How Does SCR Firing Angle Affect Circuit Voltage Graphs?

In summary, the conversation discusses the correct method for determining the maximum value of VRL in a circuit regulated by SCR. This includes using radians for alpha and multiplying the rms value by the square root of 2. It is also mentioned that the calculator must be in the correct mode and that the alpha after the sine function should match the mode. Finally, it is mentioned that there is no need to prove the equality of V34,max and Vrl,max, but it can be stated in the proof.
  • #1
Femme_physics
Gold Member
2,550
1

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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  • #2
Hmm...anyone?
 
  • #3
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.
 
  • #4
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?
 
  • #5
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?
 
  • #6
Easy...just multiply it by square root of two! As I did!
 
  • #7
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?
 
  • #8
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
 
  • #9
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!
 

What is a basic SCR circuit?

A basic SCR (Silicon Controlled Rectifier) circuit is an electronic circuit that uses an SCR, a type of semiconductor device, to control the flow of electric current. It allows current to flow only when a control signal, such as a voltage or current, is applied to the gate of the SCR.

How do you draw a graph for a basic SCR circuit?

To draw a graph for a basic SCR circuit, first identify the input and output signals. The input signal is typically a voltage or current applied to the SCR's gate, while the output signal is the current flowing through the SCR. Then, plot the input and output signals on a graph with time on the x-axis and voltage or current on the y-axis. The graph should show how the SCR turns on and off in response to the input signal.

What is the significance of the graph in a basic SCR circuit?

The graph in a basic SCR circuit is important because it shows the behavior of the SCR in response to the input signal. It helps to understand how the SCR turns on and off, and how the output current is controlled by the gate signal. Additionally, the graph can be used to calculate important parameters such as the SCR's triggering voltage and holding current.

How does an SCR turn off in a basic SCR circuit?

An SCR turns off in a basic SCR circuit when the current flowing through it decreases below a certain threshold, known as the holding current. This causes the SCR to go into its high-resistance state, effectively stopping the flow of current through it. Additionally, an SCR can also be turned off by removing the gate signal, which causes the SCR to go into its off state.

What are the advantages of using an SCR in a basic SCR circuit?

There are several advantages of using an SCR in a basic SCR circuit. One advantage is its ability to control high currents with a small amount of gate current. It is also able to switch quickly and has a high power handling capacity. Additionally, an SCR is a reliable and low-cost device, making it a popular choice in many electronic circuits.

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