I have now read enough applied circuit diagrams that I believe I'm missing something that is obvious to those skilled in the art: Where an abstract diagram has a few components, a complete wiring diagram of the same circuit is absolutely littered with capacitors in the microfarad range.

(To do: Find and insert some example images here.)

I can sort of understand how these capacitors would smooth out voltage drops, (and I guess there are usually a few diodes to deal with voltage spikes)?

But how do engineers come up with these applied circuits? Are there unwritten rules they just know, like, "Yep, gotta put a microfarad on this side of a diode, that side of a transistor, and three of them in parallel by this sort of switch..."? Or do they wire up a prototype, start scoping every junction, and add them based on actual signal noise they observe? Or is more prophylactic, e.g., "I wouldn't want a lot of voltage noise here, so why not add some capacitance just to be sure?"

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    \$\begingroup\$ Does What is a decoupling capacitor and how do I know if I need one? answer your question? \$\endgroup\$ – Andrew Morton May 14 '16 at 21:08
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    \$\begingroup\$ @AndrewMorton - Wow, that is very illuminating. It might be the whole answer. I see, "Put bypass caps everywhere if space and cost allow," "on every power input" (though why power supplies don't provide adequate capacitance isn't addressed), and "decoupling caps as specified by ICs." Is that all accurate and complete? \$\endgroup\$ – feetwet May 14 '16 at 21:18
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    \$\begingroup\$ The power supply cannot compensate for the induction of the current path between it and the component. The induction acts to slow down the recovery of the voltage to its required value. A capacitor immediately next to the component has less inductance in the path and so can reduce the change in voltage at a higher rate. (As far as I understand the operation.) \$\endgroup\$ – Andrew Morton May 14 '16 at 21:33
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    \$\begingroup\$ Capacitors at the supply are not adequate because there is trace inductance and resistance between the supply and the load. \$\endgroup\$ – The Photon May 14 '16 at 21:37
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    \$\begingroup\$ Yes, in most cases it's not even a thumb rules, just a habit. Actually, not many people bother with the question "why". By the way, same happens even where calculation may be used, like in switching supplies. Then you ask the designer, why the capacitor is so big, can't he take a cheaper one, amd he says "no idea, i always use 100uF". We all hope that IC designers recommend decoupling because they know something we don't. But frankly, i lost my trust in them too. Their recommendations for hard components like FPGA are barely realistic, so it always looks they just escape responsibility. \$\endgroup\$ – Gregory Kornblum May 14 '16 at 21:47

In my experience, bypass caps are required next to modules that can switch state,(flip-flops, memory, etc.). If you don't do it, your circuit very likely will experience erratic behavior, and even failure. This is a lesson I learned on my own when my designs were working erratically and were highly susceptible to electrical noise. After that, I paid more attention to, and followed the recommendations of the module designers.
Then there is the matter of the project schedule. Usually they want the circuit/design working "yesterday." So, you have to decide, do you want to spend time determining where you need bypass caps or making/improving your design? If you take the recommendations of others, then you don't have to spend time on that.
If you work for a large company, other people will verify and test your design - which is good. If not, then you will have to do it all yourself, leaving the user exposed to any mistakes you might have made. An example of this, that comes to mind, is the Hover-board. There is a major design flaw with it, which was not caught (or ignored), before releasing the design to production.
So, yes, learning from the mistakes and experience of others, is part of becoming a better designer yourself.


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