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Transformerless power supply

Discussion in 'Electronic Basics' started by Alessio Sangalli, Feb 17, 2006.

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  1. Hi, I've read a lot about this topic both on the web and usenet.

    I have a dimmable electronic ballast for fluorescent tubes. It uses a
    voltage ranging from 1 to 10V as an input for the dimming control. The
    input is isolated and will absorbe less than 0.6mA of current.

    Now, I'd like to build a cheap and space-saving circuit to control this

    I'm considering a transformerless because
    1) I can isolate it very well
    2) The ballast has its own isolation transformer
    3) I need to supply very little current.

    I live in Italy, 240V AC @ 50Hz.

    Which circuit should I use? I was thinking to a 78L10 and a 10KOhm
    potentiometer to set my output. What about the capacitive reactance and
    the resistors/diodes in between?

    Ciao, Thank you
    Alessio Sangalli
  2. Phil Allison

    Phil Allison Guest

    "Alessio Sangalli"
    ** Shame none of if sank in.

    ** FORGET the transforemerless idea - it is plain dangerous for a BUNCH
    of good reasons.

    Buy a plug pack type DC supply - 9 volts @ 100mA nominal will do - since
    with no load the voltage will be or 12 or more.

    Then add the voltage reg IC, control pot etc - nice and safe.

    .......... Phil
  3. kell

    kell Guest

    A "transformerless power supply" uses an impedance to limit current,
    then a SHUNT, like a zener diode, to clamp the voltage. I don't think
    you would want to use a series regulator.
    There's also the fact that if you use a cap for the current limiting,
    it will pass transients and burn stuff up, like your rectifiers and
    whatever it is you're trying to run off it. Series resistance is
    absolutely a must. In fact, it's better just to use resistance,
    Let's say you want 10 milliamps (.01 amps) out of your "t-less" supply
    so you can just put that through a potentiometer that you can adjust to
    get your dimmer controlling voltage. Well, use a power resistor
    instead of a cap; it's not really a problem to do it that way because
    your resistor is only going to create about 2.4 watts of heat. Use a
    resistor 24 kilohms, or some number in that range that is available to
    you. Can be off by a factor of two, or more... but you need to
    calculate heat accordingly. Look up ohm's law etc.
    Put a ten or twelve volt zener across the line after the resistor, and
    a smoothing capacitor. You now have a little ten or twelve volt power
    supply limited to about 10 milliamps, so you can put that across a pot
    and take your control voltage from the wiper.
  4. Phil Allison

    Phil Allison Guest


    ** The metal shaft of that pot could easily become be live at 240 volts AC.


    Don't do it.

    .......... Phil
  5. Chris

    Chris Guest

    Second the motion. Don't do it. More enterprising contrivers of
    contrivances have come to grief from this whole concept of saving a
    wall wart or transformer for "just a few milliamps" than just about
    anything else.

    If you can't make a safe mod, don't make the mod.

    This is goofy. Don't do it.

  6. Deefoo

    Deefoo Guest

    Did you try a 9V battery and a potentiometer? It is not 10V but maybe it
    will do.

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