[maemo-developers] [maemo-developers] Measuring power consumption of 770

[From: Bob Lees]

On Wednesday 15 March 2006 10:59, Teemu Nikkilä wrote:
> On Tuesday, 14. March 2006 22:42, Claudio Scordino wrote:
> > > Why don't you just wire a battery to the battery pins and connect your
> > > meters to these wires?
>
> I'd investigate this approach, maybe even building a battery "emulator" - a
> good quality 3.7 V regulated voltage source, if this is enough to fool the
> charging circuit.
Following on from this, the best way to measure these currents and their 
change would be to put a precision(1% tolerance or better) 1ohm resistor in 
series with the supply and measure the volts across the resistor.  This is 
typically more accurate than using the current scale on a dvm and as noted 
below is a better way if you are going to use a scope or a2d to monitor the 
current.
The volt drop across the resistor would be V=I*R, so in the case of 300mA 
would be 300mV and therefore you might want to increase the supply voltage to 
take account of this.  The power used in the resistor would be I^2*R which in 
this case would be 0.09 watts so a 1/4 (0.25)watt resistor should be fine.
Caution: it was a long time ago:( that I leaned Ohm's Law and its derivatives, 
so caveat emptor ........
>
> > The voltage and the current provided by the battery is much different
> > from the values provided by an electric cable connected to a socket (220V
> > and 50Hz here in Italy).
> > That's why our measurement tools wouldn't work with such a small
> > current...
>
> Without knowing your tools let's screen the problem - the battery is 1500
> mAh and it can deliver power to the device for hours, say 5 h. This means
> the average current draw during normal operation would be around 300 mA,
> most of which goes to the screen backlight and radios, I guess.
>
> If and when your algorithm can save some power, the difference in current
> would be very small and rapidly changing anyway. To make an exact
> measurement, you would probably need very good measurement equipment and
> also you would want a data logger with reasonably fast A/D converter to see
> and record what really goes on during the execution.
>
> -Teemu
>
> > Many thanks,
> >
> >                Claudio
> >
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