Re: Power Supply Negative/Positive

From: Ken Smith (kensmith_at_rahul.net)
Date: 07/14/04

  • Next message: John Larkin: "Re: Observing single electron flow" 
    Date: Wed, 14 Jul 2004 03:27:51 +0000 (UTC)

    In article <f86e538b.0407120758.1095d5ee@posting.google.com>,
    joeyla <joeden1@o2.ie> wrote:
    >Hi there. I'm pretty new to electronics and have question.
    >
    >I have a project using Analog switches and I need +5v and -5v.
    >
    >I have tried using ICL7660, but I need to supply over 100mA to both
    >supplies and this device does not work. I tried using a potential
    >divider and a 7805 and 7905 tapping the center as ground,with no
    >success either.
    >
    >How can I get +5 and -5 volts to my circuit without 2 power supplies??

    You can't really but the supplies can share parts such as the others have
    suggested.

    If you have an isolated power supply to run the circuit on, perhaps you
    can do it like this:

    Schematic just to get the idea:

          ----------
    ------! LM7805 !------------- Sort of 5V
          ----------
              !
              !------------------ Sort of GND
            /---/
              ^ 5V zener
              !
    ------------------------------ Sort of -5V

    If the input power supply is isolated, you can hook the "Sort of GND" to
    the real ground. If not, perhaps you can keep the whole thing +5V above
    the real ground.

    Now for some details:

    There must be more load on the +5V than the -5V. If there isn't naturally
    you need to add a resistor to make it such.

    The 5V zener can be a bit of trouble because you may need a fairly high
    power one. If this is the case, you can help it out with a
    transistor.

        -------------------
         ! !
       /---/ 4.3V !
         ^ !
         ! !/ c
         ---------! NPN power transistor
         ! !\ e
         / !
         \ !
         / 100R !
         \ !
         ! !
        -----------------

    You have to put bypass capacitors on the LM7805 and across the -5V supply. 

    -- 
--
kensmith@rahul.net   forging knowledge
  • Next message: John Larkin: "Re: Observing single electron flow"

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