<div dir="ltr"><div>Roman,</div><div><br></div><div>I got sidetracked for a bit with holiday guests etc., but I revisited this and I believe all the issues I was having were due to my poor implementation of a single-transistor version. Your version as originally posted seems fine with being shorted to any rail, wire-ORs beautifully, and sags very little under even unrealistically demanding loads. The only downside I've found so far (as I'm fine with the parts count/cost) is that it seems to max out around 12 or 13 KHz (I hope to re-measure tomorrow). If there's a way to increase the speed, that would be great; if not, that may just be the compromise for offering the wire-OR functionality.</div><div><br></div><div>Thanks,</div><div>-Paul</div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Thu, Nov 28, 2024 at 4:31 AM Roman Sowa <<a href="mailto:modular@go2.pl">modular@go2.pl</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Yes, there is a good way to limit the current, now that I know you use <br>
7805 especially for this. Just insert a resistor before 7805. If it's <br>
powered from +12, then 200 ohm resistor from +12 to 7805 input will <br>
provide about 30-40mA current limit.<br>
<br>
BTW, shorting the output to either GND or -12V should not make any <br>
difference in dissipated heat for 5V regulator. If that happens, <br>
probably your base resistor is too low. It's the output transistor that <br>
should take the heat on itself in this circuit.<br>
You didn't even mention accidental short to +12V. Your poor 78L05 would <br>
not approve.<br>
<br>
PNP was used there to make gate output go very closely to 5V rail and <br>
also to provide good ORing capability. You could use NPN working as <br>
follower too, but then you loose 0.7V and walk on a thin ice having <br>
outputs in off state with 5V reversed voltage (from other ORed outputs) <br>
on the emiter.<br>
<br>
Roman<br>
<br>
W dniu 2024-11-26 o 20:34, Paul Glass-Steel pisze:<br>
> Roman,<br>
> <br>
> Thanks for this - I breadboarded it this morning and it does work well. <br>
> I did note that, though the circuit itself doesn't mind being shorted to <br>
> ground or even to the -12V rail, that doing so does heat up the 78L05 I <br>
> was using. Is there a good way to limit the current in fault conditions, <br>
> without compromising the voltage under load too much? Also, I'd be <br>
> interested in the technical reasons behind using PNP here, though the <br>
> inverted logic isn't a problem for me.<br>
> <br>
> Thanks again,<br>
> -Paul<br>
> <br>
> On Tue, Nov 26, 2024 at 4:29 AM Roman Sowa <<a href="mailto:modular@go2.pl" target="_blank">modular@go2.pl</a> <br>
> <mailto:<a href="mailto:modular@go2.pl" target="_blank">modular@go2.pl</a>>> wrote:<br>
> <br>
> I can understand that you want the gate stay at constant level even<br>
> with<br>
> 30 such outputs OR-ed, hence the 1k inside feedback loop. But if you<br>
> don't need well defined current limit as in this case, why not just use<br>
> PNP transistor? Short circuit current will be limited then by base<br>
> current times beta. Just enough not to blow the transistor up in<br>
> case of<br>
> actual shorting outout to GND<br>
> <br>
> See the attachment<br>
> <br>
> And if your circuit offers reversed gate output somewhere anyway, then<br>
> you can even omit T2.<br>
> I know opamps may be cheaper now than transistors, but let's not<br>
> overkill things.<br>
> <br>
> Roman<br>
> <br>
> W dniu 2024-11-25 o 17:42, Chris McDowell via Synth-diy pisze:<br>
> > K one more.<br>
> ><br>
> > What happens here is the more outputs you OR together in this<br>
> way, the<br>
> > lower that 22k looks and the more voltage develops across your 1k<br>
> > resistor, which basically wont ever really matter. I still think<br>
> > tempting your op amp to overshoot or whatever funky thing it<br>
> wants to do<br>
> > with your diode and cable capacitance is unnecessary given that a<br>
> 220R<br>
> > resistor in series with the diode would handle it. or put the<br>
> parallel R<br>
> > and C from output to minus input like you know you're supposed to :P<br>
> ><br>
> > in my personal system, everything has some manner of 100k to<br>
> ground on<br>
> > the inputs and I would leave the 22k out entirely. I guess what I<br>
> would<br>
> > /actually/ do is build an explicit OR module.<br>
> ><br>
> > Cheers,<br>
> > Chris<br>
> ><br>
> >> On Nov 25, 2024, at 11:20 AM, Chris McDowell<br>
> <<a href="mailto:declareupdate@gmail.com" target="_blank">declareupdate@gmail.com</a> <mailto:<a href="mailto:declareupdate@gmail.com" target="_blank">declareupdate@gmail.com</a>>><br>
> >> wrote:<br>
> >><br>
> >> This is like swapping your output resistor with a diode, which<br>
> means<br>
> >> your capacitive load is only, yknow, capacitively loading when the<br>
> >> diode is conducting. I don't think there is a lot of value in<br>
> putting<br>
> >> the output resistor in the feedback loop for gate outputs, as we'd<br>
> >> usually do that to increase DC accuracy at the receiving<br>
> equipment. I<br>
> >> would move your output resistor out of the feedback loop to take<br>
> >> stability out of the conversation. You then have something pretty<br>
> >> normal that will OR with similar outputs, up to some limit where<br>
> all<br>
> >> your 100ks (and whatever all the various input resistances) in<br>
> >> parallel are too low a resistance to get your gates across.<br>
> that's a<br>
> >> lot, though.<br>
> >><br>
> >> Cheers,<br>
> >> Chris<br>
> >><br>
> >>> On Nov 24, 2024, at 12:42 PM, Paul Glass-Steel via Synth-diy<br>
> >>> <<a href="mailto:synth-diy@synth-diy.org" target="_blank">synth-diy@synth-diy.org</a> <mailto:<a href="mailto:synth-diy@synth-diy.org" target="_blank">synth-diy@synth-diy.org</a>>> wrote:<br>
> >>><br>
> >>> <br>
> >>> I'm hoping to improve on the typical transistor emitter-follower,<br>
> >>> wire-OR capable gate output buffer that many of my modules<br>
> >>> have inherited from CGS and NLC. I've come up with this:<br>
> >>> <a href="https://tinyurl.com/259pt83t" rel="noreferrer" target="_blank">https://tinyurl.com/259pt83t</a> <<a href="https://tinyurl.com/259pt83t" rel="noreferrer" target="_blank">https://tinyurl.com/259pt83t</a>><br>
> <<a href="https://tinyurl.com/259pt83t" rel="noreferrer" target="_blank">https://tinyurl.com/259pt83t</a> <<a href="https://tinyurl.com/259pt83t" rel="noreferrer" target="_blank">https://tinyurl.com/259pt83t</a>>> - it<br>
> >>> seems happy on my bench, the voltage is stable under a lot of<br>
> fan-out<br>
> >>> to multiple inputs and it seems ok with long cables / capacitive<br>
> >>> loads. But I thought I'd check here to see if I'm missing<br>
> something.<br>
> >>><br>
> >>> Thanks,<br>
> >>> -Paul<br>
> >>><br>
> >>><br>
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</blockquote></div></div>