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Mono from 1/2 x 4013

Discussion in 'Electronic Design' started by Terry Pinnell, Oct 3, 2004.

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  1. Fred Bloggs

    Fred Bloggs Guest

    With your timing, a 1M resistor in series with RESET is more than
    adequate. Win Hill's claim of that race/noise-prone minimalist circuit
    is overstated. It is NOT a 'standard' circuit and my library of
    historical logic design goes back further than his. He is basing his
    opinion on some non-critical application in a cheap mass-produced gizmo
    he reverse engineered. I advise against using that 'thing' of his in any
    application that can be disrupted by "runt pulse" behavior. The circuit
    is just plain anti-digital, and a little wisp of a few uA of clamp diode
    current for any MSI IC is a non-event.
  2. Fred Bloggs wrote...
    I don't have time now to refute Fred's mis-characterizations and
    incorrect facts and arguments point by point, so I'll be forced
    to just say WRONG and leave it at that. Anyone who's interested
    can observe the juxtaposed claims and investigate for themselves.
  3. I repeated the simulation with various diodes types. As luck would
    have it, the type I chose for my earlier test (the 1N4148, which I use
    as default for Si small signal types out of habit), gave by far the
    largest spike. Some gave no measurable spike at all. Here's the
    summary of my arbitrary selection:

    Type Spike amplitude
    ------- ---------------
    1N4148 1.40
    1N914 0.75
    1N4001 0.00
    1N458 0.17
    1N3070 0.08
    1N34 (Ge) 0.00

    BTW, what type did you use?
  4. Terry Pinnell wrote...
    Put a 3.3k resistor in series with the diode.
  5. John Fields

    John Fields Guest

    1N4148, and for the rest of it: 470K, 0.47µF Mylar, Motorola

    BTW, the period was 170ms, not the 270ms I posted earlier, and I used
    an HP54602B to view the waveform.
  6. I got around to breadboarding it today. Agreed, no spike. But did you
    notice the small 'excursion', with or without the diode? With Vcc of
    9V it's about 700 mV high and lasts about 300 us. 5V supply gave about
    330 mV.

    I used 0.47µF Mylar and 1M with a couple of types, HCF4013BE and
    CD4013BCN, giving about 250 mS.

    BTW, do you follow Win's suggestion to add a 3k3 resistor in series
    with the diode?

    Can you remind me what advantage is offered by the diode please?

    Fred: I'd closed up the workshop before I realised I'd forgotten to
    try your version; next time!
  7. Thanks. I thought at first that you must be saying there was a real
    spike, and that this would remove it. But then I realised you meant
    only the simulation.

    Indeed - just tried it with the resistor, and it removes it. Can you
    clarify what's happening here please?
  8. Terry Pinnell wrote...
    .. ____
    .. + --|D Q|---+---+--- timed pulse OUT
    .. IN ----|> | | _|_ t = 0.7 RC
    .. __ |__R_| R /_\
    .. __/ |_____|___| diode that needs
    .. | a series resistor
    .. edge- C
    .. trigger |
    .. gnd

    I have no idea why your simulation shows a spike, but in real life
    the capacitor-discharge diode without a series resistor creates a
    supply short on the Q pin at the oneshot-timeout flip-flop reset.
    More accurately, it forces a full supply-voltage signal across
    the logic-low-level Q output, causing a high current through the
    ground pin of the CMOS logic IC until the capacitor is discharged.
    A resistor added in series with the diode limits this current, and
    only modestly decreases the maximum duty ratio the timer can handle.

    BTW, in the drawing above I show the oneshot time** as t = 0.7 RC,
    but actually this time ranges from 0.35 to 1.2 RC, because the CMOS
    gate threshold Vt ranges from 30 to 70% of the supply voltage Vs.
    Of course the RC value tolerances must also be considered. The very
    poor accuracy of this circuit is one reason it's usually used only
    for short non-critical time delays in a logic circuit (if the value
    of C is much over 500pF, then a series resistor should be added to
    the flip-flop's R terminal, as discussed elsewhere in this thread).

    In contrast to the 4013 timer inaccuracy of +70% to -50%, the common
    MC14538 oneshot inaccuracy spec is nearly 10x better, +6% to -8.6%
    (at 10V). Clearly this is a much better CMOS part to use for timers.

    ** The timing formula is t = k R C, where k = ln (1 / (1 - Vt/Vs)).
    If the threshold voltage is exactly 1/2 of the supply, then k is the
    natural log of 2, or k = 0.69315 or 0.693, which is a number many of
    us have memorized.
  9. John Fields

    John Fields Guest

    Looks like an undercompensated scope probe.
    Sure. Without it the period of the one-shot would vary with the charge
    left in the cap, which would depend upon the time between when the
    one-shot timed out and when it was triggered again. That is, the
    diode steers current around the resistor when Q goes low, discharging
    the cap quickly.
  10. Ah, so it does, thanks - another puzzle solved.

    As you've probably seen, Win's now given a detailed explanation.
    Understood, thank you.
  11. Many thanks, Win.
  12. Fred Bloggs

    Fred Bloggs Guest

    There is nothing to "try"- I have been using the circuit for a long
    time. There is no problem with a few uA of clamp diode current of any
    duration. But then re-trigger recovery time has never been an issue-
    although it is easily adapted to this circuit too.
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