Re: power measure

On 2005-09-19, John Fields <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 20 Sep 2005 00:08:55 +1200, Jasen Betts
><jasen-b@xxxxxxxxxxxxxxxxxx> wrote:
>
>>On 2005-09-14, cgrahl@xxxxxxxxxxxxxxxxxxxx <cgrahl@xxxxxxxxx> wrote:
>>> Hi!
>>>
>>> I need to measure the power of some devices. I'm thinking to made a
>>> digital ampermeter, since I already know the voltage (220 V), I only
>>> need to measure the current to get the power (according to the formula
>>> P = I x V), right?
>>
>>only if it's 220V DC if it's AC power can flow the opposite direction during
>>some of the cycle meaning that VxA is more than the watts used.
>
> ---
> If it's AC, the voltage across the load alternates as does the
> charge flowing through it. Power though, continues to be dissipated
> in only one direction. For example, an incandescent lamp doesn't
> light up for one half-cycle and then dim the next.

I was thinking of an reactive load... I'm surprised you didn't catch that
with a reactive load for some period of the cycle the current oposes the
voltage, during that period the instantaneous wattage of the device is
negative. - energy is flowing from the device into the supply.

> No. You need to measure the voltage and current and, if necessary,
> convert the values read to RMS, then measure the phase angle
> between the voltage and current, get the cosine of that angle and,
> finally, multiply the voltage, the current, and the cosine of the
> phase angle together to get power.

how do you compute phase diference when there's no gaurantee that the
current looks anything like a sine wave it seems to me that that, it would
be harder to compute than calculating the average of a number of
voltage.current products.

>>for
>>this you're going to want a microcintroller that knows how to multiply.
>>(since frequent ADC readings give better accuracy, and software multiply is
>>typically 5-20 times slower than hardware multiply.
>>
>>not being familiar with the PIC line I can't reccomend a particular device.
>
> Well, then, how about recommending one you _are_ familiar with?

Atmel ATmega48 - 20 MHZ risc microcontroller with plenty of 10-bit A-D
channels.

Bye.
Jasen
.

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