Very basic power supply question

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

  1. Kara48

    Thread Starter New Member

    May 5, 2010
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    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
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    2,536
    E X I = W

    Voltage X Current = Watts
     
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  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    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:
    [​IMG]
     
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  4. Kara48

    Thread Starter New Member

    May 5, 2010
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    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
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    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
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    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
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    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
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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
    5,201
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    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
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    Wow, an Ohms tattoo...that would be a conversation piece! Thank you for your help. Back to my studies.
     
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