120V AC to low voltage DC converter

Discussion in 'The Projects Forum' started by ke5nnt, Feb 23, 2010.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    384
    15
    I'm going nuts and need a bit of help.

    I want to convert Utility AC voltage (~120V 60Hz) into a manageable DC voltage (anywhere from 5 Volts up to say 15V). There are a billion pre-made devices that do this for cell phones, laptop computers, etc that are dirt cheap. I want to build one myself to integrate into a project I'm working on that will supply about 500mA.

    I'm looking at transformers and I can't find anything that's not huge or $50.00...

    Where can I learn to build a regulated constant voltage power converter?

    I have looked over a lot of stuff including some schematics, including this one which I realize is overkill on current, but just for reference. I understand what it's doing but the transformer used in that project is $60.00 and about 4 cubic inches big. Plus, this project uses 27 parts. Looking inside this "wall wart" cell phone charger that inputs 120V AC and outputs 5.1V DC 0.7A it has maybe 20 parts and the transformer is less than 1 cubic inch.

    Can someone point me in the right direction for this?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Phone charges uses switch mode method.
    Unless you are familiar with SMPS, you cannot tackle them.

    First try to find a transformer that could supply around 10VAC @ not less than 500mA, too much current is useless if you are not going to use em. Any where between 500mA to 1A will suffice your needs.

    Transformer input should be rated to your local supply voltage.
    After that we can go on on how to get DC from it.

    On the other hand a phone charger that can supply 5V will also might suite you depending on your project. If the adapter is small then it is a SMPS type power supply. These uses ferrite core transformers and are different from the 50Hz or 60Hz type transformers.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    896
    A cheap AC/DC adapter has an output of 500mA max. But its output voltage is not regulated. A linear voltage regulator IC can be added for only $.50.
     
  4. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
    384
    15
    I found a transformer by Hammond, which specifies VA rating at 10, 115V in, 12.6V out at 800mA. I understand VA to be "Volt-Amps" which is something like Watts but different in that Watts is the "real" power drawn by the equipment and VA is "apparent power". Which is a bit confusing to a transformer newbie, but that's why I'm here.

    Anyways, assuming that transformer would work, I'm at 12.6V AC which I'm guessing then needs to be passed through a "full wave rectifier" to turn it into DC?

    I'm trying to understand how power supplies work, so what's next?

    Thanks.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Yes, but the pulses from the full-wave rectifier need to be smoothed into DC with a fairly big capacitor.
    The DC will be too high at 16.4V with a load or maybe 22V without a load.
     
  6. Jack_K

    Active Member

    May 13, 2009
    115
    0
    Go to Radio Shack and buy a 110v to 12VAC transformer, a bridge rectifier diode module, an LM317 or similar voltage regulator, and some 10,000 mdf 25 or more volts capacitors.

    Then read the Radio Amateurs Handbook (it appears you're a ham) on how to wire it all up. If you don't have the Handbook, use Google to seach for power supply projects. There are literally millions out there.

    Jack, K5TGJ
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    The transformer would suffice if you can tell me the max load you want or the power requirements of your project.
    Post these specs and we can go on, a bridge will suffice but so will 1N4001 diodes too. The diodes I think you can salvage from junk TV boards or ham radio boards as such.
     
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