Question

A transformer for a laptop computer converts a 120-V input to a 24-V output. If the output current for the transformer is 1.8 A, find the input current.

Answer 1

Power out should equal power in. Power is Voltage times current. output = 24volts * 1.8amps = 43.2 watts. input = 43.2watts / 120volts = .36 amps

Answer 2

Note the ASSUMPTIONS in the above solution:

#1. 100% efficiency transformer:
#2. Purely RESISTIVE Load


#1 implies: Input AC Power = Output DC Power
#2 implies: AC Power = V.I

Also note that AC Voltage and Current under consideration are RMS Values. The peak value is in fact the RMS Value multiplies by the Square Root of 2.

Answer 3

I certainly respect that observation and it made me second guess myself. In design work one would be considering many more factors such as coupling efficiency of the transformer as well as losses from the bridge and caps etc. However the 120V is an RMS measurement already and not peak. This is easily shown by measuring with a scope compared to a voltmeter. My assumptions are okay for this question. So I'm going to stick by my answer. Thanks for the input

Answer 4

Sure, patrickdixon, indeed your answer is entirely correct :)
I just felt like pointing out some context of a real life aspects - always good to know these!!