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Ultra-simple, low-power 3V->1000V converter

Discussion in 'Electronic Design' started by Haude Daniel, Mar 2, 2007.

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  1. Haude Daniel

    Haude Daniel Guest

    Hello folks,

    Frequently I need to make sure that things are well isolated from one another
    at voltages of 500V. We have an ultra old, clunky, heavy Rohgde & Schwarz
    thing for that purpose in our lab, but unfortunately people tend to run off
    with it without leaving note of where it is.

    So I thought I'd build my own, small, battery-operated thingy. I'd like it to
    run off 2 AA cells. So I need something that makes 500V out of, say, 2-3V.
    Output current max. 1 microamp. The output need not be regulated, nor do I
    care if it is 490 or 510 V, but I'd like it to have a fairly reproducible max
    voltage on the unloaded output. So I'd run the primary side from a regulated
    rail.

    Of course the output will collapse under load. That's a desired feature. I
    just need a qualitative reading.

    My ideas so far: battery -> low-power 5V boost regulator (copied from a Maxim
    or LT appnote) -> push-pull switcher to some intermediate voltage -> diode/cap
    ladder to output terminal. To measure the current I'd hang some TIA with a
    mechanical ammeter to the other terminal.

    The thing I'm unsure about is the HV generator. Right now I'd tend to do a
    1:30 step-up followed by a voltage doubler (5V push-pull makes 10V p-p input
    swing, times 30 makes 300 V, * 2 makes 600 modulo losses -> 500). It's be
    great if the whole thing wouldn't draw more than 10mA (I can see you
    micropower guys spill coffee all over your keyboards here. You'd probably
    design it without as much as a power switch).

    Thanks,

    --Daniel
     
  2. daceo

    daceo Guest

    you may be able to do some thing with an old tv or monitor line output
    flyback transformer?

    daceo
     
  3. Hi Daniel,
    I took for a similar project a simple oneway camera with a flash and use
    this one for such purposes. The final voltage in the capacitor is around 350
    V but easy tuneable, it works from a simple AA cell with a batteryclip and
    includes a simple discharge unit ;-)

    Very easy to built, very cheap (go to the next photoshop and ask for thes
    empty cameras or buy anew one, you may need a bill ;-)
    Viel zu kompliziert Daniel ;-)

    Marte
     
  4. Sounds similar to a camera flash circuit. A visit
    to <http://www.repairfaq.org> might give some ideas.
     
  5. Your proposed push-pull aspect rules out many simple single-IC
    solutions, yet would be required for a 2x output-doubler scheme.
    If you give up on both of those, a one-step flyback circuit can
    be used. Thirteen years ago I made a little HV supply using an
    LTC LT1172 and a small stepup transformer wound on an RM8 core
    with an A160 gap. It had to provide about 900mW to the load,
    which was a fast high-voltage opamp.* Other slower low-voltage
    ICs would be better suited for your lower-power requirement.

    BTW, the camera flash-bulb crowd has nice low-power circuits.
    There're web sites describing converting a disposable camera's
    HV generator for general-purpose use.

    * The overall circuit was to synchronize a light-phase shifter
    to the middle of the shutter-opening time of a video camera in
    a specialized motion-sensing microscope. It showed a blank
    field (from phase cancellation), except for any movement that
    occurred during the shutter's open time in each video field.
    The shutter could be open for 100us (1/10,000 shutter speed),
    and we'd catch and highlight any motion during that time. To
    change the phase of the light at mid-shutter, I used an LM1881
    video sync chip to grab each video line's back-porch pulse,
    and used complex cPLD circuit to calculate the right lines to
    generate shutter-open and mid-shutter signals. An additional
    voltage-programmed delay circuit allowed fine tuning to the
    exact shutter midpoint. I programmed the cPLD to work for all
    of the Sony camera's shutter times, from 100us to 8 and 16.7ms
    (60 per sec). The phase-shift pulse was tuned in amplitude to
    drive an optical modulator crystal via the HV opamp. They had
    the microscope optics ready before I got the electronics done,
    and a cry of "waiting for the RIS-177" became the focus of
    several lectures during the Institute's annual review.
     
  6. Haude Daniel

    Haude Daniel Guest

    Oh I don't care about the circuit topology. This is just the first thing
    that crossed my mind.
    Those are good points to start. I've got some RM's kicking around
    somewhere. For this application an RM 6 would probably big enough.

    Time to start up LTSpice. This ought to be a fun project.

    Thanks,

    --Daniel
     
  7. Haude Daniel

    Haude Daniel Guest

    I've thought about that, too. I'll have to check no-load power
    consumption.

    --Daniel
     
  8. John  Larkin

    John Larkin Guest

    Somebody makes a fairly cheap handheld DVM/hipot tester that does all
    this already. I think I've seen it in Mouser or Digikey or somewhere
    like that. I'll look.

    John
     
  9. Hi Daniel,
    Do you really think, your homebrew design will top a ready to use design?

    Marte
     
  10. colin

    colin Guest

    Id use a mc33063 in flyback mode, this gives you 40v swing,
    a 4:1 transformer will be very easy to make, giving you 200v swing,
    too high ratio means problems with leakage inductance and capacitance,
    a 5 * multiplier is easy to make with 200v diodes, wich will give you 1000v.
    ofc you might want to make it a bit more than 1000v to allow for losses etc.
    the chip will regulate it.
    ive done similar thing several times and it works quite well, with very
    small footprint.

    Colin =^.^=
     
  11. You are in Germany? Buy a kit from Kemo like the ion kit which comes with a
    nice pi wound transformer.

    http://www.kemo-electronic.de/en/bausaetze/b137/index.htm
     
  12. James Arthur

    James Arthur Guest

    Charles Wenzel has a number of designs close to what you seek,
    nicely laid out here:
    http://www.techlib.com/electronics/highvolts.html

    Best,
    James Arthur
     
  13. James Arthur

    James Arthur Guest

  14. Terry Given

    Terry Given Guest

    Go buy one of those $10 bug-zapper tennis-racket style flyswats, and rip
    out the electronics. and maybe change the quadrupler to a doubler, or a
    singler :)

    Cheers
    Terry
     
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