Help with making an AC/DC Power supply

Discussion in 'The Projects Forum' started by JasonP, Oct 9, 2009.

  1. JasonP

    Thread Starter New Member

    Oct 9, 2009
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    Hi there,

    I am looking at creating an AC/DC Power supply to output max 3Amps
    I will be using a (UK) 230v - 2 x 24VAC 3.2VA Transformer. Just want to also know does this mean it will give me 3.2VA per 24VAC secondary winding ?

    I need to know what diodes I can use for the bridge rectifier. I have been looking at the Vishay SBYV28-50-E3. Could I put 2/3 1N4003-E3 in parrallel as these are rated at 1amp ? Would putting them in parrallel give me the 3amps I need ?

    And what regulator will I need. I was looking at an LM338T. it is rated at 5amps, but is it effiecient ? Any suggestions would be great.

    I need it to output at most about 19v DC.

    Thank you
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Hi
    you need to get a new transformer. Then a transformer is labeled 3.2 VA 24 volt. It is made for giving out (3.2/24) ampere maximum. You should also get a large cooling fin for your LM338T. If you need 19 volt max perhaps a 18 volt transformer would be better. Using a commercial bridge rectifier is also convenient. Because they are easy to mount on a chassis/cooling fin for better cooling.
     
  3. KMoffett

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    Dec 19, 2007
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    Yes, it would give you 3.2VA pre winding...but:

    3.3VA/24V=0.133 amps

    You cannot get 3.0 amps out of that transformer.

    VA (volt-amps) can be interpreted loosely as watts...volts times current...the "power" the transformer winding can deliver, not the "current". For 3A @ 24V you need greater than 72VA.


    Ken
     
  4. t06afre

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  5. KMoffett

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  6. JasonP

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    Oct 9, 2009
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    Right ....

    Then can someone tell me when you buy those standard power supplies that are eg 18v 1000ma they are small .... what components are they using ?

    How do I make a power supply that will give me 2-3amps in a small form factor ?
     
  7. KMoffett

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    Dec 19, 2007
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    For small, high current AC-DC power supplies you need to look at "switch-mode power supplies" (SMPS).

    Ken
     
  8. JasonP

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    Oct 9, 2009
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    Could you point me in the direction of building something like that ?
     
  9. steinar96

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    Apr 18, 2009
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    hmm, successfully building a SMPS requires alot of experience in electronics. Since you are asking about basic linear power supplies i really doubt you want to indulge yourself in that overhead just yet.
    If you are curious though they have entire books on the subject of SMPS. Google is also you friend.
    But be warned that the complexity factor is tenfold compared to a simple linear regulator.
     
  10. KMoffett

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  11. t06afre

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    It may also require components that is hard to find for the hobbyist. May I ask what you are planing to do with your power.
     
  12. KMoffett

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    Dec 19, 2007
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    Jason,

    I think I have to agree with Steinar96. Someone who doesn't understand the VA rating of transformers is going to be lost in SMPS design. Though I would not discourage you from learning more about linear and switch-mode power supplies.

    A step back...what are you intending to power with you power supply?

    Ken
     
  13. JasonP

    Thread Starter New Member

    Oct 9, 2009
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    Looking at all of the details on SMPS I am beginning to agree with you all. :)

    I have already converted an old PC ATX Power Supply into a bench power supply. And it works really well.

    The power is needed to drive a whole lot of Power LEDS for my fishtank lighting system. I built the drivers already, but I need a little extra power than 18v to allow me to add one last Power LED onto the array (hence the 19volts). I might just try do something with the +12v and -12v lines (giving me 24v at the moment) to bring it down to about 19v. I was thinking a voltage regulator but could a plain old resistor do the trick ?

    I would still like to build a linear power supply, but from what you have been telling me is that the Amps would be pretty low, not even close to 1amp for basic usage. Is there no way to build something that would at least give me that, or at least amplify the amps ?

    Thanks so much for all your help so far.
     
  14. JasonP

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    Oct 9, 2009
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  15. steinar96

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    Apr 18, 2009
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    That circuit will be capable of 2 amps. There is no mention of VA in that one. But since the transformer can handle 3A at 20V we can calculate it as a 60VA transformer. But if you want to actually draw 3 amps you should get a transformer capable of 3.5-4 amps (70-80VA) Because you dont want it breaking down at it's maximum capacity.
    That circuit is a very basic linear power supply. It fares alright in terms of delivering the power you need but the regulation might not be as good. But as long as the thing you are powering can take some voltage ripple then you should be fine.

    The transformer in this circuit can handle 3 amps, however the diode bridge can only handle 2A. So the diode bridge actually limits the current output.
     
  16. JasonP

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    Oct 9, 2009
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    Thanks Steiner,

    I think I will give up on the idea of creating a usefull power supply, maybe just as a little side project to see if I can do it. Some of the transformers are about 2kg in weight when you start wanting those Amps.

    What would your suggestion be to change the output on the Bench Power Supply ? Would a regulator be better than just a resistor ?
     
  17. t06afre

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    May 11, 2009
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    Many power supply for laptop computers are in the 18 to 20 volt range and may deliver that amount of power. I guess you or one of your friends have one or more that is not used, laying around some place
     
  18. steinar96

    Active Member

    Apr 18, 2009
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    Yeah the transformers are quick to get heavy. That is why at higher amperages people turn to SMPS.
    The simpler ones such as buck or boost are without transformers and the more complex ones can use pretty small transformers even at high power outputs because as the operating frequency increases (20-500khz) the inductive and capacative component sizes can be reduced.
    SMPS's are usually always run at least 25khz to keep human auditable swithing noise away.
    The transformers are inductive elements so as the frequency they are operated at increases, you can decrease their size.
    Mains operate at 60hz, requiring a fairly big transformer even for medium power applications.
    SMPS operate at 25-500khz so they can decrease the transformer size dramaticly for the same power output.
     
  19. soda

    Active Member

    Dec 7, 2008
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    Hi

    Here is a complete layout for a lm338. I use it for a bench supply.

    SODA
     
  20. blueroomelectronics

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    Jul 22, 2007
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    Why are there three LEDs? (they are all hooked up the same)
     
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