What voltage appears across the primary coil of this transormer

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AI Thread Summary
The discussion revolves around calculating the voltage across the primary coil of a step-down transformer when 120 V is applied to the secondary coil. Initially, the user attempts to use the transformer ratio incorrectly by adding voltages instead of applying the correct ratio method. The correct ratio derived from the transformer’s operation indicates that if the secondary voltage is 120 V, the primary voltage should be calculated as 120 V multiplied by the ratio of 20, resulting in 2400 V. The participants clarify the misunderstanding, emphasizing that the relationship between primary and secondary voltages is based solely on their ratio, not addition. Ultimately, the correct voltage across the primary coil is confirmed to be 2400 V.
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Homework Statement


A step down transformer produces a voltage of 6.0 V across the secondary coil when the voltage across the primary coil is 120 V. What voltage appears across the primary coil of this transormer if 120 V is applied to the secondary coil?


Homework Equations



Vp/Vs=Np/Ns=Is/Ip .... ? (not sure if that is relevant really)

The Attempt at a Solution



i know Vs/Vp = 120/6

i think i don't really know what to do with the 120 v applied to the secondary coil
i added 120 to the 6 = 126 that means 6 was multiplied by a factor of 21
so i multiplied the top value of the ratio (120) by 21 and got 2520 v
but I am really not sure if that's right at all? what does "120 V is applied" in mathematical terms?
 
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You've got everything so right, I'm not sure how you can go wrong. Vs/Vp=120/6=20. So if Vp=6V, Vs=20*Vp=120V. Now suppose Vs=120V? Where is the 21 factor coming from? You don't add the secondary to the primary, it's just a ratio.
 
Last edited:
ok i get it i made a stupid mistake
so its really 120 X 20 right?
so the answer is 2400 ?
 
carrieunder said:
ok i get it i made a stupid mistake
so its really 120 X 20 right?
so the answer is 2400 ?

Yes.
 

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