[volt-nuts] Cheap Oscilloscope measurement of small current?

[Peter Loron]

Well, there's no hard goal of battery life or consumption. However once
I get things working, I do want to optimize power consumption to get
good battery life, or be able to run it from a small solar cell.

I'm thinking that for the initial work, I can simply use a regulated
bench supply as the power source.

I'll take a look at those MAX parts...might be worth checking out.

Thanks.

-Pete

On Fri, 2010-11-05 at 09:08 -0500, Oz-in-DFW wrote:
> I'm doing quite a bit of this and like most things, the answer is "it
> depends. " 
> 
> What I'm doing right now is using a resistor in series with the _input_
> of a low leakage micropower Low DropOut (LDO) regulator because this (15
> uA) circuit will run off of a regulated supply.  The LDO has a steady
> state consumption of a few microamps and that is largely a function of
> input voltage and operating temperature at the low operating currents
> I'm dealing with.  My workshop temperature is constant enough.  This
> works well below a uA.  I just use my plain old Tek scope with two
> channels in differential mode.  If I neede teh extra channel I'd brew a
> diff amp or use something like this:
> 
> http://datasheets.maxim-ic.com/en/ds/MAX9938.pdf
> 
> I believe you have a much more complex problem.  You will be operating
> from battery or solar supplies that likely have a defined voltage and
> internal resistance over time.  This can have a profound impact on
> operating time, particularly if subtle considerations are as significant
> as you imply they are.  You'll either need to take a lot of measurements
> and model the supply carefully, or you'll need a good simulator of the
> supply. 
> 
> On 11/4/2010 6:07 PM, Peter Loron wrote:
> > Hello, all. I'm working (very slowly, but working) on some designs that
> > will be battery (and possibly solar) powered sensors with a wireless
> > data backhaul (likely 433 or 915MHz).
> >
> > As part of my never ending quest for more tools, I'm starting to educate
> > myself on what I may need to get measurements on current usage over time
> > for these devices. I want to be able to quantify power usage while it is
> > asleep, sensing, sending data, etc..."does changing this code shorten
> > the radio on time?"
> >
> > I'm sure Agilent would be happy to sell me a fancy oscilloscope current
> > probe for more than my condo cost, but I'm looking for something more
> > cost effective.
> >
> > Suggestions from the hive mind?
> >
> > Thanks.
> >
> > -Pete