Question

Calculate the current drawn by the primary of a transformer 90% efficient which steps down 220 v to 22 v if the output resistance is 440

Answer 1

The current drawn by the primary will be 5.55 miliampere

Explanation :

on the secondary side , voltage, V₂ = 22 v

Resistance, R = 440

we know that current = voltage/resistance

so I₂ = 22/440 = 0.05 A

Output power = Voltage x current

= 22 x 0.05

= 1.1 watt

since the efficiency of the transformer = output power/input power

=> 1.1/P₁ = 0.9

=> P₁ = 1.1/0.9 = 1.22

we know that

input power = input voltage x input current

=> P₁ = V₁ x I₁

=> I₁ = P₁/V₁ = 1.22/220 = 0.0055 A = 5.5 mA

Hence the current drawn by the primary will be 5.55 miliampere