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Power Supply Negative/Positive

Discussion in 'Electronic Design' started by joeyla, Jul 12, 2004.

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  1. joeyla

    joeyla Guest

    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??

    Thanks in advance,
  2. Ban

    Ban Guest

    Here is a small schematic for a transformer without center tab. The current
    capability is around 150mA per side. View\fixed font

    | |
    | | |____| | +5V
    1N4001 - | | |
    63mA ^ | + | ---
    ____ | ### | ---
    o-|_--_|-. ,---+ --- | |220n
    )|( | |1000u|16V |
    120Vac )|( | | | |
    o--------' '---)-----+-----+------+------+---------o
    8V/500mA | | + | | 220n GND
    - ### | ---
    1N4001 ^ --- _|__ ---
    | |1000u/16V | | |
    |____| -5V
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  3. Ryan Wheeler

    Ryan Wheeler Guest

    look for some designs in here:
  4. Jamie

    Jamie Guest

    use a transformer with a center tap and a bridge rectifier.
    for your 5 volts you will need at least a 12 Vac with CT.
    the center tap goes to your common/ground/
    both the - and + outs of the bridge should have a E-Cap on them
    around 1k or more uf. each goes to ground for the common connection.
    beware of polarity, they are placed as indicated on the package.
    the + to the + and the - to the - etc..

    the outer leads of the secondary goes to the ~ ac inputs of the
    bridge rectifier.
    the - out goes to the input side of the 7905 which is the #2/case
    and the + out goes to the #1 leg of the 7805.
    the outs of both the 79 and 78 #3 legs are your -/+ lines with the
    common/ground for either.
  5. joeyla

    joeyla Guest

    Cheers guys, I have plenty of ideas here to work from.

    Thanks for your great advice.

  6. Ken Smith

    Ken Smith Guest

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

    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

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

    roma Guest

    (joeyla) wrote in
    Your answers is here
  8. Norm Dresner

    Norm Dresner Guest

  9. Pinchy

    Pinchy Guest

    This is quite basic. First buy a transformer with a tap in the middle
    on the secondary. This will become the ground. Connect a
    diodebridge between the other 2 ends of the secondary using the marks
    indicating AC . On the (+) of the bridghe, connect a capacitor to the
    ground en repeat this for the (-) of the bridge side. Watch polarity

    Now you get 2 seperated circuits with a ground in the middle. Connect
    the 7805 on (+) & ground resulting in +5V. Repeat for the 7905
    from(-) to ground resulting in -5V

    one remark: pinlayout for 7805 and 7905 are not corresponding, use a
    datasheet !

    Succes (draw a schematic first)
  10. Guest

    Hi Guys, thanks again for your help and suggestions.

    I am plesently supprised at how helpful people can be out there, it's
    for a beginner like me to have access to more experienced minds.

    I've made some progress with my project, which is a dart's scoreboard
    by the way. I need the +5 and -5 volts to power a set of six 4inch
    7-segment displays which is controled by a MAX7219.

    I have managed to get -4.4v from a 5volt power supply by using a
    MAX660 (2 acutally in parallel). I won't be happy until I get the full
    -5v though as the Dot Point on the 7-segment display is too dull. So I
    will try out some of the methods posted here until I succeed, I want
    the display as bright as possible.

    Thanks again,
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