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

[mzzj]

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.

You get rid of voltage ripple but still have "somewhat big" peaks on current measurement depending on your shunt resistor value.
OP's needs or scope shots are bit open to interpretation but guessing from the scope shot he has 1ms peak every 1 second?   

4000uF cap with 1ohm shunt would reduce 400mA 1ms peak alone to ~90mA peak over the ~0.4mA  average. 

[ziplock9000]

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..

The most demanding part is a WIFI module. As stated earlier, the largest peak is 300-400mA.

[ogden]

The most demanding part is a WIFI module. As stated earlier, the largest peak is 300-400mA.

Hey, when instead of chatting you will solder capacitor on your device input terminals and actually try something?

[David Hess]

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.

[ziplock9000]

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.

[David Hess]

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.

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"... 

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]

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.

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.

[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]

It's a good job I used the phrases "I imagine" and "I assume"... 

Unfortunately the DMMS I own are way off on MAX and MIN compared to my scope, thus I'd not trust the average either.

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]

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.

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.