How Does SCR Firing Angle Affect Circuit Voltage Graphs?

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Discussion Overview

The discussion revolves around the effects of SCR firing angle on circuit voltage graphs, specifically in the context of a circuit designed to regulate a load using an SCR. Participants explore the relationships between various voltage measurements and the implications of using different units (degrees vs. radians) in calculations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Participants discuss the differences between SCR and TRIAC in the context of circuit regulation.
  • There is a focus on calculating and graphing voltages (V12, V34, V56, V78) related to the SCR firing angle.
  • Some participants express uncertainty about the correctness of their calculations, particularly regarding the maximum voltage (Vrl,max) derived from the RMS value.
  • There is a discussion about the implications of using radians versus degrees in calculations, with some participants noting errors arising from mode mismatches in calculators.
  • Participants explore the relationship between RMS values and maximum values, particularly in the context of sine waves.
  • One participant suggests that they may not need the RMS value for their problem, focusing instead on the maximum voltage.
  • There is a proposal to prove the relationship between V34,max and Vrl,max, with some participants discussing the nature of mathematical proofs in this context.

Areas of Agreement / Disagreement

Participants express varying levels of confidence in their calculations, with some agreeing on the relationships between voltage values while others remain uncertain about specific errors. The discussion does not reach a consensus on the correctness of all calculations, particularly regarding the use of degrees versus radians.

Contextual Notes

Participants mention potential errors related to calculator settings and the need for consistent units in calculations. There are unresolved questions about the implications of these errors on the overall understanding of the circuit behavior.

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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