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'[EE]: Sensitive micro-ammeter?'
2003\11\22@183346 by Philip Pemberton

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Hi,
 I'm trying to get some power consumption figures for a PIC-based
battery-operated (CR2032s believe it or not) device. The problem is, although
I have a PSU that can provide the 3V-5V Vcc voltage, the built-in ammeter is
useless for low-current circuits. I've pulled out my Fluke 25 DMM and it
doesn't register ANYTHING when the PIC is sleeping - it just sits there
reading "0.00 uA".
 Has anyone here found a way to measure the current of a PIC in sleep mode?
I need better figures than zero to work out how long the batteries will last.

Thanks.
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2003\11\22@205310 by steve

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On 22 Nov 2003 at 23:30, Philip Pemberton wrote:

> Hi,
>   I'm trying to get some power consumption figures for a PIC-based
> battery-operated (CR2032s believe it or not) device.

Put a 1k (or more) resistor in series with the battery. Measuring a few
millivolts is easy and the voltage drop is inconsquential in operation,
except that it will limit the current if there is a short circuit.

Steve.


==========================================
Steve Baldwin                          Electronic Product Design
TLA Microsystems Ltd             Microcontroller Specialists
PO Box 15-680, New Lynn                http://www.tla.co.nz
Auckland, New Zealand                     ph  +64 9 820-2221
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2003\11\22@215900 by Russell McMahon

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> I need better figures than zero to work out how long the batteries will
last.

You possibly don't:-)
If you have a meter that will resolve 1 uA and it doesn't read then you
presumably have less than 1 uA drain.
A 1 uAs drain is 8765 uAh/year or 8.8 mAh per year. If long life is the
issue and the PIC quiescent current then even a modest cell is going to give
you many years so it MUST be a Lithium one. A CR2025 = 20mm x 1.6mm and 150
mAH odd. At even 10 mAh/yr that's 15 years battery life, or in excess of the
10 years off shelf life.

My point is that IF quiescent current is < 1uA and IF it predominates in the
power consumption then almost any battery you can think of will last for
about its shelf life.

>   Has anyone here found a way to measure the current of a PIC in sleep
mode?

So, the trick is to see what the quiescent current is. I'm not sure what the
bottom range is on the Fluke 25, but I'll assume it really can resolve 0.1
uA (let alone 0.01) as you suggest. Sounds like your processor is comatose,
not sleeping :-).

Why not try a capacitor as a power source and measure it's voltage drop with
time.
1st set meter to a suitable VOLTAGE range, charge the capacitor to Vdd (3.6v
or whatever), disconnect power source and note voltage drop from meter load
over say 5 minutes. You can approximate the result with a linear solution or
solve the very simple exponential decay equation.

Rule of thumb linear an amp drawn from a  farad will drop a volt in a
second.
A uA drawn from a 1 uF will drop a volt in a second.

Use a good quality non polar capacitor (mylar or similar).
Start with 1 uF.

A meter with a  100Mohm input resistance will drop a uF by a volt in about
100/V seconds.
Reversing the meter leads and repeating would be wise at this sort of
currents as offset current, even with a fluke, is potentially significant.

Do that a few times with both polarities.

Now connect the PIC and try again.

Where current varies and you get substantial occasional peaks and long
period of quiescent current you can feed the system through a largish
resistor with a suitable filter cap on the equipment side. If RC time
constant is long compared to peak current pulses repetition rate then the
resistor will tend to supply the average current and the capacitor will
supply the current peaks. With a similar filter cap this is something like
the current the battery will tend to see in use.


       Russell McMahon

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