6502.org

I suppose the first thing to say is that all advice, if sincerely given, reflects the habits and experience of the person giving it. My feeling is that distraction is a potential problem, which is one reason to try to apply some discipline to learning how to do something. Another reason to apply discipline is to build knowledge and understanding in stages. Trying to learn everything all at once seems to me like it will make it more difficult. Iteration and repetition helps, too: aiming to learn everything from a single ambitious project seems to me very... ambitious. A series of projects, of increasing sophistication and difficulty, seems a better idea.

As for limiting yourself to 50 things a year, I'm on record as estimating that I manage about one productive weekend a year. People can be wildly different, but I'd be impressed to see 12 good projects a year from anyone.

But, as they say these days, you do you, and hopefully you'll get somewhere you want to get to, and also enjoy the journey.

I guess if you want to think of electrical potential like gravity, it’s maybe worth looking at as a function of position. I think https://www.youtube.com/watch?v=j3GrOKre__0 does a good job of advocating this point of view. I guess we have different definitions or we disagree on both of these points. To me a “signal” has a clear definition as the measure of voltage or current over time. You can see a signal on your oscilloscope; you can “amplify” it with a transistor; you can send it through space with an antenna. The bits we send from a CPU to a RAM component are signals; in digital we exclusively concern ourselves with “logic signals” and try to define when the signal has to be read to make our imaginary clean world of 1s and 0s, but signals are not an abstraction, they are the underlying voltage levels and they are messy when things aren’t working. Similarly, “voltage” is a measurement of a signal value at a particular point in time between two particular locations; it’s what we plot on our oscilloscopes. And it does take time to move around, because we are essentially measuring the force being applied by an electric field and the electric field builds up in the wires and in the space between them wires at something approaching the speed of light - so it’s moving, albeit very, very quickly. For steady state circuits we can pretend it’s everywhere like you describe but for fast transient circuits — like our computers running GHz speed — we can’t. If you take this view, why don’t the electrons from the positive terminal of a battery flow into the negative terminal if I connect two batteries but don’t close the circuit? If there are “holes” in the negative terminal, and “free electrons” in the positive terminal, they should flow without a circuit…as indeed they do when you charge a comb on your hair and then touch it to a neutral object which is first attracted to the static and later repelled when the charge equalizes. But connecting two batteries in series does not ruin them. I think that’s a bit unfair to Franklin, who was 100 years ahead of electron theory and had to make some arbitrary choices about which particles moved and which were stationary. What’s a bit more surprising to me is that he had the notion that charge moved only in one direction (which I’m not sure is strictly true if we think about ions in solution subjected to a potential).
Actually I think it’s clearer to say that electromagnetic fields propagate quickly. The electrons themselves cause this propagation of the field by moving tiny distances; they do not “zip around” the circuit. In fact, the thing that matters is the distance between the point where the voltage is applied and the point where you’re interested in seeing if the electrons are moving, and what kind of dielectric fills that space. The wires dominate in channeling the fields to the destination, but there could be short cuts that don’t involve the wires and are problematic. Eventualy the answer is ground. But initially, the electrons that are already there can just move around slightly to create the electric potential. This will happen whether there is a circuit or not, I believe. I think the return path is via the path of least inductance but I have a weak understanding of this point. I also don’t think it is the circulation of current that causes the ground levels not to match. Are you sure you are correct in your explanation?
I think for my purposes getting as far as “electrons are tiny magnets that create electric and magnetic fields” will be sufficiently low-level in the stack of turtles, with possibly a tiny splash of relativity to understand how the charges create magnetic forces. I’m OK with doing the math to understand it at this level; but I don’t need to fully grok the new model as long as my partially constructed model can explain the problems I’m having at any given time. Because at the end, I do agree with you: at some point, I want to be building things, not studying

I’m really sorry to lose your insight and maybe at some time point you’ll change your mind and chime in again.

I’ll reiterate that I do not think I amt resistant to learning the theory. I just want to focus my learning on deeply understanding theory that is in my way at any given moment, rather than gaining a surface understanding of a broad swath of ideas that will enable me to build faster but without true understanding of what the causes the issues that I get to avoid by following best practice without understanding why it is the best practice.

Sorry for not being clear! I'll try to distill my question(s) into true/false questions so that they're easier to answer both in terms of time required and in terms of figuring out what I'm trying to ask. My goal in asking these questions is to confirm my understanding or shine a light on my misunderstandings, not to put people on the forum to the trouble of writing tomes in an attempt to educate me (heck, in your case you've already written and published your tome, it's totally unreasonable to expect more!). I do appreciate people's attempts to explain, too, of course, but I know it's very time consuming: please feel free to provide one-word true/false answers (or no answer at all).

In any event the questions from that post in true/false form are:

1. True or false: The electric field represents the force applied to charges in space, but it takes time to set up the electric field surrounding any conductor, and the time required depends on the “inductance”.
2. True or false: This setup time exists whether or not the circuit is open or closed
3. True or false: The electric field charges the wire to the potential that we’ve applied if the circuit is open
4. True or false: The electric field causes current to flow if the circuit is closed

If the answer to any of the above is "false", I need to go back and re-review the content that makes me believe these things to identify whether I have misunderstood it or if the content is wrong.
Great, this would appear to correspond with my question 3 and imply that it is true, validating my understanding.
This appears to indicate I have a misunderstanding. I thought that when we connect a voltage source to an open wire, the electric and magnetic fields would get established over time. Then if we remove the voltage, the magnetic field would collapse. However based on what you're saying now I think that the magnetic field collapses once the fields settle down: that is, we connect voltage to the wire, the electric and magnetic fields get set up down the length of the wire at for the sake of argument 0.6c, bounce around a little and then settle to a 0 current situation where the electric potential is now available at all points on the wire but there is no longer a magnetic field. When the circuit is closed and current flows, the magnetic field will set up along the length of the wire. It seems to me that this setup must occur "backwards", that is the voltage source on a long wire sets up an electric field that propagates from the voltage source along the length of the wire to its open end, but when we close the circuit the magnetic field will propagate from the open end back to the voltage source as some of the free electrons shuffle toward the end of the wire.

[True or false?]: As a separate matter, it is clear to me that a capacitor acts like a short circuit turning into an ever stronger resistor causing current to flow while its electric field gets set up, while an inductor (any wire or coil) acts as a open circuit turning into an ever less resistant resistor while its electric field gets set up. That'd better be true or I really don't understand
Fill the water pipe with bucky balls? ... indeed, I have completely abandoned the water pipe way of thinking about it as misleading and wrong.
I'll have to review my data sheets and maybe be more careful about not using a mishmash of whatever parts I have in my parts drawers.
This seems to correspond to what I was trying to say/ask of Paganini, that it's the path of least inductance not the past of least resistance that guides the current on the return path. Or maybe another way to say it is that as the fields in the ground plane need time to get set up, the field will set up closest to the signal trace first and so current will flow right underneath the signal trace initially, perhaps spreading into a wider and wider flow over time, but if the current flow is short lived fundamentally this should result in the current flowing the same path as the trace because there is no time for the field to spread out over the ground plane, and this is not the path of least resistance, which could be directly across some shorter straight line in the ground plane, but a path determined by the mutual inductance ... which is why I questioned Paganini's point.

In case it's at all helpful, when I used to maintain a hydraulic model in my head, I would imagine a balloon attached to the pipe for a capacitor to ground. Or, a stretchy rubber membrane halving a hollow sphere, for a capacitor which joins two signals.

For inductance, imagine some kind of turbine or water wheel in the flow, perhaps with a flywheel attached. It will resist changes in flow, and store energy in its rotation, and deliver it back. For inductance between signals, couple two turbines to the same flywheel.

I don't really bother with a hydraulic model any more. And, although I do have a maths degree, I've never used equations beyond Ohm's Law and Kirchoff's Law. Never studied or memorised Maxwell's. I have sometimes considered graphs - for example, for transistor characteristics.

Although my approach to the universe in general is to apply physics, when it comes to hobby electronics, I think in terms of simple switch-level models, and with rules of thumb otherwise. (See the topic "Techniques for reliable high-speed digital circuits" for my amateur take.)

The one thing I picked up early on from formal training is the idea of single-clock synchronous design, whereby the clock is the only signal where edges matter. And the time immediately before and after the active clock edge is the only time which matters. This is very powerful, has served me well, and suits HDL design styles very well too.

Well, what you say here is true, so far as it goes. But a signal is like a number: it's an *idea.* A signal has no physical reality itself: it's a way of thinking about a physical reality that lets us do something useful.
This is wrong, though. Voltage is simply a measure of potential between two points. There is voltage between your feet and your carpet, and in the wires going to your toaster. No signals there! What you see on your oscilloscope is whatever you set it up to measure. You could measure frequency (think radio) and still be looking at a signal. Or you could measure current. I'm under the impression that old logic families represented signals with current, not voltage, but I am not strong on that.
They might! But remember, we don't know anything about how many free electrons (or electron holes) are at the negative (positive) terminal of the batteries. All we know is that, when comparing each battery's *own terminals* there is a 9V potential between them. When you put two batteries together as you describe, there might be a *brief* current flow, like charging a capacitor.
They do, in fact. Electrons move very very quickly. How quickly depends on a lot of things (some of them already mentioned, like the size of the wire and what it is made of). I think what you're referring to is that an individual electron does not travel a very long distance. For the "last" electron in the circuit to move into the place of the "first" electron, all the electrons in between have to ripple one "step," like marbles in a pipe, which happens almost instantly, while the individual electrons displace almost no distance. (There's a cool video where someone actually made a "marbles in a pipe" circuit to demonstrate the principle, but I'm having trouble finding it!) So, our signals travel around at nearly the speed of light, the electrons themselves move at millions of miles per hour, but are only displaced in space a few inches or yards.
Good point; I meant to include length in my list. It is maybe not *the* thing that matters, bit is *a* thing that matters. In the Primer, Garth reiterates over and over that a good practice to avoid many problems is to keep your wires short!
Yes, but maybe I could have stated it better. Electrons do take the path of least resistance; this is basic electricity. In DC circuits there is no reactance to worry about. However, as I described, when our ICs are switching very quickly they become AC circuits, and the return path is the path of least *impedance.* Impedance is the AC equivalent of resistance. A ways back you expressed confusion about impedance at the ground connection of a signal originator. The point I was trying to get across, badly I guess, is that your ground return network needs to be thoughtfully designed because if the connection between ground pins of the sender and the receiver is not the path of least impedance, the signal return path will find other ways to go, including through intervening ICs.


Do you know about the website "All About Circuits?" There is a kind of (free!) online textbook there that covers the basics of AC, DC, and explains terms like resistance, inductance, reactance, and impedance. It's what I used when I first got started in the hobby. It was enough to help me establish some "rules of thumb" like Ed describes so that I have some stable hobby boards (and some not so stable ones! )

I tried to take a look but was a bit put off by the home page. Is there a direct link to the online textbook you mean?

Hmm. I replied to this thread yesterday morning, but the toobz seem to have eaten it! Anyway, here (again) is a direct link to the All About Circuits online textbook: https://www.allaboutcircuits.com/textbook/

And here is a link to an article that seems pertinent to this thread: https://www.allaboutcircuits.com/techni ... impedance/

Also, I remember a video someone posted a while back in which an experimenter had some kind of scanning device pointed at a PCB so you could actually see the current return path. They demonstrated a variety of situations, such as what the current return path does when it encounters a void in the ground plane. It was very cool. I tried googling around for it with no luck. Does anyone remember better than I do where that video came from?

I think Dave Jones has done this - maybe this video, where he traces a Gigatron built two ways:
EEVblog #1176 - 2 Layer vs 4 Layer PCB EMC TESTED!
Edit: Amusingly, it's excerpted here, as Dave himself was searching for it in his own content.

Hi Garth! I am curious about this one. We tend to place more emphasis on the quality of the ground return network than on Vcc. Why is that? I was reading Dr. Howard Johnson the other day, and came across this: This suggests to me that the Vcc network should be just as important as the GND network. Why isn't it?

I had the same thought - my answer (to myself) is that the nature of inputs is that they are referenced to ground: how large the voltage is, whether or not it's over some threshold, whether it crosses some threshold.

It might well be possible to make circuits which detect signal vs power rail, but we rarely see those.