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Re: Primary- vs. secondary-current feedback



Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 10:30 PM 11/21/2006, you wrote:
Tesla list wrote:
Perhaps this should be in another thread, But you post reminded me to ask, What is the difference between VA and Watts? The formula for power is P=VA so aren't they the same?

-Callum Brieske

This page sort of tells "why":

http://www.etasys.com/Main/ABCs/CalcVAWATTS.asp

But in general, unless the load is a "perfect" resistor, the resistance will vary over time. For inductive or capacitive loads, this variation is just a time offset. The current might lead or lag behind the voltage. For say a switching power supply, it might not make any sense at all!

So "VA" generally refers to the RMS voltage multiplied by the RMS current. This is very nice in sizing how big of transformer or other equipment a circuit might need since the raw VA number is always worst case.

But "watts" is "real power". If the voltage and current are both sine waves that are 90 degrees off from each other (pure inductance or capacitance) then the "real" power is zero since they cancel... Real power can also be thought of as how much heat the load dissipates.

This report shows the different signals for a simple flood lamp and a nasty spark gap motor.

http://hot-streamer.com/temp/Kill-A-Watt/Kill-A-Watt.html

http://hot-streamer.com/temp/Kill-A-Watt/Tek00005.gif

Voltage is yellow, current is blue, and power is red. It is not so easy figuring out power without a computerized scope, as used in this case, that figures out 10,000 data points in real time for you.

Even though the voltage is "tame" the current can be "anything". The current can even go backwards for a "negative" resistance... This is why everyone wants "True RMS" meters to show the "real" voltage and current numbers regardless of how screwed up the signals are.

In general, VA is a worst case number and watts is the "real" number taking into account that the voltage and current signals might be very screwed up.

Hope this helps ;-)

Cheers,

        Terry