accurately measuring integrated power consumption

Winfield Hill · 2019-05-31T09:30:20+00:00

What's a good instrument (need make & model number) to measure the integrated power consumption of my complex micro-controller controlled bee-hive monitor? It rapidly goes in and out of multiple modes, turns 74 sensors on/off, pulses LEDs to 50mA, draws 65mA to heat micro-miniature hot plates, etc. Average current is from 0.5 to 3mA, but is hard to measure. It spends much of its time drawing microamps, and this must also be accurately integrated over time. Going through my considerable collection of smart, highly-capable instruments, there's not one that will make continuous, accurate integrated-current power-consumption measurements. The closest are awesome 6.5-digit bench multimeters, that can be set to measurement integration times of 100 PLC (about 1.6 seconds), but these are discontinuous measurements, with gaps, not suited for the job. Yes, there are fuel-gauge ICs available, but we need to purchase an instrument, with a wide dynamic range / peak handling capability, plug it in, and take readings, so we can experiment as we work, programming the beehive-monitor's software. Not even my new awesome-looking Keysight E36312A data-logging bench supply can do this simple task. The word, "integrate" doesn't appear in the manual. Nor does the word, "average" appear. OK, I have this little $15 USB stick, with a cute display, that includes a running mA-hr value. If I open it up and do some cut-jump hacking, maybe it could be integrated into my bee-hive monitor's power system. What's its peak-handling capability, who knows. Sheesh! Maybe my 66321B "Mobile Communications DC Source", although the word, "integrate" doesn't appear in its manual either. OK, it will "average" discrete measurements. Not very re-assuring. It can fill up a buffer with 2048 data points... Arrghh! -- Thanks, - Win

May 31, 2019 31 Replies

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Winfield Hill 7 years ago

I have several nice Keithley SMUs, maybe the answer is to code a sampling-integrating mode (they connect to your computer using Java). But then, why not use an HP 34970A logger? Hey, maybe I should ask Legg about that, he's the expert with those machines.

Thanks, - Win

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Winfield Hill 7 years ago

Thanks, Fred! Aha, so the secret was "coulomb counter", this unleashes what I was looking for. Maxim suggests the MAX17211 as a replacement, but I prefer the MAX17201 I2C version. Eval versions for both are available.

This looks better, although the 100uV offset spec is a huge 0.2% of 50mV full-scale, a real problem with my high dynamic range. But I guess it can be trimmed. Too bad they don't give an offset drift spec. Hopefully it's 100x lower.

Arrow has DC590B in stock. I can hook the output pulses up to a counter. Yeah, no software or programming required!

Thanks, - Win

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Winfield Hill 7 years ago

STM32_power-shield, p/n X-NUCLEO-LPM01A $70 in stock. Yes, that looks very good as well, ready to go.

Thanks, - Win

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Winfield Hill 7 years ago

Nothing, it's very good, but I'd have to build one.

Thanks, - Win

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Winfield Hill 7 years ago

Yes, a good suggestion indeed.

Thanks, - Win

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jrwalliker 7 years ago

How about an ultracapacitor (bypassed with something smaller) in the feedback loop of an op-amp to make a precision integrator?

John

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Winfield Hill 7 years ago

Yes, John, right. So many easy ways to do it by designing and building something. I was hoping for an instrument we could purchase, with a few controls and a mAh display.

Thanks, - Win

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Clemens Ladisch 7 years ago

Most of TI's MSP430 eval boards have "EnergyTrace", a software-controlled DC/DC converter that counts charging pulses. Google says there is a standalone mode where you do not need to debug an MSP program.

Regards, Clemens

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Sjouke Burry 7 years ago

Those caps have an awful memory effect. They can store a lot of charge, but with a huge error because of memory.

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jrwalliker 7 years ago

Thanks. That's good to know.

John

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Chris Jones 7 years ago

I'm not sure whether this thing is any good, but might be worth a look:

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Reviewed here:

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other alternatives:

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You could low-pass filter the current waveform (e.g. put a big capacitor across the meter), then it would not be necessary for a DMM to have an integrating characteristic, it would just have to sample the waveform faster than twice the highest frequency component present in the filtered waveform.

Another idea: use a fast log converter circuit (e.g. based on p-n junctions) and digitize the output of that with a low resolution but fast ADC such as a DSO. Then you can figure out the current at each time point with very high dynamic range, though only modest accuracy. That should still get you the total coulombs within a few percent, over pA to mA with no range switching.

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