I will be taking a basic beginners electronics test, and I found a question on the internet that I don't understand. The question is: What size power supply would best be suited for a 12 VDC operation with a 0.2 amp current draw? The answer is 5 watts. How is this calculated?

 

E X I = W

Voltage X Current = Watts

 

Ohms law is something you will need to become familiar with.

Tattoo it in the brain. And remember WIRE.

What is WIRE you ask? Well here you are:

 

I am familiar with OHMS law, but in this problem, E x I = W which calculates as 12 x .2 = 2.4 not the answer of 5 which is the answer stated on the practice test. Maybe the practice test answer is in error? Thanks for your help and input.

 

In my opinion it's a badly formulated question, one could probably make a case for "best" in a economic view or "best" in a reliability view, or any other view for that matter.

God knows how they got the 5W though, unless they are operating with a fixed efficiency ratio *shrug*

 

Is there a preface to the question that says use a power supply rated twice your load?

Or is there a multiple choice that said something like:

a) 750mW
b) 5w
c) 925 degrees
d) rice krispies

?

If that was the case, It would be 5.

 

Okay, I'll assume the practice test is in error and forget about that particular problem. Thanks!

 

Retched: No, there isn't anything written about twice the load. I wrote the problem above as written on the practice test, so I think it is just incorrect after the responses I've received. Thanks for your input!!

 

Your welcome.

And if you really get an Ohms Law tattoo, send me a pic.

 

Wow, an Ohms tattoo...that would be a conversation piece! Thank you for your help. Back to my studies.