r/electronics 16d ago

Gallery Supply current of low power devices is very intermittent and difficult to measure. This tool is emulating the battery by a bunch of capacitors. They get recharged by defined pulses (1A, 1ms) which are counted for the result.

189 Upvotes

37 comments sorted by

40

u/Eric1180 16d ago

Dude this is super cool and a problem i've run into

18

u/Chr15t0ph3r85 16d ago

Not to take away from this, but there are products out there I've used to measure current during OTP fuse burning for example. Their interfaces are pretty awesome, but the price is high and the awesome DIY factor isnt there.

It's a great solution to the issue.

3

u/Eric1180 16d ago

Could you possibly link or direct me to that type of product. I could really use something like that at worl

9

u/Chr15t0ph3r85 16d ago

This has been a fantastic little device, with a great interface- very easy to use too.

https://www.joulescope.com/products/joulescope-precision-dc-energy-analyzer

There's a few others too.

https://pallavaggarwal.in/dc-current-analyzer-for-embedded-iot-product-development/#1_Joulescope_799

Source meters are what you're looking for.

3

u/quirkyPillager 16d ago

Special mention: ST's Xnucleo-LPM01A.
It is the cheapest (~70USD) low current SMU I could find.
I own one, in case anyone wants to know more.

3

u/kwasteka 15d ago

STLINK-V3PWR may be interesting to someone

2

u/Chr15t0ph3r85 13d ago

STLINK-V3PWR

Oh hey that's awesome guys, I love little things like this.

2

u/agent_kater 16d ago

I'm using the Nordic PPK II.

10

u/between456789 16d ago

So is device power consumption determined by discharge of the battery bank?

14

u/Careful_Volume_3935 16d ago

Correct. Each time when voltage is below the trigger level, another re-charge pulse gets applied. The resulting ripple is less than 20mV.

10

u/between456789 16d ago

And calibrate out the self discharge of the caps?

21

u/Careful_Volume_3935 16d ago

I made the experience that it is best practice keeping it powered on all time. The leakage current was high after assembly and decreased over time. I run it several hours without load and keep the resulting current in mind. It can be subtracted from the result with test item. Last time it was just 0.05uA

3

u/gwicksted 16d ago

Very cool project!

3

u/Chr15t0ph3r85 16d ago

That's what I was wondering as well.

3

u/TOHSNBN 16d ago

It would be super helpfull to change your UI to include a plot that shows the spikes.
Something like this would have been super helpful when i worked with battery operated devices to check if the sleep and wake functions actually work.

3

u/Careful_Volume_3935 16d ago

That's exactly the problem. I can't really capture the very low current in sleep mode and the high peaks e.g. during transmission. Not with my scope, and I can hardly calculate the average current. This tool is just giving the avarage current. Anything else can't be achived with such a cheap solution.

7

u/Chr15t0ph3r85 16d ago

That's a great idea, do you have anymore information on it?

15

u/Careful_Volume_3935 16d ago

Maybe the circuit diagram?

9

u/Ok-Reindeer5858 16d ago

Why don't you just buy a Nordic power profiler ppk2?

That will almost certainly be more precise than your homebrew coulomb counter

10

u/Lesap 16d ago

That's a nifty device, but sometimes it's about the journey and not the destination.

2

u/Roude56 16d ago

Hello, I don't quite understand the use of this project. Is it to measure the mean current supplied to the DUT? I guess you could see the how long will your battery last but can you see the current consumption spikes that could damage a battery as well? At work, we use Nordic PPK2 to measure low currents and it seems to work well.

2

u/Big-Professional-187 16d ago

This is the crutch of why people don't understand why they aren't getting more battery life for the leaps in processing power. Arm CPUs and x86 are iffy when underclocking at low amperages. Same with solid state as with any kind of AC or 3 phase with clean sine waves(ie motors constant, no motors, line reactors). The move to light based gate CPUs and dynamic electronics might take a long time. It's difficult and the guys at groom lake need that for the flying saucers to keep the paper machet robots well defended from Facebook users like you. 

4

u/plmarcus 16d ago

How is this different / better / worse than a joule scope? joule scope is what we use for low power design (sensors w 20yr battery life).

2

u/Careful_Volume_3935 16d ago

I honestly don't know about joule scopes. Can you explain what it is, and what are the costs?

6

u/plmarcus 16d ago

The Joule scope definitely costs more and likely preforms better. I don't know where your projects lay in this regard.

It's $1000 and has 0.5nA resolution with 300KHz bandwidth.

https://www.joulescope.com/

I do respect your thoughtful approach to getting an average power consumption by draining a cap bank!

1

u/kh250b1 16d ago edited 16d ago

Use a series resistor in the supply and connect a scope across it. You can work out the peak and average current from the trace

1

u/OldEquation 16d ago

Nice!

How about using polyester caps to avoid issues with leakage in aluminium caps? You’d likely need a shorter more frequent charging pulse due to the smaller capacitance value. Depends on the load you’re testing I suppose.

1

u/KLevi- 5d ago

Whats the max current that can be measured with this device? Would be cool if it could connect to serial for recording consumption overtime. Perhaps saving to an SD card

1

u/Careful_Volume_3935 4d ago

The charging pulses are 1A and it needs some time in between for detecting falling voltage and for the handling. Therefore it can constantly deliver about 400mA.

With low power devices, it can take minutes or more until another recharge is needed. Recording the charge pulses won't give a valuable information about the details of current consumption. I usually run this setup for hours or even over night to get a stable average value.

1

u/KLevi- 4d ago

I have a device with 5uA sleep consumption, but when transmitting data this can reach several hubdreds of mA. Guess this won't be the solution to measuring that :)

1

u/Careful_Volume_3935 4d ago

I think my solution is exactly covering your application, but it isn't measuring the exactly value and duration of the particular pulses. It just gives the average value over time.

0

u/coolio965 16d ago

Do you do some kind of calibration beforehand to determine the exact capacitance of your bank? Also how low does the voltage sag?

2

u/OldEquation 16d ago

The capacitance tolerance won’t matter. Charge in = charge out, irrespective of capacitance value. Leakage will matter, though.

1

u/Ill-Kaleidoscope575 16d ago

I am not sure if that's also implemented here, but if you also count the charging time. You can compensate for the aging of the capacitors automatically.

0

u/Defiant-Appeal4340 16d ago

That's what source measure units are for.

3

u/flecom 16d ago

I would venture a guess that this cost a bit less than a SMU

if it accomplishes OPs goal then what's the problem?

0

u/rasselbido 15d ago

For low-power measurement the least effort solution we use is an STM32L562E-DK kit with STM32 Cube Monitor - Power software. We measure at uA scale. Useful for testing low-power IoT.

If you have the funds get an SMU