Engineering How Does SCR Firing Angle Affect Circuit Voltage Graphs?

AI Thread Summary
The discussion revolves around the impact of SCR firing angle on circuit voltage graphs, specifically focusing on a circuit regulating a load (RL) with an SCR fired at a 90-degree angle. Participants analyze voltage values, including VRL and its maximum, emphasizing the importance of using radians correctly in calculations. Confusion arises regarding the relationship between RMS and maximum values, with clarification that multiplying the RMS value by the square root of two yields the maximum value for complete sine waves. The conversation concludes with participants confirming their understanding of the calculations and the relationship between different voltage measurements in the circuit. Overall, the thread highlights the significance of accurate mathematical representation in analyzing SCR circuits.
Femme_physics
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Homework Statement


http://img444.imageshack.us/img444/3062/firing.jpg
(in Hebrew it says "circuit to regulate firing angle")

1) Note the main difference between SCR and TRIAC.

2) The following is a circuit to regulate the load RL, regulated by SCR. The SCR is fired at an angle of 90 degrees. As well, the transformer reduces the voltage at a 4:1 ratio

Draw graphs to scale of the voltages:

V12
V34
V56 (Vrl)
V78 (Vscr)

Denote significant voltages and times

The Attempt at a Solution



http://img837.imageshack.us/img837/1682/voltages2.jpg

http://img845.imageshack.us/img845/5499/voltages1.jpg

http://img13.imageshack.us/img13/3389/voltages3.jpg

http://img259.imageshack.us/img259/73/voltages4.jpg
 
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Hmm...anyone?
 
Well... there are no smileys in there... nor jokes... and no pretty pictures...
Actually it looks a bit unattractive...

And to top it off, most of it looks correct.
It's only your ##V_{RL,max}## that does not appear to be correct.
 
Well... there are no smileys in there... nor jokes... and no pretty pictures...
Actually it looks a bit unattractive...

Ahh... I see, then, I'll make sure to prettify my solutions henceforth with all sorts of eye-candy spectacles :wink:

And to top it off, most of it looks correct.
It's only your VRL,max that does not appear to be correct.

Really? But...but I used radians! I converted from degrees to radians and used radians for alpha...not good?
 
Femme_physics said:
Really? But...but I used radians! I converted from degrees to radians and used radians for alpha...not good?

You used radians correctly. :)
And so you've found the rms-value of VRL.
How did you get VRL,max from there?
 
Easy...just multiply it by square root of two! As I did!
 
What does it mean to multiply by the square root of 2?
For what kind of signal would you get the max-value from the rms-value?
 
What does it mean to multiply by the square root of 2?

It gets us from the RMS value (somewhere around the middle of the wave) to the max value (top of the wave). We do it by multiplying by the square root of two

For instance,

If Vrl = 27.5 volts
Vrl max = 27.5 multiplied by √2 = 38.89 volts

For what kind of signal would you get the max-value from the rms-value?

Waveform signal
 
Not generally - only complete sine waves.
Look in your graph... what max-value does V_RL have?
 
  • #10
I like Serena said:
Not generally - only complete sine waves.
Look in your graph... what max-value does V_RL have?

Oh, yea, it's 13.5√2

But that's what I wrote, I just forgot what I wrote :)
 
  • #11
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  • #12
Wait, I think I understand

1) The calculator most be in degrees mode
2) The alpha after the sine function is written in degrees, the alpha without the function is written in radians
3) V34max = Vrl max
4) V34rms does not equal Vrl(rms)
 
  • #13
Femme_physics said:
I'm still not sure what I did wrong. I decided the use the formula again this one in degrees mode.

Got a math error!

So I guess I'll stick to radians with this formula?

If you have pi in a formula, pi will be in radians.
So anything you add or subtract has to be in radians too.
Can't subtract degrees from radians.


Femme_physics said:
Wait, I think I understand

1) The calculator most be in degrees mode
2) The alpha after the sine function is written in degrees, the alpha without the function is written in radians

Well, it does not matter if the calculator is in degrees mode or not.
But if you take a sine, you have to fill in an alpha that matches the degrees/radians mode.


3) V34max = Vrl max
4) V34rms does not equal Vrl(rms)

Yes! That's it! :approve:


Here, look, this is what happened to me after an hour of trying to figure it out :P

http://img855.imageshack.us/img855/1420/zasnul.jpg

Looks like that nap helped a lot!
Do (take pictures like) that more often! :biggrin:
 
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  • #14
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  • #15
Femme_physics said:
:) :) Will do!

You know, I figured I don't even need Vrl (rms) for this problem, only RL (max.

BTW, how do I proof that-->

Can I just say that...

Yep, you can say that.
In a "proof" you should state the conclusion that Vrl,max is the same as V34,max.

But since when do you have to prove stuff?
As it is, you can see what Vrl,max is after drawing the graphs.


Btw, I'm reading that the SCR is tired.
Perhaps it should take a nap too.
 
  • #16
:) thank you, you master of electronics, you!
 

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