How to Calculate Currents in a Simple Transformer?

[mogley76]

Homework Statement

A simple transformer with negligable looses has a primary coil of 2000 turns and a secondary coil of 150 turns. if an AC current of 240Vrms is applied to the primary, calculate the secondary and primary currents if the secondary is connected across a resistor of 300 ohms.

Homework Equations

none given

The Attempt at a Solution

i Know this much : vs/vp = Ns/Np
so..
150/2000 * 240 = 18vrms which is vs
then v=ir so
Is = 18/300 : 0.06A
Ip = 240/300 : 0.08A

is all this right or am i horribaly wrong?

 

[gneill]

The resistor is in the secondary circuit, not the primary, so you can't just divide the primary voltage by that value to find the primary current.

What you can do is recognize that the power delivered to the resistor in the secondary must ultimately be coming from the primary current source, and if there are no losses in the transformer, then the power should balance on both sides.

 

[mogley76]

so secondary wattage= Is * Vs : .06A*18Vrms which is 1.08 w
and primary current = 1.08/primary vrms of 240 which is 4.5E-3 A?

 

[gneill]

Looks reasonable. You might convert your current values to mA for clarity.

 

[mogley76]

great! thanks!