If a transformer steps down a 120-V source to 6.3 V and is connected to an 8 ohm loadWhat is the value of the impedence reflected back to the source?

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valentin68 | College Teacher | (Level 3) Associate Educator

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When a load resistance is connected to the secondary of a transformer a part of it (which can be less or bigger then 1) is reflected back to the primary, and is seen by the voltage source that supply power to the primary coil.

To determine how much of the load resistance Rs is reflected into primary (seen as Rp by the voltage source Vp that drives the primary) we write down the Ohm's law:

`R_p = E_p/I_p = (n*E_s)/(I_s/n) = n^2*(E_s/I_s) = n^2*(U(Rs))/(I(Rs)) = n^2*R_s`

where n is the transformer rapport (otherwise said the turns ratio)

`n =n_p/n_s =E_p/E_s =120/6.3=19.05`

Therefore the reflected resistance of  a load Rs =8 ohm back to the source is

`R_p =19.5^2*8 =2902.5 ohm =2.9 kohm`


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