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Ground loops and TTL supply from PSU questions

Discussion in 'Electronic Design' started by Jon Danniken, Nov 24, 2006.

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  1. Jon Danniken

    Jon Danniken Guest


    I have two questions.

    First, do I need to be concerned with ground loops for computer circuits? I
    am planning on taking 5V and Ground from a molex off of the PSU to power a
    small circuit that I am building inside of the computer case. The circuit
    will use that ground as a reference for a voltage (2V or so) coming off of
    the video card adaptor (AGP port).

    Am I going to be okay referencing the voltage from the video card to the
    ground wire from the PSU, or should I reference it to something on the video
    card itself? If I do need to reference it to something on the card itself,
    would it be okay to tie the ground on the video card to the ground on the
    molex from the PSU?

    My second question concerns using the 5V output from the PSU as the Vcc for
    a TTL chip. Should I use a voltage regulator to ensure that the supply is at
    5V (and not too far over), or is it generally okay to just use the 5V as it
    comes off of the molex?

    Thanks for any insights into these concerns,

  2. At the risk of being wrong (as always possible), I'd get the signal from
    that AGP differential.
    Then you can use normal ground from the connector, and any ground current
    will not interfere with the input.
    No, use differential input, ground side diff input on AGP card.

  3. Rich Grise

    Rich Grise Guest

    I disagree here. Jon should either use the +5 from the PSU, and just
    capacitate it smooth, or run a regulator off the +12. Any +5 regulator
    with only a +5 supply will put out less than +5V, because there is no

  4. Oh I agree, when using the 5V you should filter / decouple it locally.
    If the app is purely digital that PSU 5V will be OK, if it involves
    any analog processing you could use the 12V or a switcher on board from some
    other supply.
    To make it short, the 'molex' IS the connector from the PSU I think?
    Or did you mean one of thsoe power connectors as to the IDE drive?
  5. Jon Danniken

    Jon Danniken Guest

    Thanks, Jan The reason I want to tie the grounds together is to be able to
    use a transistor (NPN) to switch in the circuit. I would drive the
    transistor from a 4013 flip/flop, and connect the 2V coming from the AGP
    card to an appropriate resistor, and then to the Collector of the
    transistor. When switched on, the transistor would complete the circuit
    through the shared ground, and change the reference voltage from the AGP
    card (desired effect)

    Otherwise, I'm thinking I would have to use a quad bilateral switch.

    Here is a crude ascii drawing:

    |--/\/\/---- From AGP voltage
    From switching circuit --/\/\/---|

    This is why I am hoping to tie the grounds in together, so that I can just
    use a transistor to switch in the circuit.


  6. Jon Danniken

    Jon Danniken Guest

    Thanks Jan, and Rich.

    Yes, the voltage will be coming off a molex connector from the PSU.. The
    chips involved will be a MAX232, a 4013 (flip flop), and possibly a 7404
    (hex inverting buffer) if I need it. There will also be a few transistors
    switching things in and out..

    I would definitely toss in a few caps across the supply as it enters the
    board, but will that be enough?



  7. I would do it something like this (discrete), differential amp.
    _________________________________________ +
    | |
    [ ] [ ]
    | |--------------------------> signal
    d d
    -- g JFETs g---------
    AGP s s |
    SIG |-------------| |
    | |
    AGP -------------------------- +
    GND | |
    | [ ]
    c |
    e NPN ---
    | \ / temp comp diode
    | current source ---
    [ ] |
    | [ ]
    | |
    ------------------------------------ GND
    No need to tie any ground together,
  8. The PC PSU will have to work between about 4.75 and 5.25 I think, mine goes down
    to 4.81 according to bios....
    That is within the TTL spec, CMOS HC[T] may have even bigger range, look it up.
    It should work!
  9. Jon Danniken

    Jon Danniken Guest

    Thanks, Jan, but if I am reading that correctly, you have the AGP actually
    turning the FETs on and off.

    In the application I am doing, the switching is done by the remote circuit
    (by a flip flop), which connects the AGP voltage to ground via a resistor.

    The reason for this circuit is to fool a voltage sensing chip on the AGP
    card into thinking that certain voltages on the AGP card are too low. This
    is done by connecting a parallel resistance to the existing resistor on the
    AGP card. By doing this, certain voltages on the AGP card will be raised,
    which is what I am trying to accomplish. (this actually works, I am just
    trying to automate it with a switching circuit so it isn't left on when it
    isn't needed)

    I like the idea of keeping the AGP ground seperate, though, the safety of
    which is my main concern.


  10. Jon Danniken

    Jon Danniken Guest

    Thanks, Jan, that will save me some needed space.

  11. Ah now I get what you want.
    Well in that case it is the other way around, the diff amp goes on the AGP
    card and perhaps drives a power MOSFET or transistor.
    Your driver signal is the differential.
    But likely only a simple MOSFET will do, 5V on enough noise margin.
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