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dual opamp & negative voltage

Discussion in 'Electronic Design' started by [email protected], Oct 28, 2007.

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  1. Guest

    Hi All
    I am getting -5V off a max232, however i need more current. Now i read
    that putting a dual opamp buffer can help me here but when i tried
    using lm328, with both omps acting as inverters, all i got was a
    positive supply. Am i missing something here or using the chip
    incorrectly somehow?
  2. Eeyore

    Eeyore Guest

    Putting it where ? Why would that help ? Where did you read this ?

  3. Tam/WB2TT

    Tam/WB2TT Guest

    Are you sure you mean an LM328. That contains a positive step down voltage
    regulator. Not sure what a buffer is going to do for you; you need to power
    it from somewhere.

  4. mpm

    mpm Guest

    Do you mean to say you need more current on the RS232 Interface, or
    are you trying to use the RS232 signal and/or handshaking lines to
    power something else?? Please clarify.

    Also, I believe a "truly compliant" RS232 device would have to output
    -7 volts to -15 volts, and +7 to +15 volts.
    Everyting in-between is "no-man's land".
    I'm pretty sure I remember reading that in the RS232 book, but I also
    know most receivers these days will work to half that number and

    Why do you need more current?
  5. Jamie

    Jamie Guest

    With out knowing your connections. I'll guess.
    You have connected the op-amps to the 5 volts + that
    supplies the MAX chip ?
    if so, that is all you'll get is 0..5 volts.
    in order to get a -5 volts out, you need the op-amp
    to operate from a +/- dual rail with respect to common.

    if you look at the MAX chip, you'll notice it has an
    inverter on it to give you the +/- rails for the chip
    it self internally.
    This current is just about enough for the standard specs.
    but all is not lots how ever! :)
    if you're operating from a single supply which I think you
    are ? You can put one of your own little inverters on board.
    either construct an osc via a higher wattage simple op-amp
    that can drive a cap and be rectified for a - Volts or, get one
    of those ready made DC-DC inverters capable of the current you need.
  6. Jamie

    Jamie Guest

    Oh don't go there unless you want the young engineers to attempt
    chopping off your weenie when you state the proper output voltages
    for 232 as they should be from years ago.
    Of course, many are use to the marketing schemes used these days to
    cut corners and end up getting serial ports on devices that barely
    output 5 volts..That saves on parts for devices that don't have the
    supplies internally for the proper voltages like laptops etc..

    so there you go!

    Glad to see that some one knows the difference.
    also, the other problem is current. it use to be that 20 ma's was the
    max output before fold back.. now, it seems that 20 ma's is what you
    get at short circuit in many implements.

    oh well.. It's all falling apart in a hand basket!
  7. Guest

    Thats pretty much what i was doing. i'll try your solution and see how
    it goes. Thanks
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