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Re: measuring current consumption in uC with very low power modes

Hi to all,

> > often i have to measure the current absorbed by small circuits,in > battery appplications Normally i deal with consumptions big as 20mA in > run mode and 2uA in sleep mode. > > One method that sometimes i use is to desconnect the battery,leaving the > circuit feeded only by a 10,000 uF capacitor,seldom and quickly measured > by an high impedence multimeter,(of course not a 10 Mohm oscilloscope > probe).Measuring the difference in voltage at a specified times i can > calculate the consumption.The cap was previously measured,to avoid > tolerance error. > > More often i need a more immediate measurement,so i feed the circuit by > a commutator switch,in parallel to 1,10 and 100 ohm shunt resistors. > The latter is constantly inserted so that the circuit is not completely > relying on itself during commutations. > When the uc goes to sleep and i need to have an hopefully realistic > rading of few uA i switch on the 100 ohm shunt,the voltage drop between > battery and circuit is below the mV,therefore irrelevant. > The voltage is read by a 6 and half digit DMM via a 60cm(2 ft)twisted > wire. > > Is this a good method or others can be better? > > Is there any advantage using an instrumentation op amp as buffer or > amplifier, wired very closely to the shunt? > > Many thanks for your attention > > Diego

I'm taking the liberty of cross-posting this to sci.electronics.design, because this is really an electronics question, not an embedded question. I think you'll get more answers from there, and many of them will even be good ones.

I think your commutation idea will suffer in accuracy if the load is fairly stiff in voltage -- like if you're driving a capacitor. In that case the current with the low resistance switched in will be higher than without.

If the circuit is otherwise isolated, the best way to do the measurement may be to put your 100 ohm resistor between the circuit ground and the negative terminal of the battery, then amplify its voltage by 10 or 100 using a chopper-stabilized op-amp. Then you can just read the voltage and scale it appropriately to current.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott
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I was going to suggest a TIA (trans impedance amp.) opamp in the ground return line, maybe with a few R's for different ranges.

I put one together to measure semi conductor I-V and I've used it for a few different things now. (AC resistance of tap water a few weeks ago.)

George H.

Reply to
George Herold

For a non-varying current, I would perhaps use a series resistor (or pot) in the supply rail with a multimeter across and adjust to get, say, around 100mV drop, then substitute the device under test with a second resistor - maybe another pot - to achieve the same drop across the series resistor.

Cheers

--
Syd
Reply to
Syd Rumpo

get one of Dave Jones uCurrent

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-Lasse

Reply to
Lasse Langwadt Christensen

Maybe with a switch to short the input when you switch R's. (would current interuptus wake up the uC?)

Reply to
George Herold

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OPs method makes a lot of sense where the average current is a lot lower than the maximum current. Many low power uPs have sleep modes where they only run about 1 percent of the time; the rest of the time only timers and such run. Greatly reducing average power and making measurement much more difficult.

?-)

Reply to
josephkk

That's an ammeter, but for most purposes an integrating function (that measures amp-hours) would be useful. I'd nominate a flyback-type power source, where a switching supply dumps a fixed charge on the output with each cycle (i.e. not a pulse-width-modulation type). Then, you just count the pulses, for a second, or hour, to know how many milliamp-hours were consumed.

Reply to
whit3rd

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