Electronics > Beginners
How to measure average power use over time for small fluctuating circuit?
(1/8) > >>
ziplock9000:
I have a microcontroller project that has a few electrically moving parts including lots of sensors and power hungry WIFI modules and such. I want to get an accurate power use profile of the device that I can use to systematically reduce the device's power usage with the aim to hopefully get it working on batteries. To be able to do this, I need to be able to measure how effective each change I make to the device's code and electronics to compare to previous states of the system. This means I need to take a power snapshot of the device over a certain time window that is at least as large as a complete cyclic repetition of its power cycle. (This just happens to be 1 minute at the moment, each time the WIFI triggers)

I immediately realised that a DMM is no good for this so I used my DSO in roll mode to measure voltage (and by way of a dodgy 1 ohm current shunt resistor) and current over time. Using the DSO's built in maths function I multiplied the two two give me power over time.

Well.. It's not good enough for the following reasons:

1) On my DSO (Rigol DS1054) the maths function only works with data shown on-screen. This is not nearly accurate enough on either axis. On the Y axis there's just simply not enough bits of data and the output is quite quantised even when I use as much of the display as possible. The problem is even worse on the X axis (time) as data will fall off the end of the screen if the roll speed is too fast and sub-pixel data will not be registered if it's too slow. There's no good happy compromise between the two either, not without massive errors anyway.

2) Even if it did work well, the data is power over time and I really want *average* power over time for a certain time window.. Say 10 minutes, 1 minute, 1 hour, whatever I choose. Currently the repeating cyclical phases is about 1 minute but that might change.


How can I proceed with this on a budget?

(Just an FYI.. we are talking about current draws that are (very roughly) 0ma, 25ma, 75ma and spikes of 300mA)

I've attached an image of my crude attempts with my DSO
mtdoc:
There are small, inexpensive power monitors that will do this. For example, I own one of these PortaPow devices and it works well.
ziplock9000:
Interesting thanks. Do they measure down to a few mA and sample in the milliseconds?
Also can you specify the sample window duration?

Thanks.
mtdoc:
Resolution is 1 microamp. I don’t know the sampling rate - but good question. You might need to contact the manufacturer. They’re in the UK.

Sample duration is manual - i.e. time it yourself... :)
ziplock9000:
I've just realised it's a UK company for once, so no huge price hike from a US import for a change, bonus!  ;D

Yep, I've just sent them an email.

1 microamp is more than enough for me, but it's the sample rate that I'm worried about. Most of the example uses are for things like charging phones that don't have spikes etc.

Anyway this is a fantastic suggestion, thanks!  :D

(UPDATE: The display seems to update once a second.. If that's indicative of the sample rate then it's far too slow :/)
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod