Current Ratio:

The current in the windings of a transformer is inversely proportional to the voltage in the windings.  That relationship is expressed in Equation (13-5).

V_P / V_S = I_S/ I_P                                                                      (13-5)

where

I_P         =          primary coil current

I_S          =          secondary coil current

Because the voltage ratio is equal to the turn's ratio, we could express the current ratio in terms of the turn's ratio, as in Equation (13-6).

N_P / N_S = I_S/ I_P                                                                       (13-6)

Example:    When operated at 120 V in the primary of an iron core transformer, the current within the primary is 4 amps.   Find out the current in the secondary if the voltage is stepped up to 500 V.

Solution:

V_P / V_S = I_S/ I_P

Next, we solve for I_S.

I_S = (V_P/V_S) I_P

I_S = (120/500) 4 amps

I_S =      0.96 amps