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high voltage by 78

Discussion in 'Electronic Basics' started by Vexator, Sep 11, 2004.

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  1. Vexator

    Vexator Guest

    How could I regulate 190 V DC whit a 78xx? I knew these devices can be used
    at this high voltage.
  2. CFoley1064

    CFoley1064 Guest

    Subject: high voltage by 78
    I believe you mean "Memento Audere Semper" (remember always to be daring or
    audacious). That's what happens when you copy mottoes off statues.

    The LMXX regulators are good for 35V maximum input voltage. You could shoehorn
    one into a scheme for a 190VDC linear regulator, but it would be more trouble
    than it's worth. Even as a reference voltage, you'd have to provide a
    preregulator to keep from smoking it.

    It's possible you saw a 7812 or 7805 as the IC that provided the low voltage
    power for the control circuitry in the high voltage power supply you were
    looking at.

    Be careful. High voltage kills.

  3. Gary Lecomte

    Gary Lecomte Guest

    It is Possible, Using a High Voltage Value, Zener in the ground lead
    to Ground. But it Requires Special Care with the Current Draw at that
  4. I figured he was thinking of the LM317, which because of its different
    arrangement, can regulate much higher voltage. I'm not sure of the limit.
    I'm sure it's spelled out in the datasheet.

  5. Hi,

    The script on Roman statues would have used the letter 'V'
    not 'U' :)

    A 'smilee' on the other hand would not be found there unless
    it was spray-painted on.

    Cheers - Joe
  6. These devices attempt to keep a constant voltage between their common
    input and output. Thus, you might be able to make a high voltage
    regulator by floating them, like this:

    Vin | |
    ----o 78xx o----o---------.
    | | | |
    '----o---' .-. |
    | | | |
    | | | |
    | '-' |
    | | |
    '--------o |
    | |
    .-. .-.
    | | | | Load
    | | | |
    '-' '-'
    | |
    GND ------------------o---------'

    created by Andy´s ASCII-Circuit v1.25.250804

    The voltage divider forces the device to attempt to create a voltage
    of xx volts across the top resistor.

    I haven't considered startup situations, or high load currents.

    Bob Monsen
  7. Tom Biasi

    Tom Biasi Guest

    You are correct sir. "In God We Trvst."
    I'm not so sure about the smilie face, those Romans were wild and crazy
  8. Vexator

    Vexator Guest

    Why does "MEMENTO AVDERE SEMPER" excite curiosity? It's 'cos latin?
    That motto was wrote by Gabriele D'Annunzio, do u know that poet?
  9. Vexator

    Vexator Guest

    thanks all. Now I understand.
    I need 190VDC, so using a 7824 and Im going to put one 166V zener between
    common and gnd, is it ok?
  10. CFoley1064

    CFoley1064 Guest

    Subject: Re: high voltage by 78
    Mottoes are curious things, I guess. I personally prefer the one etched on the
    sword of Oscar of Gordon, Hero of Heinlein's "Glory Road" (1963) (view in fixed
    font or M$ Notepad) ;-) :

    "Dum vivimus, Vivamus"
    "While we live, let us live!"

    Very appropriate for those who want to vigorously seek out and enjoy the best
    things in life, as well as those working with high voltage.

    But seriously, the idea of just placing a 166V zener in series with the GND
    terminal of a 7824 isn't going to cut it. Among other things, a 1uF output cap
    is required to stabilize the output. At turn-on, you'll have 190V between IN
    and OUT, and smoke. There are other reasons why it just won't work, but that
    will do.

    If you really need 190VDC at small current (0 to 15 mA), you might want to try
    something like this (view in fixed font or M$ Notepad):

    Q&D 190VDC Supply 4700 Ohm
    20 Watt 192VDC
    _/ ____ 1N4007 ___ +
    o -o/ o--|_--_|-. ,-------. ,--o-----o---->|-o-|___|-o-------o
    1/4A )|( )|( | | | + |
    )|( )|( | | ### -
    .-'|( )|'----. | --- ^
    | |( )| | | | | |
    '-.|( )|,--' | | ___ | String of
    )|( )|( | .---|___|-o | Sixteen
    )|( )|( | | | 100 | 1N4742
    o----------------' '-------' '----o-' | 2W | + | (12V Zener)
    120/240V Pri | ### -
    12V Sec | --- ^
    X 2 | | |
    1N4007 -
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Here's the drill. This circuit will work if you've got 120VAC or 240VAC line
    voltage. Get two dual-primary transformers of 12VA or higher rating. The ones
    pictured are 12VAC secondary at 1A. Use the first one to "back-feed" the
    second, resulting in an isolated high AC voltage at the output (in the
    illustration, I assume 240VAC, so the "secondaries" of the second transformer
    are wired in parallel to give 120VAC output).

    Use two diodes and two 22uF, 250VDC caps as shown to give you 340V pk., which
    is dropped by the 4700 ohm 20 watt series resistor to give you about 192VDC
    acrioss the string of 16 ea. 12V, 1W zeners. If your output's a little high or
    low, tweak by replacing the last zeners in the chain with 10V (1N4740) or 15V
    (1N4744). This circuit is good for 0 to 15 mA, won't crash and burn if it
    accidentally short-circuits, and is isolated from the line. This can be
    improved by putting a 100K 1/2 watt resistor across each of the 250V caps to
    help with load sharing. But it's a good start.

    Again, please be careful here. The switch and 1/4A fuse above are not
    optional. Even isolated high voltage can be dangerous. If you don't know what
    you're doing, stop and find out. The "stupid" questions are only stupid if you
    don't know and _don't_ ask them. Have somebody else in the room while you're
    fiddling who knows where the switch is. And do remember Nikolai Tesla's
    advice, and use one hand to grab your belt behind your back whenever the
    voltage is on.

    DVM VIVIMVS, VIVAMVS (I prefer Fluke ;-) )
  11. colin

    colin Guest

    You would probably have to use a high voltage transistor to ensure the input
    to the regulator remains within the 35v rating.
    you may find that even with a zener, or floating the regulator, the change
    in unregulated voltage may exced its rating otherwise.

    If you look through the many voltage regulator spec sheets youl find some
    have an amasing colection of novel and useful circuit applications.

    Colin =^.^=
  12. Gene

    Gene Guest

    Uhm, do you really think that one has to be "wild and crazy" to created an
    empire that lasted 1,000 years?
  13. Rich Grise

    Rich Grise Guest

    The only "limit" on either is the voltage across the chip itself. You could
    use a 7812 to regulate 190V, as long as the supply doesn't go over 225V or
    under about 193, and you have a stiff reference at +178V. (Include all the
    rest of the standard caveats/disclaimers, of course.)

  14. Rich Grise

    Rich Grise Guest

    I can corroborate this. When I was in the USAF as an electronics tech, we
    were required by regulation to take off all jewelry, i.e. rings/watches,
    and keep one hand in our pocket. They didn't even like us to wear our dog

    Another little thing you can do is ground one finger while poking
    around - like, let your little finger rest against the chassis, as
    if you're steadying the probe - that way, when you get zapped, you
    get a burn on your hand, rather than cardiac arrest.

    Good Luck!
  15. Vexator

    Vexator Guest

    Hi Chris,
    thank you! ...

    **** The "stupid" questions are only stupid if you don't know and _don't_
    ask them **** that is a good motto!!!

    thanks again if you lost time to me, your advices are my precious.
  16. Tom Biasi

    Tom Biasi Guest

    No. But it helps
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