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.