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Electricity, Electronics, and Communications 289

Eff VI /(VI )
se se
prŁ prŁ
Expressed as a percentage and denoted Eff , the efficiency of
%
the transformer is:

Eff 100 VI /(VI )
se se
prŁ prŁ
%
PS turns ratio

Let N prŁ represent the number of turnð in the primary winding
of a transformer; let N se represent the number of turnð in the
secondary winding. Then the primary-to-secondarð turns ratià
is denoted P:S and is given by:

P:S N /N se
prŁ

SP turns ratio

The secondary-to-primarð turns ratià of a transformer is de-
noted S:P and is given by:

S:P N /N prŁ 1/(P:S)
se

Voltage transformation
Let V prŁ represent the rmð sine-¨ave AC voltage across the pri-
mary winding of a transformer (in volt0. Then the rmð sine-
wave AC voltage across the secondary, V se (in volt0, neglecting
transformer losses, is given by either of the following formulas:

V se (S:P) V prŁ

V se V /(P:S)
prŁ

Impedancł transformation
Let S:P represent the secondary-to-primary turnð ratio of a
transformer; let Z R j0 represent a purely resistive (zero-
in
in
reactanca impedance at the input (across the primary windinɘ
Then the impedance at the output (across the secondary wind-
inÉx Z , is also purely resistive, and is given by:
out