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

[Jean-Pierre Desrochers]

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> 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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