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How to measure average power use over time for small fluctuating circuit?

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mzzj:

--- Quote from: ziplock9000 on September 23, 2018, 03:39:14 pm ---
--- Quote from: mzzj on September 21, 2018, 10:34:23 am ---One trick you can use for very low average currents is to use capacitor as a charge integrator. Measure voltage at the start and after 1 minute, calculate current from voltage drop and capacitance.
(For best results measure the capacitance)

If the capacitor size gets non-practical for higher currents or longer durations you can use Li-ion battery as a charge integrator.
Li-ion coulombic efficiency(amh-hours in vs amp-hours out)  is very high, for a good quality cell something in the range of 99% to 99,9% (you discharge 1Ah and charge 1.001Ah)
Measure the current during charge and charge to exactly same voltage where battery was when you started the discharge and you can get better than 1% accuracy.

--- End quote ---

What size capacitor would you recommend? 90% of the time the device is using ~26-40mA, with spikes up to maybe 400mA. The power cycle repeats itself after 1 minute, but I'd like to get a few repetitions of this if possible to average out over several cycles.

Thanks.

--- End quote ---
Bit high current for ordinary electrolytic cap but totally doable with ultracapacitors
Lets say that your circuit uses about  50mA average and you want to measure 5 minutes = 60*5*0,05 = 15 Amp-seconds, if you use 15 farad capacitor the voltage would drop 1 volt.
1 volt drop is bit exessive unless its battery powered equipment, maybe aim for ~0.3 volt drop if its supposed to run from 5v fixed voltage supply, that would mean 45F ultracapacitor.
2x100F ultracapacitors  in series would handle 5 volt supply and give you 50F capacitance. Leakage current might need a further look depending on case.

Or just filter the crap out of it with enough big electrolytic after the shunt resistor, 1 ohm shunt and 100000uF capacitor will filter the 400mA 1ms peak down to 4mA

iMo:

--- Quote ---What size capacitor would you recommend? 90% of the time the device is using ~26-40mA, with spikes up to maybe 400mA.
--- End quote ---
You may try with two 600F 2.7V supercaps wired in series. That may give you a few minutes with 2A spikes max, with a drop from 5V to 4.5V. It may cost you $80 however.
Doublecheck the calculation, estimate only.

ogden:
If we assume that 0.1V drop is tolerable during 1ms spike @0.4A:

(0.4Amp * 0.001sec)/0.1Volts = 0.004 farads = 4000 uF. No need for supercapacitor.

iMo:
The OP wants to measure over several minutes, afaik. Also, he/she is using a few "electrically moving parts", maybe servos. Therefore 2A peaks are possible..

ogden:

--- Quote from: imo on September 23, 2018, 06:07:37 pm ---The OP wants to measure over several minutes, afaik. Also, he/she is using a few "electrically moving parts", maybe servos. Therefore 2A peaks are possible..

--- End quote ---

Several minutes? What you are talking about? If you need power supply for several minutes - don't use capacitor but power supply!  :D You know - one that is mains powered, ok? Bulk capacitor is needed to store enough energy for high current spike which is specified by OP as 0.4A, to decrease dV/dT - so you can sample current/voltage at lower sample rates.

If you wonder what I and OP are talking about here, you are advised to read my  post:


--- Quote from: ogden on September 21, 2018, 10:42:28 am ---
--- Quote from: ziplock9000 on September 21, 2018, 09:02:50 am ---Yeah that would work, but I was hoping for something a bit more elegant. It's looking more and more like a new Arduino based project maybe based on TM7705 based Data Acquisition Module. But I'd have to read up more on it.

500sps might be good enough:
https://www.instructables.com/id/Arduino-16-bit-500SPS-Dual-Channel-DAQ/

--- End quote ---

You don't need ultrahigh sped sampling in case only thing you want to measure - average power. The trick is - additional bulk capacitor between current shunt and device under test, to smooth current spikes and be able to run lower sampling rate.


--- End quote ---

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