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Question for the experienced folks

Discussion in 'Electronic Basics' started by zalzon, Jan 3, 2004.

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  1. zalzon

    zalzon Guest

    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.

    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.

    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?

    Thanks
     
  2. I dunno about everybody else, but I concentrate on the
    signals. Electrons just carry charge, and how much goes
    where, when, is what usually matters.
    The ends; load first to see what it does, then inputs
    (signal, power, and ground) to see what it has to work with.
    Nope. Break it into blocks (amplifiers/whatever or logic
    gates as appropriate) and see what happens to the signals
    between input and output.
    Learn to see the blocks at a glance. Some draftsmen seem
    to do their best to make it difficult to identify a given
    block by routing lines all over the place to make another
    block more obvious.
    I'm old-fashioned (electron flow), but only about
    power-supply-related stuff and toobs. It really doesn't
    matter as long as you stay consistent.

    Mark L. Fergerson
     
  3. I think I generally try to see where the _signal_ is going. For an
    overall view, I don't really care where the electrons (or mythical
    positive charge carriers) are going unless I have to get into the gory
    details of the operation of an individual stage.
    I think I tend mostly to use conventional (positive) current.
    Negative (electron) current was invented as an aid to teach
    technicians how vacuum tubes work, since you can't really explain the
    inner workings of a vacuum tube using conventional current, but it
    isn't needed with semiconductors (it isn't really needed with tube
    circuits either, unless you are trying to explain the inner workings
    of the tube (particularly cathode ray tubes!!)).
     
  4. I think some (or a lot) of what you are experiencing is because you
    are knew to it. You seem to be analyzing each little piece, because
    it is all new, but once you've become more familiar with it all you won't
    be bothering.

    Don't ask me how to get to the point, because it's been so long, and I
    suspect it just grew organically. But you basically look at a schematic
    and picked out the familiar. There's the input, there's the output, there's
    the power bus. Oh, I recognize that IC, it does this..., so that gives
    a clue about what the circuit is doing. That transistor over there is
    a low power type, while that one is a power transistor. You know these
    things because you've looked them up in the past, and the details stick
    with you. You've understood what little parts of the circuit are doing
    because you've looked at them in depth in the past, but likely in
    the form a a simple schematic that only covers that one stage.

    Often you will not need to analyze the whole thing, because much of
    it will be familiar. But if none of it is familiar, then a large schematic
    is of course very daunting.

    And of course, a lot of us grew up with magazine articles (and books
    for that matter) where there was a description, sometimes detailed sometimes
    not, of what was going on. In the internet age, I suspect that may no longer
    be the case, with many people extracting schematics from such articles
    and simply posted the schematic alone.

    Michael
     
  5. Divide and conquer..
    first identify where the power comes from..
    Isolate that systema and remember without proper power and biasing you are
    dead..
    second find the ouput and first input..
    then try to imagine how a signal gets thru or how it is generated...
    the rest is support and glitter...
    you might spend some time studying basic modules such as amplifiers,
    oscilllators etc..
    good luck
    hank wd5jfr
     
  6. Chan

    Chan Guest

    The easiest way is probably to take up a 3 year university course
    and study EE properly. Then, when you leave you will still not
    know how to look at a large schematic, but at least you will have
    the underlying knowledge, and you will
    be able to start gaining real experience. Otherwise you may find
    you are spending your time wondering how things work, without
    realising the approach to solve these puzzles.

    The proper way is the easiest way in the long run.
     
  7. I'm not an expert. However, I've been getting much better at this in the
    last few months.

    What I've found is that most circuits are built out of 'standard'
    subcircuits. After analyzing some of these, you get to know these standard
    subcircuits, and at that point, you begin to see the circuit at a higher
    level of abstraction.

    There are exceptions to this approach, but its a pretty good starting point.

    For example, there was a circuit posted on a.b.s.e. recently as a 'teaching
    exercise' by Jim Thompson. Unless you recognize the common collector
    amplifiers and the current mirror, you have no place to start. Trying to
    understand this by analyzing the transistors with ebers-moll is pretty
    difficult. However, by looking at the different amplifiers, you can get some
    sense of what its doing.

    I think a basic text on different subcircuits, like the first 3 or 4
    chapters of "The Art of Electronics", for example, gives you the ability to
    recognize these standard transistor and opamp subcircuits.

    Regards,
    Bob Monsen
     
  8. Dave

    Dave Guest

    OT question: What is a.b.s.e? Sounds like ng I need to check out...

    Thanks,

    Dave
     
  9.  
  10. Nells

    Nells Guest

    ha........uahhhhh Well that means you need to know what compnents do and
    certain configurations are noticable if you ever had any electronics
    theory, it takes years to learn from an aspect of disecting a schematic
    diagram.................i wish i could help, but their are far too many
    areas to cover with you. There are logical approaches however you still
    have understand your instruments, and waveforms etc........

    Nells
     
  11. (snip)

    I break the circuit down into functions that I can briefly describe
    and mentally try to keep the functions I have figured out separate
    from what I am still working on. It helps if some conventions have
    been followed in drawing the schematic, and if they have not been
    followed, it sometimes helps if I redraw sections that are giving me
    problems. My favorite conventions are that positive supply voltages
    are above more negative supply voltages, so I can picture the positive
    convention current sort of falling through the circuits. The second
    convention is that forward signals pass through the circuit from left
    to right, and feedback signals from outputs back to inputs passes
    right to left. Of course, there are exceptions that do not easily
    fall into these layout standards.

    The other big one is to know the action of each of the components
    cold, treating each like a black box. For instance, you should be
    able to predict how a transistor will react to any sort of bias,
    regardless which leads appear to be inputs and which appear to be
    outputs. Mathematical rigor is not nearly as useful as a general idea
    about what happens to one lead if you know something about what is
    going on at the other leads.

    I start where ever the concept of a signal is most obvious. For
    amplifiers, this might be the input or the output. For oscillators,
    the output is usually the obvious signal, and I work my way back into
    the circuit, assuming it is producing the expected signal, and figure
    out how that outcome i being produced. Microprocessors often do not
    make a lot of sense unless you have some idea of the program. with
    various pins being able to be analog inputs, digital inputs, digital
    outputs, and some other functions. A lot can be deduced by what is
    connected to the pins, but when someone has been especially efficient
    at minimizing hardware, it can be a puzzle without looking at the
    code.
     
  12. zalzon

    zalzon Guest

    Thanks all for your insight. The common theme seems to be it comes
    with experience and knowledge of circuits commonly used as building
    blocks.
     
  13. Wayne

    Wayne Guest

    If your really lucky you can find a detailed circuit description to go along
    with the schematic.

    This is what I loved about magazines like Popular Electronics, Nuts &
    Volts... etc.
     
  14. Wayne

    Wayne Guest

    If you can identify a main component, you can search the net for a data
    sheet which
    can give you detailed info and sample circuits.
     
  15. Mantra

    Mantra Guest

    This is exactly correct. These are what Architects and Programmers
    call "Design Patterns". Very little about any given circuit are
    actually terribly unique.

    1. Signals *usually* flow left to right. Power (positive to negative)
    *usually* flows top to bottom.

    2. High level signal processing is often done the same way in every
    circuit of a certain type (e.g. radio receivers, op amps, etc. have a
    predictably sequence of stages)

    3. There are a finite number of ways to "do" amplifiers, oscillators,
    etc. so the transistor level implementation is recognizable which
    helps to identify high level stages. E.g. for amplifiers you have CC,
    CE, CB and variant on those like cascade, diff amp and neut pairs.
    Learn these rules of thumb.

    4. Component-level elements (resistors, capacitors, transistors,
    diodes) have particular characteristics that have highly predictable
    uses. Capacitors always suggest blocking DC and passing AC with the
    intent to preserve or eliminate one or the other. Transistor emitters
    are always low impedance and are often used in places where having low
    impedance has some advantage; the converse for collectors. More rules
    of thumb.

    5. Thevenin and Norton theorems often seem ridiculously simplistic and
    useless to new engineers but they are worth their weight in gold. If
    you can assume linearity, the entire universe can be reduced to a
    single ideal source and impedance - sort of the EE equivalent to
    Archimedes' lever; he needed a fulcrum, we need linearity. Collapsing
    stages in you mind to a Thevenin or Norton can often help "seeing" the
    circuit - consider the transistor terminal impedance rules of thumb
    above in a multistage amplifier.

    5. Train youself to be able to write loop and mesh equations "on
    inspection". This means being able to look at something like a
    transistor output circuit and write Vcc-IcRc-Vce-(Ic+Ib)Re=0 without
    labeling currents or polarities on a piece of paper - just write out
    the equations as you trace the loop or mesh with you figure. If you
    can do this, you won't get bogged down with analysis mechanics. Sort
    of like when you can finally play piano without "thinking" about which
    keys to hit.

    6. Remember that the human brain has difficulties with intuition of
    anything that isn't linear. The math goes from conventiently simply
    to virtually impossible when you go linear to nonlinear. Even master
    analog circuit designers do nonlinear transistor design with the
    simplest models in linear form. That's the whole bit about Q-points,
    linearization and biasing. SPICE is for when you need to know what
    the nonlinear effects *actually* are. Up to that point, between you,
    me and the mouse in my pocket, we'll just assume the entire world is
    linear.
     
  16. fpd

    fpd Guest

    I feel Mr. Black's pain. I, too, look at schematics and think "Eeesh" as I
    try to trace each possible path of the electons/holes. I have the same
    problem as he does of not recognizing "chunks" of the whole picture. I've
    learned a little more by reading Nuts and Volts (same name as their web
    site), but they quickly get over my head... funny, same is happening with
    this news group... "Basics" to me is different from what most of you folks
    are posting. Am I on the wrong news group? It would be cool if the
    benevolent of this group would occasionally post a flash-card sized "chunk"
    of a circuit. This would be good for the Intermediates out there - "they"
    say the best way to learn a subject is to try to teach it. With time,
    novices like I will have a nice cache of flash cards, and hopefully some
    "chunk" vision like the big boys. Thanks, and good luck Mr. Black.
     
  17. But haven't you found, like me, that there are many sources of that
    sort of 'compartmentalised explanation' on web sites? And posts to
    this and sci.electronics.design and alt.binaries.schematics.electronic
    are sometimes accompanied by schematics with explanations.

    For example, my own posts (whether in enquiring or helping mode), are
    often complemented by schematics posted to my electronics web page at
    http://dspace.dial.pipex.com/terrypin/indexpersonal.html. Or to
    alt.binaries.schematics.electronic. Of the 300 or so files in that
    list (which happens to be fairly up to date at the moment), quite a
    few are schematics. And a fair proportion of those are accompanied by
    notes and simulations or actual output waveforms showing how I think
    they work. Maybe that resource could help you? At present the files
    are not categorised, but most filenames are reasonably meaningful, so
    selective browsing could be possible. Alternatively, specify a topic
    and I'll suggest any potentially relevant web files.

    Also, posted circuits are often analysed in depth by the real experts
    here. A shining example in this news group is John Popelish. A
    structured collection of his thorough and lucid explanations over
    recent years, with associated schematics, could constitute an
    electronics tutorial course that would be hard to improve on.
     
  18. fpd

    fpd Guest

    Yes, these groups do help, but the pure quantity is hard to sift. Also, I
    have no idea who are the experts and who are the posers. I read a little
    (books) and then surf a little, in hopes I a light bulb turns on. I still
    find it hard to understand why a Basics news group is far beyond basic.
    However; that I must accept.
     
  19. Have you posted any questions about electronics basics? I don't
    remember seeing you here. I don't think there is any question about
    electronics that is too basic to ask, here.
     
  20. 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.

    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.

    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


    (((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
    William J. Beaty SCIENCE HOBBYIST website
    http://amasci.com
    EE/programmer/sci-exhibits amateur science, hobby projects, sci fair
    Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci
     
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