At 11:32 PM 27/05/2013, you wrote:
>I'd love to be able to find something like
>http://www.powerwerx.com/digital-meters/dc-inline-watt-meter-power-analyze=
r-powerpoles.html
>but
>for smaller currents.
>
>I currently have several products which are used in solar power
>environments and one of the most common questions is actual power
>consumption.   I can (somewhat) measure instantaneous power consumption bu=
t
>telling what a reasonable long term average is for a device which consumes
>less than 1/10th of a watt and is powered by 24V (aka 4mA of current) is a
>real challenge when all the devices you find have a resolution of ~10mA an=
d
>are designed for amps of current and dozens or hundreds of watts.
>
>I'm tempted to simply use a logging voltmeter of some sort with a current
>shunt resistor, but that leads to an entirely different set of problems
>related to using most of the logging voltmeters that I've had - that is,
>the sample rate is completely atrocious when you want to be able to captur=
e
>what may be small spikes in the current load.
>
>What does everyone else do when they need to measure low-current
>consumption over a long-term average?
>
>-forrest

Since the voltage is pretty much fixed, you just need average
current for average power drawn from the source.

If the sampling meter is competently designed, it should have an
analog anti-aliasing filter which will take care of this for you,
and you can just integrate the sampled data digitally.

Failing that, slap a BFC across the shunt s.t. the time constant
is >> the sample period. Eg. 100R/10uF+ for 10kHz sample rate
and you'll probably get close enough.

If you want to measure (and validate predictions) for products
you've designed in detail, looking at various pulse-y modes,
an oscilloscope is advisable. See TI's AN092 (on BT measurements,
but equally applicable to other situations). Ideally, you'd use
a digital scope with a deep enough memory to cover the events of
interest and a $$$ current probe.

--sp


--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.