Re: 15C LE current draw issue Message #16 Posted by Eric Smith on 14 Sept 2011, 4:52 a.m., in response to message #11 by Katie Wasserman
Quote:
The only important thing is to use a current meter with a low voltage voltage burden
Yes, that is VERY IMPORTANT. If you use a typical cheap multimeter, the burden voltage of the meter will result in the voltage to the calculator being too low, and either the calculator won't work at all, or you'll get a possibly inaccurate measurement. The inaccuracy of the measurement is caused by running the calculator at less than the expected voltage. If the calculator does not use a switching regulator, it will probably draw less current at lower voltage (almost proportionate). If the calculator uses a switching regulator in boost configuration, it will draw more current at lower voltage. Either way you're not getting a representative sample for the intended supply voltage.
Note that the burden voltage depends on the current range used on the meter, even with autoranging meters. Rich and I have run into that problem with our calculator prototypes on a handheld DMM. On some ranges the burden is low enough for everything to work OK, but at the lowest current range, necessary for accurate measurements of sleep current (i.e., under 20 uA), the calculator will not work *except* in sleep mode.
I'm now using an Agilent 34410A. The burden voltage on the 100uA and 10mA scales is <0.03V, which is great, but on the 1mA and 100mA scales it is <0.3V, which is barely acceptable.
That looks like an HP 34401A on Katie's bench. It has a burden voltage of <0.1V for the 10mA range, which is reasonably decent, but <0.6V on the 100mA range, which I would consider unacceptable. I use the 34410A rather than a 34401A partly because I wanted lower current ranges than 10mA, and partly for the lower burden voltage.
Inexpensive multimeters often don't even provide specifications for burden voltage.
To really do a good job of measuring very low currents, you need something like DaveJ's µCurrent.
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