# Circuit to control a full wave via single SCR

1. Apr 21, 2012

### Femme_physics

1. The problem statement, all variables and given/known data

http://img543.imageshack.us/img543/7906/ddd2e.jpg [Broken]

To translate from Hebrew from left to right: Net Voltage 220 Vac, Pull Down Transformer, full wave rectifier (marked with a, b, c, d), below it is a "load" (or resistance), and to the right it says "a circuit to control the angel of ignition".

3. The attempt at a solution

I am first asked to draw a full rectifier that includes 4 rectifying diodes, and include where "a", "b", "c", and "d" is there.

Did I get it?

http://img859.imageshack.us/img859/2743/ddd1s.jpg [Broken]

Last edited by a moderator: May 5, 2017
2. Apr 21, 2012

### Ouabache

Hi FP
Something looks amiss in your attempt.
Look where your input is to the full bridge rectifier (first diagram).
(input is coming from the secondary winding of the transformer)
How is it labelled? Do you see the corresponding position in your
second diagram for the input signal?
(hint, you haven't labelled those points yet).

When you say pull-down transformer, i would expect the voltage
at the secondary winding is lower than the primary voltage. In your
second diagram you show the input to the rectifier the same as the
primary voltage (220-230V)

On the output side, your d is labelled correctly, but the other
side of the load resistor is not b. Do you see what it should be?
(see first diagram).

You may notice you have indicated a & d at the same node
in your circuit. Did you really mean to do that?

I suggest looking up full wave bridge rectifier (FWBR) on the web.
There is a more standard representation of the diodes in a FWBR that makes it
a whole lot simpler to analyze in a circuit.
(hint: see 2nd circuit diagram (with 4 diodes) on the link i gave).

Last edited: Apr 22, 2012
3. Apr 23, 2012

### Femme_physics

The pull-down transformer is 10:1 but they only let me know that in the second part of the question. The first part just wants me to figure out the rectifier circuit.

I'm pretty sure I follow you. I see a is the positive end and b is the negative end. As far as the outputs-- "d" I'm glad you confirmed because it made sense to me even when rethinknig the problem. And as far as "c", it makes sense to me it's just the other end of the load.

http://img209.imageshack.us/img209/148/ablast.jpg [Broken]

Last edited by a moderator: May 5, 2017
4. Apr 23, 2012

### I like Serena

Looks good.

(Quick eh? )

Last edited: Apr 23, 2012
5. Apr 23, 2012

### Femme_physics

LOL, insanely quickly!

Now the second question wants me to draw a graph with an appropriate scale of the voltage the falls on the load, while mentioning voltages and times.

The transformer reduces the net voltage at a 1:10 relaton
Alpha (the ignition angle) is 40 degrees
The net voltage acts in frequency of 50 hertz

http://img811.imageshack.us/img811/6298/vppg.jpg [Broken]

In the graph, alpha = 40 degrees, beta = 140 degrees

Url max is same as Vt max

Last edited by a moderator: May 5, 2017
6. Apr 23, 2012

### Ouabache

Nice Job (on part 1)!!

on part 2:
What is the behavior of the voltage coming out of the Bridge Rectifier?
I don't see that in your graphs.

Do you know what affect the SCR has, on the shape of the output signal?
Once you fix the graph of the rectified voltage, the "on" voltages you've indicated look fine.

If you're saying the amplitude of voltage = $Vpp\sqrt{22}$, I don't figure that.
(Hint: you have Vrms = 22V)

I agree, the period ($\tau$) of the signal at frequency of 50Hz equals 0.02sec,
but your graph does not indicate that.
(Hint: what part of a period does $\alpha+\beta$ equal?)

Last edited: Apr 23, 2012
7. Apr 23, 2012

### Femme_physics

My bad, I meant 22(Square root)2, not square root of 22.... *slaps forehead*

You're right, it doesn't let current go through the negative end. Besides, the rectificer assures there is only pulse on the positive end....ah... I see what I did wrong now.

Alpha and beta only apply to the positive period, because it's the only period that exists

I'll fix it with graphs soon :)

8. Apr 23, 2012

### Ouabache

Good !!
sounds like you're on a roll.
Well a full period equals 360 degrees, whether voltages are positive or negative.
You've already stated the values of $\alpha$ and $\beta$, which is useful in this context.

Trivia: we sometimes use ג,ב,א as variables.. I've noticed them in a few engineering texts

Last edited: Apr 23, 2012
9. Apr 24, 2012

### Femme_physics

Last edited by a moderator: May 5, 2017
10. Apr 24, 2012

### Ouabache

(א) I agree with D1 D4 on the Positive half cycle. However I disagree with
the diodes that you chose to conduct the negative half cycle.
(Hint: look in the reference I gave you in post #2).

(ב) How come you have $\sqrt{22}$ on your graph again?
You told me the correct peak voltage in post #7.
I take it you understand how an SCR affects output voltage across a load, from previous material.
That part looks good, where it cuts off and on.
The shape of the output voltage also looks much better. Is that all they ask?
You may also want to label the peak voltage on the U_RL plot.
The x-axis labeling looks great!!

(ג) Looks interesting, were you given the equation for this part, or did you derive it?
Given the equation you used; I plugged in the numbers we found for this example
and came out with the same numerical result.

I can't remember exactly where, but when I mentioned it to some friends, they tell me they also had seen Hebrew characters in some texts. Now Hindi would be some interesting variables: ह, ज, क ..

Last edited: Apr 24, 2012
11. Apr 24, 2012

### Femme_physics

Sorry-- I miswrote, it's D2 and D3. :)

12. Apr 24, 2012

Yep :)