A transformer connected to a 120 V (rms) ac line is to supply 15000 V (rms) for

Question

A transformer connected to a 120 V (rms) ac line is to supply 15000 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 6.50 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 6.50 mA? W (c) What current rating should the fuse in the primary circuit have? A

Explanation / Answer

a)

Es = 15000 V

Ep = 120 V

Ns /Np = number of turns in secondry / number of turns in primary = Es/Ep

Ns /Np = 15000/120 = 125

b)

is = current in secondary = 6.50 x 10-3 A

P = power = Es is = 15000 x 6.50 x 10-3 = 97.5 watt

c)

using the equation

ip /is = Ns/Np

inserting the values

ip /(6.50 x 10-3 ) = 125

ip = 0.8125 A

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