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Multiple Input Pull-down

Discussion in 'Electronic Basics' started by FyberOptic, Sep 22, 2008.

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  1. FyberOptic

    FyberOptic Guest

    I'm having a bit of a quandary. Since I'm still somewhat of a
    beginner, I hope somebody more experienced can help me out!

    I have three TTL logic chips, with one input on each one all connected
    together to a single input signal. When that main signal wire is
    connected to ground, the logic chip signals are all low as expected.
    When I remove the ground connection though, they seem to all go high
    on their own without being connected anywhere (aside from to each
    other). This isn't my problem exactly, but I'm curious if someone can
    explain it.

    So I need a pull-down on that main input signal to keep the inputs
    it's connected to forced low unless a high is explicitly introduced.
    The main input signal itself will be connected to a pin coming out of
    a chip elsewhere, which is switchable between being an input and
    output. At startup, that pin is an input, so it's not outputting a
    low like I want, hence the need for a pull-down.

    Where the actual problem comes in is that when I add a "normal" pull-
    down resistor value between the input signal and the ground, it
    behaves as if it's not connected to ground at all. I've tried a few
    different ones, from 100k, to 50k (didn't have a 47k), down to 4.7k.
    I couldn't figure it out. Then finally I tried a 1k, and it seems to
    properly pull it down low. Keep in mind that this isn't even
    currently connected to the other chip I mentioned which will default
    to an input at startup, this is just the input signal being connected
    to ground or left floating. Is having to use such a low pull-down
    resistor because I have three inputs connected together? Is there a
    resistance inside the IC I'm not taking into account when connecting
    it up like this? My assumption is that a "normal" 10k-47k pulldown
    value assumes only a single input is being connected to. A possible
    worry I have though is that the 1k resistor is kind of low and could
    be wasting power (since it will be able to run from batteries), but I
    dunno.

    I'd appreciate any light anyone can shed on how these things work!
     
  2. Jasen Betts

    Jasen Betts Guest

    10K for a normal TTL pull down, but with three inputs involved you
    might need 1/3 of that or 3.3K

    also the TTL thresholds are near grouind so you need to pull harder to
    go down that you do to go up.
    if it's to be battery operated, CMOS might be a better choice than TTL.
     
  3. It's called floating. The state is usually undefined but because of unknown
    factors it could set it to either state. That or internally there is a
    pullup.

    (are you sure it's continuously high and not toggling back and forth very
    rappidly?)
    My bet is that there is an internal pullup. By adding your pulldown it works
    in parallel resulting in a voltage divider.
    What is the logic family? (e.g. what is the numbers on the chips)

    Some chip families work better than others.

    As Jasen mentioned, I'd end up using some form of low power cmos rather than
    bipolar. You need to choose the right family if you are worried about power
    loss. Some families are high speed which you might not need.

    In any case the cmos families(or equivalents) are usually a direct
    replacement so there is not that big a deal. (there are a few issues with
    have to do with thresholds but I doubt you'll have those problems)

    If I were you I'd probably look at a few alternatives to bipolar.

    http://en.wikipedia.org/wiki/7400_series

    somewhere there is also a chart that shows the power consumption
    differences.
     
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