# Homework Help: Effocoency Question

1. Sep 20, 2007

### JeeebeZ

1. The problem statement, all variables and given/known data
The power supply for a stereo amp develops an output of 22 volts at 2.8 amps. If the overall efficiency of the power supply is 78%, what is the input current that it will draw from a 117 volt source.

2. Relevant equations
P = EI

3. The attempt at a solution

Eo = 22 V
Io = 2.8 Amps

Po = 22 * 2.8 = 61.6 watts

Pi = 61.6 / .78 (efficiency) = 79.0 watts
Ei = 117 V

Ii = 117 / 79.0 = 1.48 amps

Basically is this right?
I thought what you put in always has to be bigger then what you get out?
if so then how can 1.48 amps in = 2.8 amps out?

2. Sep 20, 2007

### antonantal

This principle is referring to the power. The power you put in is always greater than the power you get out. This is not true for the current or the voltage. You can have a lower input current @ a higher input voltage as in this case.

Also you have a mistake. When you wrote

you actually meant Ii = 79/117 because Ii = Pi/Ei

3. Sep 20, 2007

### JeeebeZ

I wrote it wrong but i calculated it right... >_<