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<p>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.</p>
<p>Roman<br>
</p>
<div class="moz-cite-prefix">W dniu 2020-10-11 o 17:26, Jean-Pierre
Desrochers pisze:<br>
</div>
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-CA">I think this added Schottky diode could protect
U12D’s input..<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-CA">It has around 0.3v of forward voltage
(conducting before 74HC’s 0.6v internal input diode)<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-CA">and 200mA max of forward current (when C11
discharges to +5v).<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-CA">Schottky diode..<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"
lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><img
id="Image_x0020_1"
src="cid:part1.F97214D5.6FEB4876@go2.pl" class=""
width="493" height="264"></span><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif""
lang="FR">De :</span></b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif""
lang="FR"> Didrik Madheden [<a class="moz-txt-link-freetext" href="mailto:nitro2k01@gmail.com">mailto:nitro2k01@gmail.com</a>] <br>
<b>Envoyé :</b> 9 octobre 2020 10:59<br>
<b>À :</b> Jean-Pierre Desrochers<br>
<b>Cc :</b> SDIY List<br>
<b>Objet :</b> Re: [sdiy] Capacitor type matters in 74HCxx
one-shot circuit ?<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">The two parameters that could matter
are frequency response (self-inductance) and leakage
current. <o:p></o:p></p>
<div>
<p class="MsoNormal">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.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">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.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">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.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">/Didrik<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">On Fri, 9 Oct 2020 at 16:42,
Jean-Pierre Desrochers <<a
href="mailto:jpdesroc@oricom.ca"
moz-do-not-send="true">jpdesroc@oricom.ca</a>>
wrote:<o:p></o:p></p>
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<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
lang="EN-CA">Does the capacitor type (Ct: ceramic
or film) matter in a circuit like this one ?</span><o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
lang="EN-CA">I’d say film is obviously better
but.. in such a circuit… ??</span><o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
lang="EN-CA"> </span><o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
lang="EN-CA"> </span><o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><img
id="gmail-m_-8135044707607254865Image_x0020_1"
src="cid:part3.B4DAA2D5.84E8914D@go2.pl" class=""
width="506" height="230" border="0"><o:p></o:p></p>
</div>
</div>
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<o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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