to 1 mV/1uA changes the measured voltage from -25 mV to -50 mVThe DMM as a Mastech MS8218 which I consider a reasonable accurate DMM for hobby projects- Jan

author: Chalcogenide
author_rank: Regular Contributor
timestamp: September 02, 2016, 08:46:35 pm
content: Well, you shouldn't connect the output voltage - to ground. Just connect the multimeter straight across the voltage output terminals. The uCurrent input is not isolated from the output, so you should not connect the output to anything but a multimeter or an oscilloscope.

author: chrille_dk
author_rank: Newbie
timestamp: September 03, 2016, 03:48:09 am
content: Sorry if my description isn't clear.The output is connected directly to my DMM - even using the pair of cables that came with the uCurrentuCurrent is inline with the positive power supply to the project I'm trying to measure- Jan

author: markce
author_rank: Regular Contributor
timestamp: September 03, 2016, 06:38:30 pm
content: Hi Jan,Did what you described above with my uCurrent (the original one).It reads 0.0mV on my Fluke 175. On a Philips bench DMM the offset isless than 20uV.

author: chrille_dk
author_rank: Newbie
timestamp: September 03, 2016, 06:40:49 pm
content: Quote from: markce on September 03, 2016, 06:38:30 pmDoes it mean that you think I received a faulty unit?- Jan

author: markce
author_rank: Regular Contributor
timestamp: September 03, 2016, 06:47:18 pm
content: The gold version is a slightly different design, to achieve higher bandwidth.From what I read on this forum, the offset with your model should be well below 0.2mV (<50uV).Please check the other topics on the uCurrent Gold here. Could be oscillating or faulty.

author: chrille_dk
author_rank: Newbie
timestamp: September 04, 2016, 12:57:03 pm
content: Quote from: markce on September 03, 2016, 06:47:18 pmI see some mentions of oscillating, mainly in the most sensitive range. I tried to connect the uCurrent to my oscilloscope. With the switch set to SHORT I see an oscillating signal at around 14 MHz and 80 mV peak-to-peak. Changing the input range/sensitivity doesn't change anything. The inputs are unconnected while testing thisIf I connect something to the input (1k resistor connected to 5V) the behavior is the same, except that the oscillating signal is now 120 mV instead of 80-100 mVSee attached file for waveform

author: chrille_dk
author_rank: Newbie
timestamp: September 04, 2016, 01:09:46 pm
content: Quote from: chrille_dk on September 04, 2016, 12:57:03 pmDid some further testing - something in my lab is causing this issue. I moved the uCurrent, DMM, microcontroller and some batteries into the living room - 5 meters from my lab and everything just works! Now I need to locate the the source of the RF noise- Jan

author: markce
author_rank: Regular Contributor
timestamp: September 04, 2016, 02:15:07 pm
content: You could also shield the inside of the uCurrent case with copper foil.

author: luisprata
author_rank: Regular Contributor
timestamp: April 23, 2017, 06:27:40 pm
content: It's not the same but maybe helps in other situations....https://www.eevblog.com/forum/projects/ucurrent-ma-range-error-using-oscilloscope-and-function-generator/msg1192329/?topicseen#msg1192329