Transformer Power supply dilemma

Discussion in 'Homework Help' started by fiatuno, Apr 4, 2012.

  1. fiatuno

    Thread Starter Member

    Jan 28, 2011
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    Hi members, I am designing a power supply with an LM338 and I have got a transformer that from 240vac gives an output of around 18V ac and a max current of 3.5 A. The power supply I need to build is 12V dc and 5A max current and for this I am using the LM338. I am asking as this regulator I have only one and I can't find easily in here so I cannot afford any errors. Is it possible to get the Maximum current of 5A from this transformer please, since it has alot of extra voltage which is 18V ac please. Thanks in advance.
    Regards,
    Fiat Uno
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You may be able to do it if the transformer has a center tap,
    but a lot depends on the actual transformer.
    You are just a bit short on voltage or current.
     
  3. fiatuno

    Thread Starter Member

    Jan 28, 2011
    47
    0
    My main problem is that I cannot simulate it neither on proteus and i need to try it by myself on the hardware itself, and cannot afford any errors. If I am not mistaken the transformer I have is an 18v 3.5A with a 5A 250V Fast blow fuse.From my knowledge on electronics, if I reduce the voltage I should get a bigger current. What I need to keep in mind is there will be a voltage drop in the LM338 and I think this will be the main problem as it will reduce my voltage available to get the 5A. I don't know if my reasoning is good. Thanks in advance for your help
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Does the transformer have a center tap on the secondary?
    Btw, a 5A fuse is overkill. 0.5A should do.
     
  5. fiatuno

    Thread Starter Member

    Jan 28, 2011
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    0
    To tell you the truth, that information is what I have found on the web on a post one made on the same power supply. I am a bit scared of ruining it if I open it. Or is it safe enough as I am used to open other electronics but never power supplies and I am very afraid of ruining it as it was expensive when I had bought it for my surround system. How do you see if it is center tapped please? it is sort of like the below link the one I am using:

    http://www.vdott.com/crms/bigpic/stock/vdott/600_699/6174_1.jpg
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Hah! That is not a transformer.
    That is a switched mode power supply.
    There is no way you can increase the output from 3.5A to 5A.

    It is always a good thing to tell the truth in the first place.
     
  7. fiatuno

    Thread Starter Member

    Jan 28, 2011
    47
    0
    Thanks very much for your help, then. Just for the sake of learning, how can I recognise if it is a switched mode power supply or a transformer if it is giving an ac voltage on the output please. Thanks in advance
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    That should be a DC supply.
    What does it say on the label?

    A 60W transformer would be heavy.
     
  9. fiatuno

    Thread Starter Member

    Jan 28, 2011
    47
    0
    and please what type of transformer would I need to use the LM338 to get the desired 12V 5A. Thanks in advance, regards
    fiatuno
     
  10. fiatuno

    Thread Starter Member

    Jan 28, 2011
    47
    0
  11. MrChips

    Moderator

    Oct 2, 2009
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    One simple solution would be to find an adapter that says 12v DC and at least 5A.
     
    Last edited: Apr 4, 2012
    fiatuno likes this.
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