# Homework Help: Industrial electronics. Question with coils and diode.

1. Aug 22, 2011

### Femme_physics

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

http://img198.imageshack.us/img198/1421/vintm.jpg [Broken]

Given that k = 2

A) Draw in a qualitative manner the following letters

Vin, VP, Vs, Uxy, UR1, UR2, UD

B) Calculate the power in R2

C) Calculate the average current through R1

3. The attempt at a solution

I'll start with a little theory. The way I see it there are 2 currents, one marked in blue and one marked in green. The current marked in red actually gets stuck, so it appeared no current hits the second circuit. Could it be?

http://img8.imageshack.us/img8/6656/mkkkmkkkk.jpg [Broken]

Last edited by a moderator: May 5, 2017
2. Aug 22, 2011

### Staff: Mentor

Consider that voltage on the primary of the transformer is a sinusoid, so it has positive and negative excursions. Similarly, the voltage across the secondary will also have positive and negative excursions and follows a sinusoidal function.

In the secondary circuit, as you surmised, the diode will block current flow when the secondary's voltage causes the diode to be reverse biased. So, perhaps you could sketch the anticipated current function for the secondary circuit for a full cycle. How would you represent this function mathematically? (HINT: you might want to break the function into more than one time interval).

3. Aug 22, 2011

### I like Serena

Morning Fp!

What do you mean by: "Given that k = 2"?

I suspect that when they asked to draw the letters in a qualitive manner, they meant as a graph, with the amplitude on the vertical axis, and the time on the horizontal axis.
Do you know how to draw for instance Vin in this manner (this looks like a sinusoid)?

I like your drawing with the blue and green arrows, which is exactly what happens. :)
And yes, in the right circuit the current following the red arrow is blocked, but as gneill already suggested, in the reverse direction it will still flow.

4. Aug 23, 2011

### Femme_physics

Hi gneil! Morning ILS

Oh, I thought it's understood that k is "transmission relation of the transformer", which is basically i1/i2 or u1/u2 or n1/n1 (I'm still not sure what's N1/N2)...

To my understand coil Vp only transforms the current in one direction, no? The direction I drew in red. I do understand there are two directions for current flow, that's why I drew the one in green and blue

I'm not sure. Can the current go like this?

http://img855.imageshack.us/img855/3387/vpvs.jpg [Broken]

Ah, thanks for clearing it up

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5. Aug 23, 2011

### I like Serena

I suspected as much, although I'm still not clear which coil has the most windings.
Do you know?

As you can see the letters P and S are used.
Those stand for "primary" coil and "secondary" coil.
n1 and n2, or N1 and N2 are the number of windings in each coil.
Although in this case I'd prefer Np and Ns to eliminate the ambiguity.

So is k = Ns / Np?
Or is k = Np / Ns?

Actually, the coils do not so much transform current, but they transform voltage (in both directions).
And due to the voltage the current will flow (if it can).
In the primary circuit the current will flow in both directions, while in the secondary circuit it will only flow in one direction due to the diode.

Yes, it can. :)

6. Aug 23, 2011

### Femme_physics

Good question! Can anyone help with it? I wish I knew!

But your preference of using Np and Ns is better IMO

Well, the question doesn't say so

Makes sense

So what's the point of the dots in the coil? u see? one Vp dot and one Vs dot? Do you see the dots I'm talking about?

7. Aug 23, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
8. Aug 23, 2011

### I like Serena

Let's assume k=Np/Ns, because usually a transformer transforms the voltage from high to low.
(Like in the adapter for your laptop, where it is transformed from 230 V to 12 V.)

I believe the dots are only markers to indicate the points where they would like you to make a qualitative drawing of the voltage (in the form of a graph with an x-axis and an y-axis).

9. Aug 23, 2011

### I like Serena

I'm almost seeing a smile.

I'm afraid the voltage of Vin does not make jumps.
It's just a fluent sine curve, it has to be since it's supposed to be a perfect voltage source.

10. Aug 23, 2011

### Femme_physics

Makes sense

I was thinking it had more significance than that, but I guess I had it wrong. Oh well.

Doesn't it depends at what loop we're looking at? But if not, I guess it should be: (ignore lack of symmetry!)

http://img812.imageshack.us/img812/1227/secondtryy.jpg [Broken]

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11. Aug 23, 2011

### I like Serena

Ignoring lack of symmetry... yep! :)

Btw, do you know how high the tops are?

Last edited: Aug 23, 2011
12. Aug 23, 2011

### Femme_physics

Well, there's

U(av)
and
U(rms)
and
U(max)

I think what's given to me is U(max) so I'd venture to say the tops are U(max)

i.e. 230V

but they ask to draw it in a qualitative manner, so that means without numbers no?

As far as Vp and Vs...

http://img42.imageshack.us/img42/2459/booooooz.jpg [Broken]

once again ignoring symmetry heh...

Last edited by a moderator: May 5, 2017
13. Aug 23, 2011

### I like Serena

Hmm, I don't know U(av). What is that?

I can tell you that U(rms) is given - I believe it usually is.

I'm not sure, what "qualitative" means exactly.
It think it means that you should not do calculations, but I think it does (or should) include which numbers go where.

Anyway, after the graphs they are asking for numbers, so I think it's a good thing if you're already aware of how the numbers apply to the graphs.

I see you've taken the smile-version of the sliced sine graph. :)

However, I'm afraid you're too early slicing it off.
The induced voltage in the secondary coil is still the full voltage, positive and negative.
I'm afraid that's how induced voltage (also known as electromotive force or emf) works.
If you want I can give a longer explanation, based on the related magnetic field.

You should take into consideration that Vs is on the side of the coil that has fewer windings, which has an impact on the amplitude.

14. Aug 23, 2011

### Femme_physics

http://img808.imageshack.us/img808/7753/mizzz.jpg [Broken]

Fair enough makes sense.

oh heh, that's how it's called?^^

Oh, you're right, I was thinking "circuit", I wasn't thinking "coils". So Vs is the same as Vp (as I've drawn Vp)

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15. Aug 23, 2011

### I like Serena

Aaah, "av" stands for "average".
Oh, well, that one is not usually used.

Anyway, I know those formulas slightly differently:
$$U_{av} = \frac {2U_{max}} {\pi}$$
$$U_{rms} = \frac {U_{max}} {\sqrt 2}$$
so your U(max) is not right yet.

Where did you get your formulas?

Almost, but not quite.
Vs is on the coil with fewer windings....

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16. Aug 23, 2011

### Femme_physics

From my textbook! I swear it says it... so it's wrong?

A bigger hint, maybe?^^

17. Aug 23, 2011

### Femme_physics

From my textbook! I swear it says it... so it's wrong?

A bigger hint, maybe?^^

18. Aug 23, 2011

### I like Serena

Errr... yes, it is wrong assuming U(rms) and U(max) are voltages.

There is an alternate formula:
$$P_{rms} = \frac {P_{max}} {2}$$
Perhaps that one was intended?

Btw, the symbol U is also used for energy, although that seems to be misplaced here.
But that might almost be applicable.

You wrote before:
So I assumed you knew...
$$\frac {Vs} {Vp} = \frac {Ns} {Np}$$

19. Aug 23, 2011

### Femme_physics

Well my textbook definitely says U and not P. But I'll ask my lecturer, for sure.

[/QUOTE]

Well, I'm now reading the comments section of the question and it says

"If both the points are in the same place, the transformer keeps the phase. If both points are in a different location, the transformer reverses the phase. I don't know what it means by "reverses the phase"..hopefully I'm translating it correctly. "

It also tells me that: "The directing of the winding determines the current's direction. If it's the same winding directon, it'll be the same direction of current. Although it doesn't say here the direction of the winding. Do I read it from the sketch?

20. Aug 23, 2011

### I like Serena

It does not appear to be given in the sketch or in the problem statement.
My choice would be to assume the winding direction is the same, meaning the direction of the current is the same.
But since they do not specify both choices would be correct.

As always you have a "voltage difference".
On the top of the coil the voltage goes up and then down (relative to the bottom or some zero in the middle).
But at the bottom of the coil the voltage goes down and then up (relative to the top or some zero in the middle).
This is what they call "reversed phase".

(Note that with reverse winding, this would also reverse the phase.)