Date: Mon, 19 Feb 2001 20:01:14
From: "neil barnes" <[EMAIL PROTECTED]>
Subject: Re: 50CT & 70CT batteries
>
>Ohm's Law should dictate that lower voltage devices will draw more current,
>thus equalizing the power drain. But the "lower voltage" concept in this
>case refers more to the internal spacing of the junctions which require
>less
>power to operate.
>
>Thanks
>
>Pres Waterman W2PW
Heya Pres,
That turns out to be not quite the case...there are lots of mechanisms
involved here...
Ohms law: V=IR - for the same resistive load, running something at a lower
voltage will cause less current to be drawn - or conversely, as power =
V^2/R, dropping the voltage drops the power as the square of the voltage. So
if you can run your processor at a lower voltage, you use less power from
the battery (and this is efficient, because the power supply dc-dc
converters can deliver the volts you need efficiently - 95-98% in good
cases)
However, it's not quite that easy - because later CPUs tend to have more
transistors in them...and that means more power :(
But the good news is: the transistors used don't use any power at all! Well,
they don't unless you want to actually switch them...the bad news? We want
to switch them faster and faster.
So what's the result? If you want to save power, you need to use an old CPU
with not many gates, built using modern technology, and clocked as slowly as
you think you can get away with. Not very exciting. Damn!
Also - if you want to clock a modern processor you find that you usually
have to increase the voltage to keep the thing working...this is probably to
do with slew rates on the chip gates - I'm not an expert on the chip design
- but if you have to up the voltage from 2.1 to 2.5 volts, you've increased
the power by 42% - and that *all* comes out as heat...which is why big heat
sinks are a good idea!
Cheers,
Neil
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