# Full wave rectifier with R load

Discussion in 'Homework Help' started by notoriusjt2, Oct 4, 2010.

1. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
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0
A single-phase full-wave bridge rectifier has a resistive load of 18ohms and an ac source of 120V rms. Determine the average power in the load.

these average power questions are killing me....

Vo=((2)(120)(1.414))/pi = 108v
Io= Vm/R = 108/18 = 6A

P=108*6=648w

so what step am i forgetting to do here... i am never taking an electronics/math class online again.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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1,089
I think that you need to find RMS voltage and current.

3. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0
so
Vrms=108/1.414=76.37
Irms=6/1.414=4.24

P=76.37*4.24=324w

would this be correct?

4. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
$P_{avg}=\left\langle \frac{V^2}{R} \right\rangle \\
=\frac{\langle V^2 \rangle}{R}\\
=\frac{V_{rms}^2}{R} \\
where\ \langle\ \rangle\ \text{is the time mean}$

Your answer is wrong, because Vrms is not connected to the average voltage with the relation you wrote. The Vrms of the rectified waveform is the same as the Vrms of the initial sine wave. Do the math and it will become apparent. Use the default RMS calculation formula.

Is that clear?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,939
1,089
Hehe, No.
Maybe you should read the question again.
"A single-phase full-wave bridge rectifier has a resistive load of 18ohms and an ac source of 120V rms. Determine the average power in the load."

Last edited: Oct 4, 2010
6. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0
yea that helps, but what do you mean when you say time mean?

so if Pavg=(V^2rms)/R
then
Pavg=(120^2)/18
Pavg=800w

is that what you mean?

7. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Time mean is just the mean of a value over a time period, mean for short. Don't bother about it.