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High AC voltage convert to DC voltage on driving IC

Discussion in 'Electronic Design' started by Electronic Swear, Feb 4, 2005.

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  1. Currently I am designing a circuit for controlling motor.
    And I need to convert a 230Vac to 12Vdc for supply low voltage IC. In
    the past, because of low power consumption for IC, I just use some
    resistors and transistor for this convertion.
    ________ ___ C E
    230Vac o-----| |---|_R_|--+----+------\ /-----o 12V
    | diode | | | ---
    | bridge | --- | ___ B
    | | --- +-|_R_|--+
    o-----|________| | |
    GND |
    12v Zener

    Now, I have changed some components and it need more current to drive
    it. And the past design is not enough to do so. Now, I want to know
    any similiar simply circuit can convert 230Vac to 12V dc with arround
    200mA current.

    If I replace the transistor to the TO-220 7812, can it supply enough
    output for me to use? Thanks~
  2. First, please go find a sturdy wall and recite the following while
    banging your head on the wall:

    "It is a very, very, very, very bad idea for a hobbyist to use a
    non-isolated power supply".

    The point you have labelled as "GND" actually has about 110 VRMS on it
    when plugged in. There is also no current limiting of any kind so it is
    a lethal combination.

    Go to your nearest electronics store and buy a "wall wort" (wall plug-in
    supply) to use as a starting point. You can either get a DC output that
    already has the diodes and filter inside (very easy to find) or, if you
    want the design challenge, get an AC output unit and build your own.
    Given your apparent level of experience, I suggest you get a DC wort.

    To you get a regulated output from a 7812, you need at least 14 VDC on
    the input when under load. The more input voltage you have available,
    the hotter the 7812 will get. Try to get a 15 VDC wort but anything up
    to 24 V should work, as long as you put a big enough heat sink on the
    7812. With 24 V in and 200 mA out, the 7812 will be dumping 2.4 watts
    and will require a decent heat sink. Google for "thermal resistance" if
    you want to calculate what kind of heat sink you'll need.
  3. Electronic Swear wrote...
    Awwkk!! Awwkk!! Troll alert! Troll alert!!!!
  4. How can you tell? I can remember doing this kind of thing when I was too
    young to know better so I know it could be real. I do _hate_ to waste
    time on trolls, though.
  5. Actually, I have changed to use 7812 on supply regulated voltage for
    low voltage IC. The circuit is as following:

    ________ ___ ______
    230Vac o-----| |---|_R_|--+-------+--| 7812 |-----o 12V
    | diode | | + | --+--- | +
    | bridge | --- | | ---
    | | --- zener | ---
    o-----|________|--+ | | | |

    Is it better to do so?
    However, the R will be in very high WATT and what can I to do so?
    Sorry, I am not familiar with such voltage convertion circuit. :(
  6. James Meyer

    James Meyer Guest

    ___ ________ ___ ______
    230Vac o--- |_C_|--| |---|_R_|--+-------+---| 7812 |-----o 12V
    | diode | | + | --+--- | +
    | bridge | --- | | ---
    | | --- zener | ---
    o-----|________|--+ | | | |

    Add a capacitor between the 230Vac and the diode bridge. That will drop
    the voltage without getting hot. Use a non-polarized (not electrolytic)
    capacitor. The voltage rating should be 400 volts and the value probably needs
    to be at least 1 microfarad. The R value can then be reduced so that it doesn't
    get too hot.

  7. Mook Johnson

    Mook Johnson Guest

    I agree with the previous poster. It is very dangerous to use power from
    the lines this way. You cold kill youself or so unsuspecting person that
    happens across your device.

    I would HIGHLY recommend using the Wall arts idea followed by a regultor or
    puttin a dedicated transformer between the 230VAC and your curcuit. A fuse
    would also be highly recommended so you don't burn the place down if things
    go wrong.

    In the current design below you will most certainly blow the 7812. When
    there is no load on the output, the input will likely rise above its input
    voltage rating and kill the device.

    If you need help sizing a transformer or wallwart for the application, thet
    good folks here would be glad to help of you give voltage output current
    requirements for the power supply.
  8. lemonjuice

    lemonjuice Guest

    Some ideas ...
    Unless you're interested in investigating your sensory and muscular
    response to electrical stimulus you need to put an optoisolator at the
    output and better still connect it to a triac or a relay... make sure
    the closing time of your relay is less then that of your heart beat to
    avoid premature death. Place a resistor and a fuse there too.
    Further precautions would be ...
    At the 240V output I'd place a fuse 100mA or whatever you need as
    current and a fusible resistance 200ohms.

    Other ideas ... to avoid heat dissipation is to use a capacitor of the
    suppressor type then a resistor before the diode bridge as power
    dissipated by a capacitor is less then a resistor of the same
    impedance voltage and current being out of phase.
    I'd connect a capacitor then a regulator or some zeners to get a low ac
    voltage in that point go for something like a total of 16volts .

    maybe this is clearer

    fuse fuse resistor
    live o-o_/ \o--|___|-------|
    resistor | |
    1k | |
    Capacitor ===
    +230v input /-\ \. to diode
    |------------- bridge
    | /´
    z zener
    A zener

    Well the part after the diode bridge is simpler... add another
    capacitor above 40uF then in parallel a Resistor plus a 12 V zener
    across which you'll have your load connection. do your calculations
    .... I = E/ 1/2*pi* 50(or60) *C .
    IMO you can do without the transistor. If you need current above 50mA
    .... your capacitors become very big and so expensive. Not much point
    then of doing without a transformer.

    Disclaimer: The author assumes no liability for any incidental,
    consequential or other liability from the use of this information. All
    risks and damages, incidental or otherwise, arising from the use or
    misuse of the information contained herein are entirely the
    responsibility of the user.
  9. Rolavine

    Rolavine Guest

    Subject: Re: High AC voltage convert to DC voltage on driving IC
    Take a look at Power Integrations, at

    This buck design works very well and is fairly cheap compared to the cost of
    the big non polarized cap you would need for the cap coupled off line supply.
    Just a guess, you would need at least 5uF for 200 ma output, maybe 10.

  10. Rob Gaddi

    Rob Gaddi Guest

    Hear hear. You specced 12V, 200mA. I'd overrate the wart to 300mA just
    to give yourself some room to play. 12V DC, 3.6W adapter. If that
    costs you $10 you overpaid for it. Doing your own off-line regulation
    when you have pretty trivial power requirements doesn't start to pay
    until you're cranking them out by the thousand.
  11. Rich Grise

    Rich Grise Guest

    No. You are still going to kill yourself or worse, somebody else.
    Here's a place to start:

    check out these links:

    Good Luck!
  12. Jamie

    Jamie Guest

    yes, the 7812 is fine for your needs how ever, your input voltage must
    be dropped down using a transformer which is much safer in the long run.
    get a transformer that will give you a drop to around 16.. 24 Vac. then
    pass that to a bridge rectifier, Cap and then 7812.
    the 7812 is rated for 1 amp and needs a heat sink., in your case of
    200 mA, keeping the input voltage 3 volts (after filtering) above as a
    min should produce a smooth stable regulation for the 7812. if you keep
    the input voltage not much above the output voltage , the 7812
    will be in near saturation state and thus can be operated with out the
    heat sink at 200 mA . the lower resistance effects will produce much
    less heat.
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