A transformer is to be used to provide power for a computer disk drive that need

Question

A transformer is to be used to provide power for a computer disk drive that needs 7.0 V (rms) instead of the 120 V (rms) from the wall outlet. The number of turns in the primary is 390, and it delivers 500 mA (the secondary current) at an output voltage of 7.0 V (rms).

(a) Should the transformer have more turns in the secondary compared to the primary, or fewer turns?

1)There should be more turns in the secondary -or-   2)There should be fewer turns in the secondary.    

(b) Find the current in the primary.
mA

(c) Find the number of turns in the secondary.
turns

Explanation / Answer

a)

2)There should be fewer turns in the secondary. as it a step down transformer

b)

I1 = (I2)(N2/N1)

N2/N1 = (V2)/(V1) = 7/120

I1 = (500/1000)(7/120) = (1/2)(7/120) = 7/240

I1 = 0.029 A = 29 mA

c)

N2/N1 = (V2/V1)

N2 = (N1)(V2/V1) = (390)(7/120) = 22.75 turns

you can round up to : N2 = 23

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