Electronics > Beginners
How to measure average power use over time for small fluctuating circuit?
David Hess:
--- Quote from: ziplock9000 on September 24, 2018, 09:08:44 pm ---I imagine they do this digitally by taking samples as mentioned in my OP. I'm going to make an assumption here that the sample rate is going to be at least 2 order of magnitude too slow for my use (which isn't really that high speed anyway) of around 1 to 10x second whereas I need at least 1ms. For example all of my DMM, bench and mobile are WAAYY off and don't catch the MAX spikes or even the MINS that are much longer in duration.
--- End quote ---
That is not how multimeters work.
Except in special cases, the input is *integrated* over a whole number of power line cycles so it is already averaged over some integer multiple of 16.7 (60 Hz) or 20.0 (50 Hz) milliseconds. The reason for this is to produce nulls in the frequency response at the power line frequency and its harmonics which considerably helps normal and common mode rejection of power line interference.
Averaging mode if the meter supports it continuously averages these results.
But the actual response of the meter before integration extends to kHz or 10s of kHz and can be determined by looking up the AC frequency response. An AC input signal of 60Hz will show 0 when measured in DC mode but a DC pulse will produce the correct average within the AC measurement bandwidth. So for instance a 0 to 10 volt DC pulse with 50% duty cycle at 1kHz will measure 5 volts DC.
Measuring minimum or maximum values are limited by the integration time of each sample. Some meters use a faster than normal integration time or include separate peak detection to capture faster events.
ziplock9000:
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.
mtdoc:
--- Quote from: David Hess on September 24, 2018, 12:29:18 pm ---My better handheld multimeter has a minimum, maximum, and averaging function which can make these measurements over arbitrarily long times. A multimeter with averaging would be perfect for this.
--- End quote ---
You’d need two of such meters, one for voltage and another for current (preferably with low burden voltage) to do this, no?
ziplock9000:
The largest cap I have is a 6800uF and it didn't make much difference. I placed it between the shunt and the DUT from the high to low rail. There was minimal smoothing
BrianHG:
Place your own shunt resistor between your power source and your device under test (DUT).
Place a series resistor of 100k and a 100000uf6.3v cap together, parallel across the shunt resistor.
Have an optional switchable 1k or 10k resistor in parallel with the 100k to allow fast charging of the 100000uf6.3v cap.
Measure the voltage across the 100000uf6.3v cap and calculate your current based on the shunt resistor.
You will have a smooth average voltage readout which you can then calculate the current from.
It should take around 1-2minutes for the average current to steady out.
47000uf 6.3v or 33000uf6.3v cap should also be good.
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