Trouble with RMS/Average values in Electronics

[crunchercrunch]

Hi there. I'm going through the NCEES 2009 sample exam and I am having trouble knowing when to use RMS, average, or AC value.

The problems I have havnig trouble with are: 106, 505, 515.

106 - The voltage source is sinusoidal, and it is asking us to provide the value of R to prevent the diode maximum current. If we are to analyze the circuit at the moment the switch closes, we must treat it as instantaneous value, meaning we have to divide the voltage by square root of two. The solution says we just use the max value of the sinusoidal voltage source. Why?

505 - It is asking for the average voltage across the 0.022uF capacitor. Sure, I get that the average value of a sinusoid is zero, but why are we using the average and not RMS? Is it because we are asked for only the average voltage and not the average power?

515 - It is asking for the minimum VA rating for the transformer. A current value is given in RMS. It is not asking for the average VA rating, so why must we use RMS in this case? Another problem I am havnig with this question is that when you use the a=(Zp/Zs)^1/2 transformer formula, why is the Zp independent of the 90ohm resistor? When you find Zp as 135ohms, shoudn't that be the total inpedance, meaning it is the total including the parallel 90ohm resistor? Yet, the solutions treat the 135ohms as a separate resistor from the 90ohm resistor. Why?

 

[Peele1]

I'll take a try at this...

I don't have a copy in front of me...

106- Diodes are generally DC elements. They will see the peak value of the voltage go through them. Max Voltage = RMS voltage * 1.41. 2^1/2 =~1.41.

505-I think that it is just seeing if you know that the average value of an AC circuit is zero. It asks for average. In another question, be on the lookout for the same thing, but with a diode clipping out the negative voltage, resulting in 1/2 RMS as the average.

515-Transformers are AC elements and VA is done in RMS values for AC. Without seeing it, I can't think about the other parts.

Correct me if I'm wrong... it's been a while.

 

[crunchercrunch]

I'll take a try at this...

I don't have a copy in front of me...

106- Diodes are generally DC elements. They will see the peak value of the voltage go through them. Max Voltage = RMS voltage * 1.41. 2^1/2 =~1.41.

505-I think that it is just seeing if you know that the average value of an AC circuit is zero. It asks for average. In another question, be on the lookout for the same thing, but with a diode clipping out the negative voltage, resulting in 1/2 RMS as the average.

515-Transformers are AC elements and VA is done in RMS values for AC. Without seeing it, I can't think about the other parts.

Correct me if I'm wrong... it's been a while.

Thanks for the reply.

For 106, if it is a diode is a DC element, then shouldn't we find the RMS value of the sinusoidal source because it will see a DC value?

For 505, I understand your explanation.

For 515, I am beginning to see where you're coming from. Can you confirm that my way of thinking is correct? Since VA is another way of saying it is a power term, it must use RMS calues because power is always done in RMS. Is that right?