[sdiy] CMOS mystery

[KD KD]

Sounds the 10k fronting 5.1V zener is to big so you get large slews,
many zeners have large capacitance's, check slew times.

2018-07-07 2:16 GMT+02:00, David G Dixon <dixon at mail.ubc.ca>:
> Hey Team,
>
> So, just a quick update on the fifth generator situation...
>
> Today, I took my old PCB and tried the two different brands of 4024 counter
> -- one that worked and the one that didn't.  On this PCB, I'm creating the
> 1/3 duty cycle pulse wave from the triangle with an LM311 comparator
> between
> 0V and +5V, so the resulting pulse wave will be exactly 0 to 5V.
>
> The result:  Exactly the same.  The CD4024BCN chip worked, and the CD4024BE
> chip did not work.
>
> Then I realized my mistake: The square wave I'm sending to the 4024
> counter's CLK input is the VCO's output square wave, which is 10Vpp, -5V to
> +5V.  D'Oh!  Of course, the input diode on the 4024 clock input is limiting
> the negative swing to one diode drop below 0V, but there is probably a fair
> bit of current flowing through that diode (about 5mA, I'd guess, given the
> 1k output resistor on the square wave output).
>
> So, I kludged on a 10k resistor from the square wave, a 5.1V zener to
> ground, and a 30k resistor to +15V.  This made the CLK input more or less
> exactly 0 to 5V.
>
> The result:  No change.  The CD4024BE chip still does not work, but the
> other ones do.  I've come to the conclusion that these CD4024BE chips are
> just defective somehow.  The funny thing is that I'm only using the Q1
> output, and it looks fine on the scope, but it obviously is doing something
> very weird to the subsequent logic gates.
>
> I'm still stumped, but I've learned a couple of important lessons.
>
> So, I'm thinking that it would have been OK just to put the 10k
> current-limiting resistor in between the raw square wave output and the
> 4024
> CLK input, and let the on-board protection diode limit the voltage, with a
> paltry 500uA of current flowing through it, which isn't going to hurt
> anything.  Am I thinking about this correctly, or is it just a whole lot
> harder and more sophisticated than my tiny brain is capable of conceiving?
>
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