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botching together a power supply -9v 0v 9v 18v without killing myself

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

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  1. bengo

    bengo Guest

    hi

    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
    paranoid.

    any help gratefully received, email or here!

    bengo
     
  2. (in <>) about 'botching
    together a power supply -9v 0v 9v 18v without killing myself', on Thu, 1
    Apr 2004:
    If these adaptors have fully isolated outputs (no ground connections),
    the you just take the junction between the first and second as '0 V'.
     
  3. soundman

    soundman Guest

    But, take care to make sure that these are regulated adapters. Your +18V
    could quite easily reach +25V or more if they are unregulated. The 9V can
    also reach 12 - 13V. While the synth will probably cope with the extra
    volts, you don't want to discover that it can't by plugging in an
    unregulated PSU
     
  4. bengo

    bengo Guest

    i'll check its not over with a multimeter i guess.

    i know i'm being dumb, i just dont get how it wont be 0v 9v 18v and
    27v if i put them in series without doing something cunning to earth
    the 9v into 0v and make the whole lot -9v 0v 9v 18v.

    i'm sure i used to be clever. i'm just being thick about this.

    b
     
  5. ----------------
    Yes you are.

    Voltage is NOT an absolute, it's relative. If you connect them
    plus to minus, all in series, and if you simply SAY that the first
    node from the end is zero volts, then that IS what it will act
    like IF YOU CONNECT IT TO YOUR DEVICE THAT WAY!!

    When you connect 4 batteries or PSUs in series, they do NOT have
    absolute voltages, they are relative to each other ONLY, they can
    be said to be 0, 9, 18, 27 *OR* -9, 0, 9, 18, *OR* -18, -9, 0, 9
    *OR* -27, -18, -9, 0, if you connect the "0" Volts as your ground!!

    It can even be -54, -45, -36, -27 if you stand it off of the
    ground with yet another supply by -27 Volts!!

    -Steve
     
  6. (in <>) about 'botching
    together a power supply -9v 0v 9v 18v without killing myself', on Fri, 2
    Apr 2004:
    There is no 'absolute' 0 V. 0 V is where you SAY it is. If you connect
    the junction of supplies 1 and 2 to the common rail of your circuit, the
    other output of supply 1 will be at -9 V *relative* to your common rail.
     
  7. bengo

    bengo Guest

    any suggestions about what current the adaptors should be?

    b
     
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