[sdiy] Capacitor type matters in 74HCxx one-shot circuit ? input reverse voltage protection..

[Roman Sowa]

Actually HC gates has lower output current capacity than its input 
protection diode can handle. So in this circuit it has no power to 
damage 2nd gate and no additional diode is needed. For long term 
reliability and to minimize spikes on power rails, one might think about 
adding a resistor between the cap and input of 2nd gate to lower the 
current flowing via input protecion diode. 10k will be just fine.

Roman

W dniu 2020-10-11 o 17:26, Jean-Pierre Desrochers pisze:
>
> I think this added Schottky diode could protect U12D’s input..
>
> It has around 0.3v of  forward voltage (conducting before 74HC’s 0.6v 
> internal input diode)
>
> and 200mA max of forward current (when C11 discharges to +5v).
>
> Schottky diode..
>
> *De :*Didrik Madheden [mailto:nitro2k01 at gmail.com]
> *Envoyé :* 9 octobre 2020 10:59
> *À :* Jean-Pierre Desrochers
> *Cc :* SDIY List
> *Objet :* Re: [sdiy] Capacitor type matters in 74HCxx one-shot circuit ?
>
> The two parameters that could matter are frequency response 
> (self-inductance) and leakage current.
>
> Frequency response doesn't really matter. It's just passing a pulse 
> through the capacitor and waiting for the capacitor to charge and flip 
> back. It's very noncritical.
>
> Leakage current doesn't really matter either. Both sides are driven at 
> all times, so you're not relying on the capacitor to hold it's charge 
> for long periods like in for example a sample and hold circuit.
>
> Something else about the circuit however, is that during the reset 
> phase U1's output will go high while the capacitor is charged. This 
> will cause the voltage at U2's inputs to go above the usual maximum 
> voltage and discharge the capacitor through the chip's protection 
> diodes. This may or may not be ok in practice but something to be 
> aware of.
>
> /Didrik
>
> On Fri, 9 Oct 2020 at 16:42, Jean-Pierre Desrochers 
> <jpdesroc at oricom.ca <mailto:jpdesroc at oricom.ca>> wrote:
>
>     Does the capacitor type (Ct: ceramic or film) matter in a circuit
>     like this one ?
>
>     I’d say film is obviously better but.. in such a circuit… ??
>
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