# Very basic power supply question

Discussion in 'Homework Help' started by Kara48, May 5, 2010.

1. ### Kara48 Thread Starter New Member

May 5, 2010
5
0
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?

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
E X I = W

Voltage X Current = Watts

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3. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
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:

• ###### Ohmslaw.jpg
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36.8 KB
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4. ### Kara48 Thread Starter New Member

May 5, 2010
5
0
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.

5. ### CoolBeer Active Member

Mar 29, 2010
40
6
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*

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6. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
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.

7. ### Kara48 Thread Starter New Member

May 5, 2010
5
0
Okay, I'll assume the practice test is in error and forget about that particular problem. Thanks!

8. ### Kara48 Thread Starter New Member

May 5, 2010
5
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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!!

9. ### retched AAC Fanatic!

Dec 5, 2009
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313
Your welcome.

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

10. ### Kara48 Thread Starter New Member

May 5, 2010
5
0
Wow, an Ohms tattoo...that would be a conversation piece! Thank you for your help. Back to my studies.