Primary and Secondary Voltages In A Transformer

[scorpa]

Hi!

I just got to my last homework question and am not quite sure how to figure it out, it goes like this:

Scott connects a transformer to a 24V source and measures 8.0V at the secondary. If the primary and secondary were reversed, what would the new output voltage be?

I'm not sure how to go about this. I know the basic ratios like

Np/ns = Vp/Vs = Is/Ip but I can't think of how to do this question. I'm sure it's super easy but I'm super dumb
Thanks for any help in advance!

 

[Doc Al]

Hint: If the transformer steps down the voltage when used one way, then what must it do if the primary and secondary are reversed?

 

[scorpa]

If they are reversed the voltage must step up, but I still don't see how to answer the question :(

 

[Doc Al]

The input voltage doesn't change. It's still 24V.

 

[scorpa]

Would the new voltage be

24/8 = 8/x
x = 2.7V?

 

[Doc Al]

Not even close. Remember: When used one way, the transformer steps down the voltage by a certain ratio (what is that factor?). So... when used in reverse, it will step up the voltage by the same factor.

 

[scorpa]

Um ok, so originally you have 24/8, and now we are stepping it up by that factor (3). So now we end up with the original 24v x 3 which equals 72v?

 

[Doc Al]

Exactly right.

From Vp/Vs = Np/Ns, we know that Np/Ns = 3 for the original configuration. Now if we swap primary and secondary, Np/Ns = 1/3. Since the input voltage is still Vp = 24V, Vs = 72V.