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Help needed safely lowering voltage

Discussion in 'General Electronics Discussion' started by ImNotLoved, Apr 16, 2012.

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  1. ImNotLoved

    ImNotLoved

    2
    0
    Apr 16, 2012
    Hi, I am in the process of making a circuit to distribute 120v of power from a outlet to a few low voltage items. I need to give the door 12v, keypad and rfid reader need 22v together and thats it. Oh and I also need 9v for my ardunio. Anyway im not sure how to safely reduce the voltage and split it in 3 ways. Im not sure if a resistor would be safe or a transformer. Any help is greatly appreciated.

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    Thanks,
    Kyle (ImNotLoved)
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,540
    1,967
    Nov 17, 2011
    Hello Kyle,
    a transformer is the best solution in this case to first lower the 120 V mains to a safe level. I don't know about the door, but the keypad, RFID reader and arduino definitely will need DC power. So after the transformer you'll need a rectifier, smoothing capacitor and probably a voltage regulator for each voltage.

    You have basically two options:
    1) use a 120 V/ 18 V transformer. Rectify and smooth to ~25 V DC (that 18 V * 1.414 because the AC peak voltage is higher than 18 V). Add three voltage regulators to generate the required voltages.
    2) use a transformer with two taps: 18 V, 10 V. add 2 rectifiers and 2 smoothing capacitors. Place the 22 C regulator after the 18 V tap, the 12 V and 9 V regulators after the 10 V tap. This will generate less waste heat in the 12 V and 9 V regulators.

    In both cases you gave to take into account the power requirements of your circuits. Each voltage regulator (and associated transformer winding) must be rated for at least the nominal current you are going to draw from it.

    Anyway, if you are not well versed in building power supplies, use three appropriately specified AC adapters ("wall warts") and leave the safety relevant handling of the high voltage (120 V mains) to the experts. This is what I'd suggest.

    Whatever you do: keep safety in mind.

    Harald
     
  3. ImNotLoved

    ImNotLoved

    2
    0
    Apr 16, 2012
    Thanks harald that reply was very informitive and helpfu. Thanks for the help.
     
  4. selanac

    selanac

    22
    0
    Apr 15, 2012
    Harald, Which Tutorial would be best that covers this information. I'd love to review them myself.

    Thanks,
     
  5. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Just because I have seen it so many times from the inexperienced that don't comprehend/understand that running two 10v devices doesn't mean you need a 20v supply... Please clarify this, and define 'together'...

    "keypad and rfid reader need 22v together"

    Note 2, most Arduinos have an on board regulator and can take, 6 to about 20 volts input, although anything over about 12 will likely burn up the regulator (or at least make it damn hot) without a heat sink... You can run Arduinos just fine off a 12 volt supply in most cases, no need for a separate 9 volt tap just for it...

    So now we have a single 12 volt supply that will run the door and Arduino, and once you elaborate further on the other devices actual needs, a solution will start to develop that could be really simple...
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,540
    1,967
    Nov 17, 2011
    Sorry, I don't know any tutorial. If you Google on or two and post the links I can have a quick look and check for quality.

    CocalCola: good idea. I didn't think about that possibility (It would never come to my mind to wire up loads like these in series).


    Harald
     
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