# Homework Help: Calculate the power in the load

1. Sep 27, 2009

### tommoturbo

1. The problem statement, all variables and given/known data

for the thyristor circuit calculate the power in the load as a percentage of max power for a firing angle of 45^

2. Relevant equations

Va=Vm/2pi (1+cos theta)

3. The attempt at a solution

theta=arcos 0.707
45^=arcos 0.707

but as there is no value for VS or Vl i dont know what the ratio between them is to calculate power any advice apreciated

Regards

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Last edited: Sep 27, 2009
2. Sep 28, 2009

### CEL

Since what is asked is the relationship of output to input powers, you don't need the voltages.
Remember that the mean power of a periodic wave is the integral through one period of the amplitude squared, divided by the period.
At the input you have a sinusoid, so you can calculate the integral. At the output you have only part of the sinusoid, since during the rest of the cycle you have zero V.
For the amplitude of the output voltage, it is either equal to that of the input voltage, or is this amplitude minus the drop in the device junction.

3. Sep 28, 2009

### tommoturbo

Man you so know your stuff. Thanks for the reply im going to re do the lesson and take on board what you have said to see if i can figure it out

thanks ian

4. May 5, 2010

### zztop1000

So presumably then expressing as a percentage

% = sin45 x 100
= 70.71%

Or am I way off?

5. May 17, 2010

### n.karthick

You must also take in to consideration that Power is proportional to square of output voltage.