Electronics > Beginners
How to measure average power use over time for small fluctuating circuit?
ziplock9000:
That's what I tried to do with a smaller cap. I'll have to order a bigger one. I've got no supercaps that are more than 3v
BrianHG:
You can use 1megaohm with your 6800uf cap. That should smooth things out, except, you will need to either work out your meter's internal resistance to correct for that huge series resistor, or, disconnect the meter from the cap, and touch the meter on the cap for 1-2 seconds to take a reading every 30 seconds or so.
You can also use a voltage follower J-Fet input opamp to get the cap voltage without a DC load.
David Hess:
--- Quote from: ziplock9000 on September 25, 2018, 12:34:19 am ---It's a good job I used the phrases "I imagine" and "I assume"... ;D
Unfortunately the DMMS I own are way off on MAX and MIN compared to my scope, thus I'd not trust the average either.
--- End quote ---
There is a big difference between min/max and average.
The input is averaged over the integration time so the min/max returns the minimum or maximum of a bunch of averages which is often useless. But the average returns the average of a bunch of averages which is exactly what you want.
Test it yourself using the trigger output of your function generator. My better handheld multimeter returns a 1% error at 40kHz.
mzzj:
--- Quote from: BrianHG on September 25, 2018, 07:14:00 am ---You can use 1megaohm with your 6800uf cap. That should smooth things out, except, you will need to either work out your meter's internal resistance to correct for that huge series resistor, or, disconnect the meter from the cap, and touch the meter on the cap for 1-2 seconds to take a reading every 30 seconds or so.
You can also use a voltage follower J-Fet input opamp to get the cap voltage without a DC load.
--- End quote ---
1 megaohm gets you in trouble with electrolytic capacitors leakage currents. Typical 6800uF cap leakage current could be anything in 5 to 50uA range at half the rated voltage.
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