accurately measuring integrated power consumption

Well, one way is to buy lots of batteries. The old Tadiran lithium cells for computers (Macintosh 6100) all went flat about three years from purchase date, and from the manufacturer's data I estimated the clock drain at 36 uA. With some sheets of brass, I shunted the current through a VOM, and sure enough, it read... 36 uA.

So, within a few percent, you could drain a fresh battery to calibrate, then after a month or year of hive instrument operation, yank the used battery and drain it, noting the mAh difference.

The battery chemistry really IS a good, reliable indicator of current drawn No little spikes or slow leaks will get past the chemical change record.

Reply to
whit3rd
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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.

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 Thanks, 
    - Win
Reply to
Winfield Hill

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!

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 Thanks, 
    - Win
Reply to
Winfield Hill

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

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 Thanks, 
    - Win
Reply to
Winfield Hill

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

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 Thanks, 
    - Win
Reply to
Winfield Hill

Yes, a good suggestion indeed.

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 Thanks, 
    - Win
Reply to
Winfield Hill

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

John

Reply to
jrwalliker

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.

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 Thanks, 
    - Win
Reply to
Winfield Hill

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

Reply to
Clemens Ladisch

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

Reply to
Sjouke Burry

Thanks. That's good to know.

John

Reply to
jrwalliker

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

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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.

Reply to
Chris Jones

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