[K12OSN] alternative power supply

Rob Owens robowens at myway.com
Fri Dec 3 11:31:30 UTC 2004

You can measure power easily with a clamp around type amp meter.    It <br>basically has a magnetic clamp that you clamp around a power wire and it <br>gives a current readout.  Multiply this by 120 (volt) to get power.  The <br>nice thing is you don't need to touch any high voltage wire.  You will <br>need to clamp only 1 wire (hot or return) but you can easily build a <br>short extension cord with wires split, or carefully split the outer <br>insulation on an existing extension cord and seperate the wires (think <br>disecting an earthworm in science class).
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This actually gives you something called volt-amps, which in DC current is the same as Watts.  But in AC it is different because we're dealing with a sinusoidal wave of constantly changing voltage and current.  To calculate average AC power, you multiply volts x amps x the power factor.  The power factor comes from the fact that the voltage wave and the current wave do not match up exactly -- the waves are offset from each other.  If you're interested, here's a website I found about it:  http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html

Anyway, I'm not an expert on this, but it looks like the power factor is a function of the load, meaning your power supply.  The only way I know of to find out your power factor is to hook the power supply up to an oscilloscope and view the voltage and current waves.  Measure the angle of offset between them--this is called the phase angle.  Take the cosine of this angle and you have your power factor.

There might be an easier way, but I don't know it.  Maybe you have an electricity teacher at the school who could help out with the formulas and the measurements.

Here's another site I found that shows how to measure AC power w/o an oscilloscope:  http://home.earthlink.net/~jimlux/hv/varvom.htm
They also mention that using a wattmeter will give you good results, because it takes all this business into account for you.



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