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Primary and Secondary Voltages In A Transformer

  • Thread starter scorpa
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  • #1
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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 :grumpy:
Thanks for any help in advance! :biggrin:
 

Answers and Replies

  • #2
Doc Al
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Hint: If the transformer steps down the voltage when used one way, then what must it do if the primary and secondary are reversed?
 
  • #3
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If they are reversed the voltage must step up, but I still don't see how to answer the question :(
 
  • #4
Doc Al
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The input voltage doesn't change. It's still 24V.
 
  • #5
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Would the new voltage be

24/8 = 8/x
x = 2.7V?
 
  • #6
Doc Al
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Not even close. :smile: 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.
 
  • #7
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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?
 
  • #8
Doc Al
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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.
 

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