Converting 120 AC to 18 or 24 DC at 30 Amps - Help

Discussion in 'The Projects Forum' started by Acshirey, Sep 20, 2013.

  1. Acshirey

    Thread Starter New Member

    Sep 20, 2013
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    I hope some one can help with this problem. I've been stumped for sometime. All help or suggestions will be greatly appreciated. Please let me know if you need any additional information.

    Issue:

    • I need to convert standard 120V AC (From a wall outlet) to 18V DC or 24V DC with a maximum current draw of 30Amps.

    Restrictions/Guidelines:

    • Needs to be fairly inexpensive design/components
    • Needs to be able to fit into an approximate 3"x3"x5" foot print
    • Output needs to be selectable between 18V DC and 24V DC
     
  2. Experimentonomen

    Member

    Feb 16, 2011
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    With these requirements(size in particular), i only see a SMPS as a viable option.
     
  3. Acshirey

    Thread Starter New Member

    Sep 20, 2013
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    Forgive my ignorance but what is a SMPS?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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  5. Acshirey

    Thread Starter New Member

    Sep 20, 2013
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    Thank you for the reply and the information link.

    Does anyone know of any "How To" guides or example circuits for creating one of these from purchased or scrounged components?
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is a kind of application that you will save a lot of money by getting it from the shelf. To build one of those requires a lot of insight and some of the parts needed is not easy to get unless order a buss load of them.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What are you trying to do?
    why 18 -24vdc?
    why 30A

    Its going to be nearly impossible (and not cheap) to find something capable of 30A in a 3x3x5 package.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I don't even see SMPS as a viable option. maybe arc reactor.
     
  9. strantor

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  10. t06afre

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    What do need all that power for anyway?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Look at Antek or on Ebay Jonango, he makes toroidal transformers and supplies, a linear supply going this route is more rugged than a SMPS and easily fixed if it ever quits.
    You can buy just the Toroidal transformer and if necessary slightly modify by adding or subtracting turns to get the right voltage.
    There is only basically three components needed.
    You need at least a 500- 600va transformer, it could be larger than your size requirements however?
    Max.
     
  12. massimj

    New Member

    May 5, 2013
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    If you must build the supply, you can put a switching supply out of your head. If you plan to buy a switching supply, 24 volts @ 30 amps should be easy to find on Ebay. Some of the supplies can be chained together, but that is something that I done by design, so it will be in the specifications.
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Nobody criticised your "restrictions" list, but I think they should have. :)

    That is a power supply with input power in the range of 850W or so, which is going to be a really big and fairly expensive supply.

    Your restrictions of "inexpensive" and 3x3x5" footprint are both unreasonable by themselves, and impossible when combined.

    Kind of like me saying I want a V8 engine that makes 800 hp and fits in a 3x3x5" box, and by the way it needs to be really cheap too... ;)
     
  14. strantor

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    Oct 3, 2010
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    What!? I did my part!
     
    THE_RB and #12 like this.
  15. tindel

    Active Member

    Sep 16, 2012
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    Here is the only real guide on the planet for making SMPS, written by one of the best analog engineers to live (and like all the greats, died too soon)... enjoy.
    http://www.linear.com/docs/4120

    I also like the suggestion of the linear supply... simple... but it gets bigger as your power requirement gets bigger.

    The good thing about a SMPS is that it generally gets smaller as your switching frequency gets higher (the transformer gets smaller). However, the faster you switch, the more difficult it is to make...

    Finally, be careful, 110V can kill you.
     
    #12 likes this.
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Linear at 30 amps... How small?

    Here is my little friend (Scarface reference), and he is only 15 amps. Weighs a mere 60 lbs.

    All the weight is from the silicon steel transformer plates! Ruler on top is 12" for reference.
     
    Last edited: Sep 21, 2013
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    No, you'll just get fried. Check ebay and buy one. Building switchers that run off the AC mains is not a project for the novice to throw together out of scrounged components.
     
  18. bountyhunter

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    I used to design power supplies for 20 years. I was so conditioned to getting impossible specs that when I read the requirements, I just smiled and nodded my head by Pavlovian reflex......

    You are correct: an 800W power supply (even an offline switcher) will probably be bigger than that footprint. More like the size of a typical PC power supply.
     
  19. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Yep, an 800W linear is going to need a forklift or at least a wheelbarrow.....
     
  20. THE_RB

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    Feb 11, 2008
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    Sorry I missed that, you might have been too subtle... ;)


    Even the ferrite transformer in an 800W offline SMPS is about 3x3x3", then you need input caps, output caps, input FET(s) and heatsinks, output rectifier(s) and heatsinks, fan, room for airflow and PCB and good 30A layout, etc.

    It's going to be shoebox sized at least, if someone is good at squeezing stuff into small spaces.

    I haven't seen a PC sized PSU good for 800W, but they might be that small now.
     
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