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hamfistedly constructing a psu without killing myself...

Discussion in 'Electronic Basics' started by bengo, Apr 1, 2004.

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  1. bengo

    bengo Guest


    maybe someone could help me?

    i've got to make a power supply for an old synth, that needs:

    200mA at -9v 0v 9v and 18v.

    i was hoping i could do it as a botch job, by connecting three 9v
    adaptors together in series, but i just can't see how i could make the
    -9v and stop it being 0, 9, 18, and 27.

    how about if i stuck an off the shelf voltage regulator on the lead
    i'll take off between the second and third 9v power adaptor, and set
    that to 9v, will that force the rest of the outputs into line?

    i'm really keen not to have to break out the calculator and soldering
    iron to build a string of voltage regulator zener diode circuits
    because i'm really busyat the moment. i'm also keen not to wire up the
    0v line to the earth as i suspect that will keep tripping the fuse,
    introduce extra noise, and possibly also kill me. maybe i'm just

    any help gratefully received, email or here...

  2. happyhobit

    happyhobit Guest

    Use your 3 power supplies, just make the 9 volt point ground. Then ground
    becomes –9 volts, the old +18 is now +9 and the old +27 is now +18.
  3. tempus fugit

    tempus fugit Guest

    Does the old synth use an adaptor? If not, I think you'll find that a wall
    wart gives unpredictable and unregulated outputs.
    You could use the other poster's setup, but I think I would be using
    requlators in there too.
  4. bengo

    bengo Guest


    i can buy regulated power supplies and check them with a multimeter,
    if it doesnt work i've just splashed money on adaptors i can use

    but is it really ok to stick them in series to get -9v? i know i'm
    maybe being foolish, and they'll kind of make -9v 0v 9v and 18v
    relative to each other, but i just don't see how they won't make 0v 9v
    18v and 27v....

    sigh, i used to be clever...

  5. Bob Masta

    Bob Masta Guest

    The power supplies don't know what you are using as a
    ground reference. Assuming each supply is the usual
    transformer-isolated type, then you can make any
    junction the ground: You can ground the top of
    the chain and call the outputs 0, -9, -18, and -27,
    or ground the top of the middle for +9, 0, -9, -18,
    or whatever you want... everything is relative to
    the point you choose as ground.

    However, depending on the circuit you are
    running, this chain may not be the best way
    to operate. That's because the current for
    one output must pass through two or more
    supplies, so you not only may not get the
    current capability you think, but you may
    get added ripple, plus modulation of the
    high voltage part of the circuit by loads in
    the low voltage part. And any current
    limiting or other protection will be messed
    up as well.

    But if you have the supplies or they
    are cheap enough, you can always
    try your daisly chain and see what happens!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  6. bengo

    bengo Guest

    any suggestions about what current the adaptors should be?

  7. John Fields

    John Fields Guest

    If you're going to post the same message to several newsgroups it
    would be better if you crossposted to them instead of posting to them
    separately. That way the answers can all be crossposted as well,
    without forcing your respondents to visit each group and post their
    answers separately.

    In this case, my response to your question is at
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