Question

# A transformer connected to a 125 V (rms) ac line is to supply 12000 V (rms)...

A transformer connected to a 125 V (rms) ac line is to supply 12000 V (rms) for a neon sign. To reduce shock hazard, a fuse is to be inserted in the primary circuit; the fuse is to blow when the rms current in the secondary circuit exceeds 5.00 mA.
(a) What is the ratio of secondary to primary turns of the transformer?
(secondary/primary)

(b) What power must be supplied to the transformer when the rms secondary current is 5.00 mA?
W

(c) What current rating should the fuse in the primary circuit have?
A

a)
Ns = no. of turns in the secondary coil
Np = no. of turns in the primary coil
Vs = voltage in the secondary coil
Vp = voltage in the primary coil
Is = current in the secondary coil
Ip = current in the primary coil

Ns / Np
= Vs / Vp
= 12000 / 125
= 96 (3 s.f)

b)

Power
= VI
= 12000 x (5.0 x 10^-3)
= 60 W

c)
Ip / Is = Vs / Vp

Ip
= Vs x Is / Vp
= 12000 x (5.0 x 10^-3) x 125
= 7500 A

a)
Ns = no. of turns in the secondary coil
Np = no. of turns in the primary coil
Vs = voltage in the secondary coil
Vp = voltage in the primary coil
Is = current in the secondary coil
Ip = current in the primary coil

Ns / Np
= Vs / Vp
= 12000 / 125
= 96 (3 s.f)

b)

Power
= VI
= 12000 x (5 x 10^-3)
= 60 W

c)
Ip / Is = Vs / Vp

Ip
= Vs x Is / Vp
= 12000 x (5 x 10^-3) x 125
= 7500 A

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