# Very basic power supply question

#### Kara48

Joined May 5, 2010
5
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?

#### Wendy

Joined Mar 24, 2008
22,208
E X I = W

Voltage X Current = Watts

#### retched

Joined Dec 5, 2009
5,208
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:

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#### Kara48

Joined May 5, 2010
5
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.

#### CoolBeer

Joined Mar 29, 2010
40
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*

#### retched

Joined Dec 5, 2009
5,208
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.

#### Kara48

Joined May 5, 2010
5
Okay, I'll assume the practice test is in error and forget about that particular problem. Thanks!

#### Kara48

Joined May 5, 2010
5
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!!

#### retched

Joined Dec 5, 2009
5,208

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