# Homework Help: Transformer with attached load: required source voltage

1. Apr 13, 2010

### Kev1n

1. A large power transformer has a turns ratio N1 / N2 of 1:5. When the primary voltage is 400v and it is connected to a resistive load its secondary current is 100A. Calculate the required value of source resistance of the supply to give a maximum power trnasfer

2. Vp = 400v, Ratio 1:5, Is = 100A

3. Vp = 400v, 1:5 Vs = 2000, Is = 100A, 1:5, Ip = 20A
R = Vp/Ip = 4 ohms.
I am a bit lost here, any pointers

2. Apr 13, 2010

### collinsmark

Re: Transformer

So far so good.

What can you say about "maximum power transfer" of a simple circuit?

3. Apr 14, 2010

### Kev1n

Re: Transformer

The Rp and Rs should be equal?

4. Apr 14, 2010

### Kev1n

Re: Transformer

Is that what you meant

5. Apr 14, 2010

### collinsmark

Re: Transformer

I guess it depends on what you mean by 'Rp' and 'Rs'. But if you define Rp as the resistance in the primary and Rs as the resistance in the secondary, then 'No'. (Transformers not only modify voltage and current, but also impedance.)

Try again by putting things in terms of Rsource and RL, where

Rsource is the source resistance of the supply (which is what you are solving for in this problem), and
RL is the load resistance, as seen by the primary source.

6. Apr 21, 2010

### collinsmark

Re: Transformer

Hello Kev1n,

Forgive me for speaking too soon in my first response. I may have caused some confusion. I seem to have skimmed over your attempted solution, and I missed a mistake.

Your calculation for Ip is incorrect in your original post. And your calculation of R = Vp/Ip = 4 ohms is also incorrect. Forgive me for the confusion of stating "so far so good."

But what I said before about transformers changing impedance is true.