A transformer connected to a 120 Vrms ac line is to supply 1

A transformer connected to a 120 V(rms) ac line is to supply 1.21 * 10^4 (rms) for a neon sign. To reduce the shock hazard, a fuse is to be inserted in the primary circuit and is to blow when the rms current in the secondary circuit exceeds 8.40 mA. What is the ratio of secondary to primary turns of the transformer? N_2/N_1 = What power must be supplied to the transformer when the rms secondary current is 8.40 mA? P = W What current rating should the fuse in the primary circuit have? I_1 = A

Solution

Voltage in primary V = 1.21 x10 ⁴ volt

Voltage in secondary V \' = 120 volt

ratio of secondary to primary truns of transformer N\' / N = V \' / V

                                                                                = (120)/(1.21x10 ⁴ )

                                                                                = 9.917 x10 ^-3

(b).Secondary current i \' = 8.4 mA

                                   = 8.4 x10 -3 A

Required power P = V \' i \'

                          = 120 x 8.4 x10 ^-3

                          = 1.008 W

(c).Current in primary i = ?

We know i / i \' = N \' / N

                    i = i \'( N \' / N )

                      = 8.4mA ( 9.917 x10 ^-3)

                      = (8.4 x10 ^-3 ) ( 9.917 x10 ^-3)

                     = 83.3 x10 ^-6 A