Transformers TRANSFORMER THEORY Voltage Ratio The voltage of the windings in a transformer is directly proportional to the number of turns on the coils.   This relationship is expressed in Equation (13-4). (13-4) V_P V_S N_P N_S where V_P  =    voltage on primary coil V_S  =    voltage on secondary coil N_P  =    number of turns on the primary coil N_S  =    number of turns on the secondary coil The  ratio  of  primary  voltage  to  secondary  voltage  is  known  as  the  voltage  ratio  (VR).    As mentioned previously, the ratio of primary turns of wire to secondary turns of wire is known as the turns ratio (TR).   By substituting into the Equation (13-4), we find that the voltage ratio is equal to the turns ratio. VR = TR A  voltage  ratio  of  1:5  means  that  for  each  volt  on  the  primary,  there  will  be  5  volts  on  the secondary.    If  the  secondary  voltage  of  a  transformer  is  greater  than  the  primary  voltage,  the transformer is referred to as a "step-up" transformer.  A ratio of 5:1 means that for every 5 volts on the primary, there will only be 1 volt on the secondary.  When secondary voltage is less than primary voltage, the transformer is referred to as a "step-down" transformer. Figure 2   Example 1 Transformer Example 1: A   transformer   (Figure   2)   reduces voltage from 120 volts in the primary to  6  volts  in  the  secondary.    If  the primary winding has 300 turns and the secondary   has   15   turns,   find   the voltage and turns ratio. Solution: VR V_P V_S 120 60 20 1 20:1 TR N_P N_S 300 15 20 1 20:1 Rev. 0 Page 5 ES-13