A step-down transformer has a turns ratio of 100 : 1. An ac voltage of amplitude 170 V is applied to the primary. If the current in the primary is 1.0 mA, what is the current in the secondary? Ans. (0.1A)



Data Given:


A step-down transformer has a turn ratios = Ns  :  Np = 1 : 100

NsNp = 1100


Supplied Voltage to the primary coil of transformer  = Vp = 170 V

Current in the primary coil = Ip = 1 mA = 1 ｘ 10⁻³ A


To Find:


Current in the secondary coil = Is = ?


Solution:

  

To find Is  (Current in the secondary coil) by using the following formula for an ideal transformer that is 
Power of primary coils = power of the secondary coil

Pp = Ps   

or

Vp Ip = Vs Is    (∴ P = VI )

or 

Is = VpVs ｘ Ip  --------equation (1)

To find Is we have to find first the Vs also So,
by using the formula between voltages and number of turns of Primary and secondary coils are as follows

NsNp = VsVp

or 

Vs = NsNp ｘ Vp

by putting values

Vs = 1100 ｘ 170 V

Vs = 1.7 Volts

Now using  equation (1)

Is = VpVs ｘ Ip 

By putting values

Is = 170V1.7V ｘ 1 ｘ 10⁻³ A

by simplifying we get

Is = 0.1 A------ Ans.

Thus the current in the secondary coil is 0.1 A

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