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Using Hex Inverters to generate 0, -5V output

Discussion in 'Electronic Basics' started by [email protected], Feb 7, 2006.

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  1. Guest

    Hi there, I was wondering if anyone can help me design a simple circuit
    to drive a switch. The switch has two states, and expects two inputs.
    To put the switch in state one, it needs inputs 0V, -5V and to put it
    in state two, it's inputs need to be -5V, 0V. So what I'd like to
    design is a driver that takes a 0 / +5V signal, and converts it to the
    appropriate output state.

    Truth Table:

    Input Output 1 Output 2

    0V 0V -5V
    +5V -5V 0V

    Now, I thought I could use a hex inverter in a kind-of negative state
    (i.e. Vcc = GND, GND = -5V) to generate the appropriate negative
    voltage, but playing around in the lab hasn't been so helpful. Can
    anyone suggest a way that I can do this? Switching times aren't
    important.

    Thanks!
     
  2. Andrew Holme

    Andrew Holme Guest

    What is the switch? Is it sensitive to voltages relative to ground, or only
    potential difference between its inputs? What power supply rails do you
    have available? Must you use a hex inverter?
     
  3. Noway2

    Noway2 Guest

    What do you have for an incoming power source? If you have a center
    tapped secondary transfomer, assuming sufficient secondary voltage, you
    could use a 7805 and 7905 regulator to generate both a positive and
    negative 5v.

    Another possibility for generating the -5v is using a small dc-dc
    converter. If you are not talking much power (current output) you
    should be able to find one an inexpensive one that will meet your
    needs.

    Once you have a + and -5V supply, you could use a push - pull or totem
    pole output. Basically you use two transistors, one to conduct in the
    postive direction and one to conduct in the negative direction. If
    this sounds like it might work, search on those terms (push pull and
    totem pole) to learn more about their structures and implementation.
     
  4. Chris

    Chris Guest

    Hi,Alex. Even though you didn't state it specifically, I'm assuming
    you don't have a +/-5V supply available. If you do, just use an LM393
    dual comparator, and place your output pullups to GND instead of the +
    supply (note that this comparator, like most, has open collector
    output).


    |
    | VCC
    | |
    | .-.
    | 22K| |
    | | |
    | '-' VCC
    | | +
    | | |
    | | |\|
    | 2.5V o------|-\ 0V/-5V
    | | | >-o-----o
    | | .---|+/ |
    | | | |/ .-.
    | | | | |
    | Vin | | | |
    | o-------)--o '-'
    | | | |
    | | | ===
    | | | |\ GND
    | | '---|-\ -5V/0V
    | | | >-o-----o
    | o------|+/ |
    | | |/| .-.
    | | - | |
    | .-. Vee| |
    | 22K| | '-'
    | | | |
    | '-' ===
    | | GND
    | ===
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    However, if you don't have the negative supply, here's one kind of
    cheesy way of generating the negative outputs, using a 74C14, 74HC14,
    or 40104 hex inverter with schmitt trigger input (view in fixed font or
    M$ Notepad):

    | ___
    | .-|___|-.
    | | |
    | | |\ | || V1
    | .-->|-o-| >O--o-||-o-|<-o--o---.
    | | | |/ || | | |
    | | --- - --- .-.
    | | --- ^ --- | |
    | | | | |+ | |
    | Vi |\ |\ | | === === '-'
    | o-| >O-o-| >O-' === GND GND |
    | |/ | |/ GND ===
    | | ___ GND
    | | .-|___|-.
    | | | |
    | | | |\ | || V2
    | '--------->|-o-| >O--o-||-o-|<-o--o---.
    | | |/ || | | |
    | --- - --- .-.
    | --- ^ --- | |
    | | | |+ | |
    | === === === '-'
    | GND GND GND |
    | ===
    | GND
    |
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Note that inverters 1 and 2 act as a buffer and inverter of the input
    +5V/0V signal. Inverters 3 and 4 are set to be oscillators, with
    diodes and caps performing voltage inverter function at the output. At
    any given time, either inverter 1 or inverter 2 will be sourcing
    current into one of the oscillator caps, preventing that inverter from
    oscillating. When that inverter stops, the output load resistor will
    pull the charge off the output cap, bringing the output voltage down to
    GND.

    It's ultra slow, it's clunky, and the output voltage will be a couple
    of diode drops short of -5V (use schottky diodes if it helps), but this
    might get you where you want to go.

    Another option you might be interested in is using a MAX232 or other IC
    to get the negative voltage and use a diode to prevent the output from
    going above 0V, or use a 4.7V zener if you can live with -4.7V/+0.6V.

    Good luck
    Chris
     
  5. Guest

    Thanks for all the responses, I'll have to go over them in detail, but
    I just thought I'd supply a little more information for those who were
    looking for it - the switch is a GaAs DC-20 GHz SPDT switch for a
    transceiver. I don't specifically need to use Hex inverters, I was
    just under the impression that that's how this kind of thing was done.
    The first version will be a bit of a prototype, and I was planning to
    just switch a power supply off an on to generate the 0/ +5V input - I
    could use 2 power supplies to generate both control signals, but I
    think in the end they'll come from some sort of logic circuit, so I'd
    like to make it a bit more realistic from the start. A do have some
    room to play on the control signals - here are the reqs:

    Low: 0 to -0.5V @10 uA Max.
    High: -5V @ 3 uA Typ. to -7V @ 10 uA Typ. (+/- 0.5 Vdc)
     
  6. Chris

    Chris Guest

    Hi, Alex. From the sketchy explanation you've given, a dual +/-5V
    power supply with a comparator to provide the switching signals is more
    realistic. Scratch the second circuit -- you'll definitely have both
    sides of the switch on, which won't do you any good if your signals are
    low impedance. Also, a lot of modern comparators have complementary
    outputs, which should make your job easier.

    Using an logic NOT gate oscillator to make an inverter has a couple of
    problems for you. First, once you get past the diode drops, you'll
    either be right at the edge of a logic high, or out of spec. Second,
    your - supply is unregulated, which means that the noise on the
    negative power supply might feed through to your signal, especially if
    it's right on the edge.

    If you don't have a negative power supply handy, you might just want to
    add a Simple Switcher, an inductor and a couple of passives to get a
    good -5V or -6V supply. Add a comparator with pullups to GND, and
    you're done.

    Good luck
    Chris
     
  7. Guest

    Hi Chris, thanks a lot for your help,

    Alex
     
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