A step down transformer has a power output of 50w and efficiency of 80%.if the main supply voltage is 200v.cal the primary current of the transformer?

First, we need to calculate the power input to the transformer using the efficiency formula:

Efficiency = (Power output / Power input) * 100
0.80 = (50 / Power input) * 100
Power input = 50 / 0.80
Power input = 62.5 W

Now, we can calculate the primary current using the formula:

Power input = Voltage * Current
62.5 = 200 * Current
Current = 62.5 / 200
Current = 0.3125 A

Therefore, the primary current of the transformer is 0.3125 A.