uCurrent accuracy - what am I missing? - Page 1
Forum section: electronics / testgear

author: int2str
author_rank: Regular Contributor
timestamp: June 28, 2016, 06:02:48 am
content: 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: 0.98680VFluke 189: 986.5mVExtech EX330: 0.986VI've changed the batter to be sure, but the reading hasn't changed.What am I missing?

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 28, 2016, 06:56:24 am
content: please change the instruments to read mA, not volts.greetingsMartin

author: boffin
author_rank: Unknown
timestamp: June 28, 2016, 07:03:15 am
content: Quote from: int2str on June 28, 2016, 06:02:48 amDid you get one from the batch with the bum reference resistors?Strikes me that was a long time ago though

author: borjam
author_rank: Unknown
timestamp: June 28, 2016, 07:06:22 am
content: Quote from: Martin.M on June 28, 2016, 06:56:24 amµCurrent is a current sensor, and it's read in volts, not amperes

author: Fungus
author_rank: Super Contributor
timestamp: June 28, 2016, 07:20:30 am
content: Quote from: Martin.M on June 28, 2016, 06:56:24 amThe entire point of a uCurrent is that you see a reading in volts, not amps.

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 28, 2016, 08:30:47 am
content: thank you for correction, I don`t have one.greetingsMartin

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 28, 2016, 09:08:47 am
content: 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

author: Fungus
author_rank: Super Contributor
timestamp: June 28, 2016, 09:09:39 am
content: Quote from: int2str on June 28, 2016, 06:02:48 am

author: Fungus
author_rank: Super Contributor
timestamp: June 28, 2016, 09:11:38 am
content: 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.

author: int2str
author_rank: Regular Contributor
timestamp: June 28, 2016, 05:01:31 pm
content: Quote from: LA7SJA on June 28, 2016, 09:08:47 amHmmm....1mV/nA = 0.008 Ohm1mV/Ua = 9.875 Ohm1mV/mA = 1.36106 kOhm

author: 3db
author_rank: Frequent Contributor
timestamp: June 28, 2016, 05:07:01 pm
content: Quote from: Martin.M on June 28, 2016, 06:56:24 amI thought the uCurrent output was a voltage.

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 28, 2016, 07:25:06 pm
content: Quote from: int2str on June 28, 2016, 05:01:31 pmI get about the same as the schematic.10 mOhm @ 1mV/mA (0.01Ohm)10 Ohm @ 1mV/uA10 KOhm @ 1mV/nAAre 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

author: int2str
author_rank: Regular Contributor
timestamp: June 29, 2016, 06:30:13 am
content: 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 - 10kOhmR9 - 10OhmWhat's the last one, is it R1 (the big one?)?

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 29, 2016, 04:18:38 pm
content: Here is the schematic:http://www.eevblog.com/files/uCurrentRev5schematic.pdfR1 is 10 milli Ohm (0.01Ohm)R2 is 10 Kilo OhmR9 is 10 OhmIt would be great if you answer this question:QuoteJohan-Fredrik

author: int2str
author_rank: Regular Contributor
timestamp: June 30, 2016, 03:23:47 am
content: 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)

author: LA7SJA
author_rank: Regular Contributor
timestamp: June 30, 2016, 05:50:50 am
content: Quote1mA thru a 10Ohm resistor = 10mV times 100 should read 1.000V1mA thru your r9 measuring 9.875Ohm = 9,87500mV times 100 will read 0.98750V - 0.000080V = 9.87492VSo the culprit is the r9 resistor that has a wrong value of 9.875Ohm insted of 10OhmJohan-Fredrik

author: guenthert
author_rank: Frequent Contributor
timestamp: June 30, 2016, 04:50:58 pm
content: Quote from: int2str on June 30, 2016, 03:23:47 amDo 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.

author: Skimask
author_rank: Super Contributor
timestamp: July 01, 2016, 04:23:17 am
content: 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.

author: int2str
author_rank: Regular Contributor
timestamp: July 01, 2016, 06:48:24 am
content: Quote from: Skimask on July 01, 2016, 04:23:17 amJust whatever you do, don't accidentally be helpful or anything.

author: LA7SJA
author_rank: Regular Contributor
timestamp: July 01, 2016, 08:57:28 am
content: Quote from: Skimask on July 01, 2016, 04:23:17 amI must be one of them, that sentence makes no sense to me (but I have no formal English education).

author: int2str
author_rank: Regular Contributor
timestamp: July 01, 2016, 10:36:19 pm
content: So, does it seem that my resistors are broken / out-of-spec?

author: wraper
author_rank: Unknown
timestamp: July 01, 2016, 10:44:53 pm
content: 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.

author: Skimask
author_rank: Super Contributor
timestamp: July 02, 2016, 03:36:15 am
content: Quote from: LA7SJA on July 01, 2016, 08:57:28 amYa, that's what I get when I don't correct the autocorrect which should've corrected the correction that I didn't correct.

author: SaintGimp
author_rank: Contributor
timestamp: September 08, 2016, 06:38:14 am
content: Quote from: LA7SJA on June 30, 2016, 05:50:50 amThis 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.

author: EEVblog
author_rank: Unknown
timestamp: September 08, 2016, 06:59:55 am
content: 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.

author: SaintGimp
author_rank: Contributor
timestamp: September 09, 2016, 03:13:47 am
content: Oh, I'm sure that your stock is generally fine, but my particular unit definitely has issues.I scrubbed the flux residue off the board but it made no difference. Then I desoldered R9 and measured it completely out of circuit - 9.70R. It is unambiguously busted. I'm perfectly willing to entertain the idea that I caused it to happen through unintentional abuse, but I don't know what that abuse might have been. It looks perfectly fine under a microscope.Anyway, I sorted through a whole bunch of 5% 10R 0805 resistors until I found one that happened to measure exactly 10.0 ohms, soldered that one into the R9 place, and now I get 0.9992 V out for the 1.000 mA DMMCheck current source, which probably means it's just about perfect considering that the UT61E seems to read DCV about 1 mV low anyway. I'm sure the tempco on this replacement resistor isn't great but it'll do for my purposes, I guess.

author: int2str
author_rank: Regular Contributor
timestamp: October 13, 2016, 10:50:03 pm
content: Took a while to get to this.De-soldered R9 and sure enough it was off.Bought a replacement from Digikey and now it reads much closer on the mA scale.Still have to replace the other resistors next.Too bad it came this way

author: JDW
author_rank: Frequent Contributor
timestamp: March 29, 2017, 10:17:12 am
content: Quote from: int2str on October 13, 2016, 10:50:03 pmI thought all uCurrent's were 100% QC tested prior to shipping. Or was that done only on the original, non-Gold edition?

author: luisprata
author_rank: Regular Contributor
timestamp: April 23, 2017, 06:31:30 pm
content: Not the same, but...https://www.eevblog.com/forum/projects/ucurrent-ma-range-error-using-oscilloscope-and-function-generator/msg1192329/?topicseen#msg1192329