🚀 go-pugleaf

In article <309e7c98.0210011751.51a56500@posting.google.com>,
 deja_bhoot2000@yahoo.com (Bhoot Nath) wrote:

>This is pretty simple question, but at the moment, I can not find a Google
>reference. I know that the residential power supply in North America is
>120 volts.
>
>Q1. Is that 120 volts PEAK of the sine wave, or is that RMS voltage?
>
>Q2. If I have a sandwich maker listed as "120V, 60Hz, 900W", how much
>    maximum current does it draw? Assuming a 100% resistive load,
>    is it  900 / (120 * alpha)
>
>    where alpha = [ integral of (Sin x)^2, from 0 to pi ] / pi = 0.50
>
>In other words, will the sandwich maker draw 15 amperes of current? If
>that is not correct, how do I compute it?
>
>Q3. How do I go about computing current drawn by other appliances which
>may not be completely resistive?
>
>No, this is NOT a homework problem!


Q1: 120 is the RMS voltage.

The line power is a sine wave so the peak voltage is about 170 volts.  
Overall, the 170V peak sine wave integrates to the same amount of power 
as continuous 120 VDC.

Q2: For appliance ratings, everything is RMS.  The sandwitch maker draws 
7.5 Amps RMS.  It's a resistive load so its current is a matching sine 
wave.  The peak current is 10.6 Amps.

Q3: Inductive and capacitive loads are more complicated.  Consider a 
cheap capacitor based power supply that converts 120VAC to 6VAC using a 
series capacitor.  You have a 6VAC 100mA load from a 120VAC source yet 
the capacitor doesn't get hot from the missing 114 VAC.  It's because 
the current is almost 90 degrees out of phase.  V and A have to be 
multiplied with the phase considered to get the real load.

DC appliances are worse.  DC appliances suck current off only the peaks 
of the voltage sine wave.  Conversion from peak to RMS becomes 
complicated and the heating of the wiring is different.

It sounds like you need a copy of "The Art of Electronics" by Paul 
Horowitz and Windfield Hill.  It's an excellent book and I use it on 
nearly every project.  Mr. Hill even hangs out in sci.electronics.design.