Calculating Power Output with PWM and 1ohm Load

[GlynnHeeswijk]

Hi
I have a question to do with using PWM
If for example I had a 1ohm load and a voltage supply of 4JC^-1, I would get a power draw of 16JS^-1, now if I had a duty cycle of say 50% I should get an average power output of 8JS^-1. However if i thought of it as having an average current of 2CS^-1 and a average voltage of 2JC^-1 then the power output comes out to be 4JS^-1, which is 4 times less and not half. If it should be the half what does the actual voltage and current average out to?

Thank you very much.

 

[berkeman]

(4V)^2/1 Ohm = 16W continuous = 4A * 4V

50% duty cycle

8W = 4A * 4V * 0.5

You can't average both the current and voltage, without taking an RMS average. RMS averages give you the right answer -- can you see why?

 

[GlynnHeeswijk]

I can see that it does seem work. However I thought that RMS using route 2 was for sine waves? Am i missing something?

Thank you

 

[berkeman]

I can see that it does seem work. However I thought that RMS using route 2 was for sine waves? Am i missing something?

Thank you

No, it's used quite generally for power calculations. You can calculate the RMS value of an arbitrary function:

http://en.wikipedia.org/wiki/Root_mean_square

.

 

[Averagesupernova]

Be sure to keep in mind that .707 * Vpeak is the RMS equivalent for sine waves only. But real actual RMS doesn't care what the waveform is.

 

[GlynnHeeswijk]

Thank you both very much for your help. I understand it a lot better now.