# Homework Help: Basic SCR circuit - drawing graphs

1. May 24, 2012

### Femme_physics

1. The problem statement, all variables and given/known data
http://img444.imageshack.us/img444/3062/firing.jpg [Broken]
(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
3. The attempt at a solution

http://img837.imageshack.us/img837/1682/voltages2.jpg [Broken]

http://img845.imageshack.us/img845/5499/voltages1.jpg [Broken]

http://img13.imageshack.us/img13/3389/voltages3.jpg [Broken]

http://img259.imageshack.us/img259/73/voltages4.jpg [Broken]

Last edited by a moderator: May 6, 2017
2. May 25, 2012

### Femme_physics

Hmm...anyone?

3. May 25, 2012

### I like Serena

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. May 25, 2012

### Femme_physics

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

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

5. May 26, 2012

### I like Serena

And so you've found the rms-value of VRL.
How did you get VRL,max from there?

6. May 26, 2012

### Femme_physics

Easy...just multiply it by square root of two! As I did!

7. May 26, 2012

### I like Serena

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. May 26, 2012

### Femme_physics

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

Waveform signal

9. May 26, 2012

### I like Serena

Not generally - only complete sine waves.
Look in your graph... what max-value does V_RL have?

10. May 26, 2012

### Femme_physics

Oh, yea, it's 13.5√2

But that's what I wrote, I just forgot what I wrote :)

11. May 26, 2012

### Femme_physics

Last edited by a moderator: May 6, 2017
12. May 26, 2012

### Femme_physics

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. May 26, 2012

### I like Serena

If you have pi in a formula, pi will be in radians.
So anything you add or subtract has to be in radians too.

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.

Yes!!! That's it!

Looks like that nap helped a lot!!
Do (take pictures like) that more often!

Last edited by a moderator: May 6, 2017
14. May 27, 2012

### Femme_physics

Last edited by a moderator: May 6, 2017
15. May 27, 2012

### I like Serena

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. May 27, 2012

### Femme_physics

:) thank you, you master of electronics, you!