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help reading schematics

Discussion in 'Electronic Basics' started by Mike, Oct 7, 2004.

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  1. Mike

    Mike Guest

    Hello,

    I'm relatively new to the field of electronics, and without a doubt I feel
    lost when reading a large schematic. I understand that being able to read a
    schematic requires thorough understanding of electronic theory, math, etc,
    but is there any type of cheat sheet or tips on where to start? Obviously,
    first step is determining power and polarity, but I think you get the gist..

    Thanks in advance!
    Mike
     
  2. The biggest help in reading a schematic is to be able to break the
    system down into smaller and smaller sections, till you have fairly
    simple functions to figure out. Of course, experience with a wide
    range of circuits is what makes this parsing possible. I suggest you
    take a fairly complex schematic, copy it and start circling
    recognizable functions with colored high lighters to see how much of
    it you can recognize.
     
  3. Mike

    Mike Guest

    Thanks, John. I'll take that algorithmic approach :)
     
  4. If you need help with a specific schematic and can post a graphic of
    it somewhere and link to it or post the graphic on
    alt.binaries.schematics.electronic
    we can talk about it and how we would pick it apart.
     
  5. John's point is exactly right.

    But, of course, you also need to have at least some of the building blocks in
    mind to do that, too. Once you pick up a few of them, you start seeing them in
    places or you start seeing them slightly modified.

    One thing that really threw me off when I was _first_ struggling (I still
    struggle a lot as I'm just a hobbyist on this stuff) was that the published
    schematics were poorly drawn for understanding. Lots of them, instead, were
    drawn for helping you figure out the wiring as you soldered or connected the
    parts. Or, sometimes it seemed, they were drawn to just make it really hard to
    figure them out.

    A class I took on electronic drafting at Tektronix in Beaverton really drilled
    in a set of very simple ideas to help me unwind the Gordian Knot of a poorly
    drawn schematic (the teacher kept throwing bad ones at us and making us redraw
    them sensibly.) The idea was to have electron flow run like an upside down
    waterfall from the bottom of the page to the top and to have signal flow go from
    left to right. (The top edge is positive, bottom edge is negative, left edge is
    where signals come in, and the right edge is where signals go out.) Also, the
    teacher pointed out to NOT "bus" power rails around -- he said that all those
    extra wires do is to distract you from the meaning. Yes, in a physical sense
    those conductors will be needed when the circuit is built. But no, they do not
    help you understand the circuit -- in fact, they tend to confuse you rather than
    help. (This last rule isn't always exactly right, because there are a few times
    in real circuits where it is IMPORTANT to show those lines explicitly -- but
    that's an exception, not a rule.)

    Now, keep in mind that there was NO prerequisite for this class that students
    knew anything about electronic circuits. It was a drafting class and many of
    them had only a glancing idea about it. So it's not like we were experts in
    anything. We weren't. But just following these rules on schematics I found
    that I was *much* better able to sit down and fathom their meaning.

    Here's an example of a schematic that is VERY BADLY drawn. It works just fine,
    though, and the wiring in the schematic is right. (What happens is... when you
    press the BUTTON for a short time, it connects RLOAD to the battery and starts
    the circuit so that it will keep the battery connected to RLOAD for some
    designed-in time even after you release the button.)

    http://users.easystreet.com/jkirwan/TimedHoldBAD.png

    Now, take a look at the same circuit redrawn according to the above rules:

    http://users.easystreet.com/jkirwan/TimedHoldOK.png

    Exact same circuit, works the same, etc. See the difference?

    Just some more thoughts to consider as you go.

    Jon
     
  6. Mike

    Mike Guest

    Thanks, John. Perhaps I can find some threads in
    alt.binaries.schematics.electronic where someone (maybe even yourself) has
    pulled apart schematics.

    Mike
     
  7. Mike

    Mike Guest

    That's a pretty clever approach to dissecting diagrams. I have been doing
    computer programming for eight years, and surenly that first diagram is
    analogous to "spaghetti" code!

    Thanks a lot for the in-depth explanation!

    Mike
     
  8. Here is an application note that picks an opamp apart into functional
    pieces to explain how the whole thing works. This is an example of
    what I am suggesting you try to do with other schematics.

    http://www.national.com/an/AN/AN-A.pdf

    Of course, the more different functional pieces you get familiar with,
    the easier this gets.
     
  9. colin

    colin Guest

    Sometimes it helps to re draw the circuit for yourself. Pick out simple
    bits that you know like regulator circuits, op amp circuits etc.. and re
    draw them as blocks so you can see how they interconect, then try figure out
    bits that are left, its not always clear what the function of various
    circuit paths are for even to the experienced engineer. at first sight the
    multiple feedback op amp filter for example is hard to figure out until you
    look at an explanation.

    Colin =^.^=
     
  10. That was my recommendation, as well, unless it's already drawn well.
    Yes. But I also realize there is a point in time where one knows almost nothing
    at all about various subcircuit sections and cannot therefore have much idea of
    which circuit a resistor or capacitor is part of, for example. What really
    surprised me and worked very well for me at that point in time was simply
    following the drafting rules I mentioned earlier. If I redrew a schematic that
    way, I found that my meager knowledge about individual components was more
    easily applied to gain an abstract idea about sections I otherwise couldn't
    fathom.

    For example, in this controlled current drive for an LED:

    +9
    |
    |
    ---
    \ / LED1
    ---
    |
    |
    |
    |/c Q1
    +CTRL------------|
    |>e
    |
    -CTRL--, |
    | |
    gnd \
    / R1
    \
    /
    |
    |
    gnd

    is much more amenable in my mind to figuring out what it does and how it does it
    (if you don't already recognize the idiom), than is this:

    ,-----c e--------,
    | \ ^ |
    | --- Q1 |
    | | |
    | | |
    | +CTRL |
    | |
    --- |
    D1 / \ |
    --- \
    | R1 /
    | \
    | /
    | |
    | |
    | -CTRL----+
    | |
    | 9V |
    | | | |
    '----------||||-----'
    | |

    They are equivalent. But what a difference it makes to get rid of the battery
    completely, arrange for the electron flow from bottom to top, signal input from
    CTRL to come from the left and the LED (which is the output resulting from the
    input) on the right side, and rid the schematic of power busing lines (which
    only distract the eye and contribute no understanding.)

    Once I started redrawing using these simple rules, not worrying initially about
    organizing into functional units (which I didn't really know that much about),
    then I found that the functional groupings almost stood out by themselves. When
    you undo the initial knots and unkink the schematic, suddenly the actual clumps
    become much more obvious because you've removed a lot of the schematic "noise."
    I would then often redraw it one more time, this time with an eye to breaking it
    along "apparent function" (this is pretty easy to see, even when generally
    ignorant, because there are few signal wires connecting distinct functions and
    there are lots of signal wires connecting bits of a single function.)

    Jon
     
  11. For me, it's more intuitive to say 'current flow from top to bottom'.
    Strictly technically inaccurate, perhaps, but easier to follow, if
    that is indeed your aim!
     
  12. Mike

    Mike Guest

    Colin, Jon, Terry -- Thanks.

    Mike
     
  13. Understood and that's probably a good way to say it! I was just trying to be
    more "physically minded," I suppose.

    Jon
     
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