Low(mA, uA) current measurement equipment - Page 1
Forum section: electronics / testgear

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 03:29:27 pm
content: I am looking to buy some equipment to measure low current drawing devices in the micro--amps / milli-amps range. I don't have thousands to spend, but can manage couple hundred. I found "uCurrent" and it's clones but a lot of them are out of stock. Are there any suggestions for equipment that I can look at?

author: Brutte
author_rank: Frequent Contributor
timestamp: May 31, 2019, 05:44:38 pm
content: All mostly depends on what accuracy you need.I would say that 1999 count $3 multimeters typically have those ranges, accuracy is 0.5%.For uC development and low power I use an analog ammeter. This is 1.5% but it drains no battery and it is very easy to read, when compared to digital.

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 06:20:20 pm
content: I am looking to develop low power microcontrollers and wireless devices, I'm not sure 0.5% accuracy means here (pardon my noob-ness). Due to the nature of these devices(sleep most of the time), I need an equipment I can track w.r.t. time. So it's fine if the unit has a logging feature so I can hook it up to my computer and look at the current against time. I found U1272A from Agilent ($389) which has capability to measure low currents and also has a logging feature. 34461A from keysight ($1121) that's on the pricier side. Are there any cheaper (sub-thousand) devices that are similar to these devices that I can look at?EDIT: There is also DMM6500 ($1140) which seems to do low currents

author: HKJ
author_rank: Super Contributor
timestamp: May 31, 2019, 06:36:34 pm
content: It is easy enough to measure low current (uA), nearly any multimeter can do it, the problems arrives when you want low voltage drop and large dynamic range, i.e. a microprocessor that wakes up from sleep once in a while.There is specialized equipment to handle that, both professional equipment and some cheaper equipment like "Joulescope" or a fancier version of the uCurrent called CurrentRanger.

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 06:51:56 pm
content: > the problems arrives when you want low voltage drop and large dynamic range, i.e. a microprocessor that wakes up from sleep once in a while.Sounds like my use case. What professional equipment can handle that? Can the U1272A be good enough or do I need to consider something like DMM6500.Current ranger looks pretty decently priced. If it does what it advertises, then I might give it a shot! Thanks for the suggestion.

author: HKJ
author_rank: Super Contributor
timestamp: May 31, 2019, 07:11:25 pm
content: Quote from: rookie on May 31, 2019, 06:51:56 pmNo "cheap" multimeter will work, you need either the equipment I mentioned above or some rather expensive equipment from the big brands.Keysight has a note about it here:http://literature.cdn.keysight.com/litweb/pdf/5992-1765EN.pdfThe DMM6500 is always worth getting, it is a very versatile meter (I have one), but like anything except specialized equipment, it has a limited dynamic range.You can do it with regular meters, but you have to split the job in two: Standby power consumption and active power consumption: Measure them separately and add together with appropriate weights.

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 08:05:02 pm
content: Thanks for the app note and your inputs ! Very helpful

author: HKJ
author_rank: Super Contributor
timestamp: May 31, 2019, 08:49:19 pm
content: I forgot to mention large capacitor with low leakage current, place one on the power rails of your device and youmightbe able to measure the average current consumption with a ordinary multimeter.

author: hamster_nz
author_rank: Super Contributor
timestamp: May 31, 2019, 08:57:34 pm
content: I've been wanting an excuse to disuses this for a while.To measure deep sleep currents for ESP32 projects and other stuff I've been using a ANG8008 on uA, and switch to short the meter out of the loop until the firmware has it in deep sleep.I would love a low-cost equivalent ofhttps://www.qoitech.com/products/standardto become available - It looks to be the Saleae of low current Power Supplies (e.g. really nice, but way more $ than I could afford!).Maybe we should get a team together and build one???

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 09:27:21 pm
content: Oh wow, that Otii device looks interesting, lil pricey.

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 10:40:53 pm
content: So far of all devices I have looked for, I just wanted to put them down here for reference. If anyone has an opinion of choosing between either, I'd love to know.uCurrentCurrentRangerJoulescopeOtiiZS-2102-ADMM6500N6705C

author: hamster_nz
author_rank: Super Contributor
timestamp: May 31, 2019, 11:11:41 pm
content: I can't edit your post, but links for each would be handy....

author: rookie
author_rank: Contributor
timestamp: May 31, 2019, 11:38:07 pm
content: added links

author: MosherIV
author_rank: Super Contributor
timestamp: June 01, 2019, 10:07:47 am
content: HiI think any dmm that has mA or uA (micro Amp) range will do that job.Even the cheap DT830 or freebie from harbour freight.Even the voltage drop in not a problem.As others have pointed out, the problem is dynamic range.That means that when the micro turns on for a short period, the measuring device may not see it.I suggest a oscilloscope with an in line resistor in the power to the micro as well as a current meter (anything that does mA or uA).Use a known resistor value, so that you can apply ohms law, ie Amp = V / RTo work out the current by looking at the V in the scope.The dmm is there to verify you are in the right ball park when on the low current reading.Another idea.In that past, I have used bench meters with averaging facility to check low power draw over time

author: Fungus
author_rank: Super Contributor
timestamp: June 01, 2019, 10:42:34 am
content: Quote from: MosherIV on June 01, 2019, 10:07:47 amNope.Any resistor that's small enough to work with an oscilloscope in the uA range will have too much burden in the mA range.

author: hamster_nz
author_rank: Super Contributor
timestamp: June 01, 2019, 12:23:42 pm
content: I was thinking about prototyping something that had a AD7193 (4-Channel, 4.8 kHz, Ultralow Noise, 24-Bit Sigma-Delta ADC with PGA) over a 1 ohm current sense resistor, maybe with followed with a diode to ground to lift both differential inputs away from the 0V rail.I was thinking of this part because I already have a Digilent PMOD AD5 in my stash-https://store.digilentinc.com/pmod-ad5-4-channel-4-8-khz-24-bit-a-d-converter.Sure, bandwidth is limited, it would need calibration and so on, but might be far better than an inexpensive multimeter, have much better dynamic range and cost far less than the other options here.If it worked out playing with with the PMOD, the ADC IC is ~USD$12 in qty 1, making it would be quite possible to make a PCB with the ADC, a small micro and a USB to serial bridge, maybe even an LCD or OLED for feedback.

author: bson
author_rank: Unknown
timestamp: June 01, 2019, 01:59:15 pm
content: I use my Keysight 34465A with the DIGI option. Pretty much everything I design has a voltage regulator on the input, so the burden voltage doesn't matter - just turn up the supply a couple of volt (if even necessary). Current in = current consumed, by definition.

author: hamster_nz
author_rank: Super Contributor
timestamp: June 02, 2019, 11:40:05 am
content: Tonight I did a bit of playing with an ESP32, a AD7193 ADC and a passive resistor voltage divider to generate low voltages.Input => ADC countShorted to Gnd => 3003.140mV => 1,345,8505.701mV => 2,444,0006.955mV => 2,987,030Without real analysis it looks like noise is about +/- 100 counts (+/-233nV).Over a 1 ohm shunt it seems that it might be a workable way to sense from about 1uA to ~20mA.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 07, 2019, 07:00:10 pm
content: Quote from: rookie on May 31, 2019, 06:20:20 pmYou need the Current Ranger. I used it to measure down to nA.Again if you are developing the MCU - this is your best bet at $120.https://lowpowerlab.com/guide/currentranger/

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 07, 2019, 07:30:05 pm
content: You can also use an SMU... like a Keithley 2400 and have power supply and meter in one... how much do they go for... $4k.. 6.5 digit resolution.

author: rookie
author_rank: Contributor
timestamp: June 07, 2019, 10:38:10 pm
content: Why would I choose an SMU like the 2400 (source + measurement) as compared to say DMM6500 (also 6.5 digit)?

author: Electro Fan
author_rank: Super Contributor
timestamp: June 08, 2019, 04:11:27 am
content: FWIW, Keithley 6500 Specs

author: Electro Fan
author_rank: Super Contributor
timestamp: June 08, 2019, 04:17:37 am
content: FWIW, Current Ranger specs

author: Electro Fan
author_rank: Super Contributor
timestamp: June 08, 2019, 04:19:46 am
content: Very different products and functionality, of course.

author: sambonator
author_rank: Regular Contributor
timestamp: June 08, 2019, 06:15:53 am
content: Quote from: Electro Fan on June 08, 2019, 04:17:37 amWhatever happened to uCurrent? Is it still in production? I don't see them available anywhere.

author: JxR
author_rank: Unknown
timestamp: June 08, 2019, 06:30:47 am
content: Quote from: sambonator on June 08, 2019, 06:15:53 amhttps://www.digikey.com/reference-designs/en/sensor-solutions/current-sensing/2637It is open sourced now, so I think you just build it yourself if you want one.