help reading schematics

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.

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John Popelish

Thanks, John. I'll take that algorithmic approach :)

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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.

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John Popelish

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.)

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

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

Just some more thoughts to consider as you go.

Jon

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

Mike

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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

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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.

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

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John Popelish

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gist..

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 =^.^=

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

Colin, Jon, Terry -- Thanks.

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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!

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Terry Pinnell
Hobbyist, West Sussex, UK

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

Jon