When you guys first look at a schematic, how do you analyse it?
I mean how do you break it down. I sort of try to trace the path of
electrons which is fine for small circuits. But sometimes in a medium
sized schematic, I get overwhelmed trying to figure out where the
electrons are going and what's going on.
I started out as a beginner by tracing electrons. After some years of
schooling I abandoned this, since that's not how circuits actually work.
In the education business, tracing electrons is called "the sequential
fallacy." Just try and figure out the brightness of several light bulbs
and switches connected in series/parallel networks and you'll rapidly find
that tracing the electrons doesn't tell you much. When the circuit breaks
into several "Y" branches, how can you know the value of current in each
branch?
Circuits instead work by Ohm's law; by Voltage Nodes and Current Loops.
If you know the voltage between the ends of a component, then you can
figure out the current inside that component. And if you know the current
inside a component, then you can figure out the voltage-drop across that
component. After lots of practice, thinking in terms of Ohm's Law becomes
automatic, and you start to be able to "see" the operation of circuits.
On the other hand, to figure out the general function of parts of a large
schematic, you simply need some experience in recognizing particular kinds
of simple circuits. For example, you might look at one handful of
transistors and see that it's a voltage regulator, so it must be and part
of the power supply on that board, while another bunch of transistors
forms a square wave oscillator, while another is an AC power amplifier.
You can draw squares around different parts of the schematic to separate
it into functional blocks.
Unfortunately beginners will have a terrible time doing this because they
lack experience in recognizing the standard kinds of circuits everyone
uses. Schematics with lots of ICs helps, since the ICs usually act as
functional blocks in the first place.
Where or what do you guys first look at when you look at a schematic.
How do you guys figure out what's going on. Are you guys "chasing"
the path of electrons as I do?
Is this something that comes only through experience or are there some
tips I could use.
Build lots of simple circuits such as "common emitter power amp" or
"Hartley Sineway Oscillator" or "threshold detector." Play with LM555
oscillators and Op amp chips and Voltage Regulators. That way you get to
know them intimately, and can recognize them by their chip numbers.
Complicated schematics are made of building blocks, and once you learn to
recognize the building blocks instantly by eye, you can break the
complexity down into a "block diagram" in your mind.
Another thing is I am used to thinking of current as flowing from the
negative to the positive. That is, I envision the electrons going up
from the 'ground' or negative to the positive terminal of the battery.
Conventional current has it the opposite way. Which way do you guys
use when you look at schematics?
Since electrons flow in some components, while positive charges flow
within others (batteries, gas discharge, electroplating, "proton
conductors" in fuel cells, etc.,) it's not correct to assume that
"electric current" means "electron flow." To be accurate, electric
current is ANY type of flowing charges. But that makes things hard to
deal with.
Also, knowing the polarity of the flowing charges is only critical if
you're under the sway of the Sequential Fallacy and performing "electron
tracing" I mention above. Think instead in terms of circuits being like
drive belts, with all circuit loops containing a belt inside the wires,
then the polarity of charges isn't so important. Circuits aren't like
hollow tubes with bullets flying through. They're more like bicycle
chains. Push a belt along, and the entire belt starts flowing.
See:
Which way does the electricity really flow?
http://amasci.com/amateur/elecdir.html
Franklin and positive electrons: he wasn't wrong
http://amasci.com/miscon/eleca.html#frkel
The advanced professionals (engineers and physicists) use a well known
method of simplifying this situation. They ignore what actually happens
inside the components and instead assume that all charges always have the
same polarity: an "unsigned" polarity, the positive one. It's mostly a
math convention: if a particular numeral doesn't have a polarity sign, we
assume that it's positive. Also, ammeters and resistors (Ohm's law) deal
in positive current, so if we avoid thinking in terms of flowing
negatives, then we avoid having to perform a "double negative" in our
minds each time we want to visualize things clearly.
And then, if the ACTUAL polarity of the flowing charges is important for
some reason, we can abandon the flow of "conventional" positive charges
and think in terms of several kinds of flowing charge. For example, in
liquid conductors there are two types of charge flows, postive charges and
negative charges flowing past each other in opposite directions. And in
transistors there are four kinds of charge flow, two kinds in the n-doped
parts and two other kinds in the p-doped parts.
You probably know that there's an older standard for charge, the negative
one. This standard evolved during the age of vacuum tubes when we mostly
had to visualize electron clouds in tubes and in wires. This made it easy
to convince ourselves that all currents really were flows of negatives,
and any components which used positive charge flow could be swept under
the carpet. Doing this makes it very easy to understand vacuum tubes, but
harder to understand the innards of semiconductor operation, or the
strange things that go on in the electrolyte between the plates in a
battery. As long as our circuits aren't dominated by vacuum tubes,
there's no big reason to declare the electron flow to be the "convention"
for charge flow.
Also see:
Typical beginners' mistakes in understanding electricity
http://amasci.com/miscon/elect.html
Collected electricity articles
http://amasci.com/ele-edu.html
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William J. Beaty SCIENCE HOBBYIST website
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