# Homework Help: Step up transformer

1. Oct 23, 2006

### nateastle

I have a homework question that states: A transformer connected to a 120 V rms ac line is to supply 16800 V(rms) for a neon sign. To resuce shock hazard, a fuse is to be inserted in the primary circuit; the fuse is to blow when the current in the secondary circuit exceeds 10.0mA. a) What is the ratio of secondary to primary turns of the transformer? b) what power must be supplied to the transformer when the secondary current is 10.0 mA? c) What current rating should the fuse in the primary circuit have?

for problem a I figured it to be n1/n2 = v1/v2 or a 1 - 140 ratio.
I am confused on how to figure out problem b and problem c. I do not know what formula to use for those problems.

2. Oct 23, 2006

### Ja4Coltrane

(Charge/time)(energy/charge)=energy/time=power

3. Oct 23, 2006

### nateastle

How do I figure out energy/time? would this be (16800 x 10 x 10^-3) or about 168 watts where 16800 is the total voltage of the transformer and 10 x 10^-3 is the current?

with part c would this be 140 times greater than the current in the second transformer?

Last edited: Oct 24, 2006
4. Oct 24, 2006

### OlderDan

For an ideal transformer, the power input is the same as the power output. So yes, the primary current would have to be 140 times the secondary current.

Check your exponent on the current.

A real transformer would not be so efficient, but unless you were told otherwise you can assume it is ideal.