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Schmitt trigger question

Discussion in 'Electronic Basics' started by G.T. W., Sep 12, 2003.

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  1. G.T. W.

    G.T. W. Guest


    I'm trying to figure what I am doing wrong when comparing the
    input/output of a schmitt trigger in an energized circuit.

    This is a MC7414 chip on a pcb, in a centerfuge operating under its own
    power supply. The circuit board is on a floating ground, I measured
    5.1 Volts (RMS) between pins 14 and 7 with a Fluke meter.

    Unfortuneately, the only o'scope available to compare input/output was
    a digital unit, first one I've seen. Everything was set to 'auto',
    there was no manual on how to confirm its settings.

    I set the first probe to the 7414, pin 1(input). A positive pulse was
    seen there. A second probe was put on the output (pin 2) to observe
    the re-shaped output. There was no wave-shaping, no squared pulse.
    Both traces appeared identical, a positive-going pulse, rounded at the
    half-power points. Further, the output was not inverted.

    Due to my ignorance with a digital o'scope, I can not say what the true
    pulse amplitude was. Using pin 7 as a ground reference for the probe,
    the digital readout indicated pin 1 was 174 volts (I looked
    very hard for a decimal point, there wasn't one).

    My questions is.

    What condition could cause a schmitt trigger to pass a signal
    unchanged, instead of re-shaping it? If the input pulse was, for
    example, 1.74 volts, and the schmitt trigger won't go high until 1.8
    volts, does it pass the signal unchanged? I had the same result with
    each gate on this chip. Thank you.
  2. JeffM

    JeffM Guest

    What condition could cause a schmitt trigger
    Not possible (risetime, propagation delay, etc),
    but the best approximation would be:
    The input low level is the same as Vee;
    The input high level is the same as the rail (minus Vsat);
    and the edges of the input are very fast.

    does it pass the signal unchanged?

    If it doesn't reach the threshold, the output doesn't change at all.


    *************** Turn-off threshold reached (6)
    v <--- Input voltage decreasing
    /| | ---> Input voltage
    / | | continues to increase
    / | | with no change
    Output drops low (7) | | in output (4)
    | |
    | | Output jumps high (3)
    | | /
    Input voltage continues to decrease <--- | | /
    with no change in output (8) =======-------->---

    0V 2V 4V 5V
    Input voltage increasing (1) --------------------------->
    Turn-on threshold reached (2)


    _________________ ........... 5V
    | |
    | |
    | |
    | |
    _______| |________ ..... 0V
    . .
    . .
    . .
    . .
    . .
    . /\ .
    . / \ . Turn-on threshold 4V
    / \ / \ .
    / \/ \ .
    / \ . Turn-off threshold 2V
    / \
    / \
    / \
    __/ \____ ..... 0V


    After it trips, it doesn't matter that the input voltage continues to increase.
    The output stays high until the input reaches the turn-off threshold.
    The noise on the input (between the 2 thresholds) does not affect the output.
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