Resistor Puzzle Solution... and a rant on schematics

Resistor Puzzle Solution... and a rant on schematics

  • Comments 12

Did you see last week’s resistor puzzle? Check it out if you missed it. Here’s the solution:

We’re not accustomed to reading three-dimensional schematics so the first step is to redraw it clearly. There are three distinct paths from A to B, colored blue, green and red. Each has a series connection of 1Ω—R—1Ω.  The 3Ω resistors are effectively in parallel with the “R” resistors. The symmetry of these 3Ω resistor connections makes their effect the same as if each is directly in parallel with one of the “R” resistors.

The total resistance from A to B is 1Ω, so each of the three legs must be 3Ω. With 1Ω on each end of the legs, the middle parallel combination must also be 1Ω. So R must be 1.5Ω in parallel with 3Ω to make 1Ω.

Was this fun?  Maybe you missed a previous puzzle, the infinite resistor network. More fun.

It’s all so much easier when we have a well-drawn schematic. Huummmm… a well-drawn schematic?

A genius former colleague in my past used to say “circuits work better when the schematic is drawn right.” He didn’t mean “drawn without errors.” He meant that it was easier to understand the circuit when it was drawn well. Nuances are easier to discern, details more easily optimized and problems are more easily resolved. So true!

I’m on my soapbox now! Have pride in your schematics. A well-designed circuit deserves a well-drawn schematic. And a poorly drawn schematic does not inspire confidence in your work.

Take care in laying out your schematic. Signals flow better left-to-right, you know. Currents flow downward. With thoughtful layout, you can minimize confusing crossovers and labeled interconnections that hinder interpretation. If you need multiple pages, make connections clear, preferably so one sheet can lie next to another and connections are obvious. Draw sub-circuits the same way each instance they occur so they are easily recognized.

Use familiar symbols. Op amps are triangles not rectangles—much easier to read this way. When possible, place components in a way that suggests a good circuit board layout. Draw symmetrically if this is desirable in the PCB layout. Label IC part numbers and include all circuit values. Number components so that they can be identified easily in e-mails or phone conversations.

So often, I see schematics without a single word of helpful annotation. A few words of explanation can add so much! A footnote explaining why a certain component is chosen or how a value is calculated can be so valuable to a support engineer a few years later. How about labeling some nominal voltages at key nodes? Show gain values and nominal signal levels. Label major blocks. If you last as long as I, you may be the beneficiary of your own good documentation as you retrace your own steps.

Some thought and empathy will make life easier for those who later must decipher your circuit. Make the quality of your schematics a signature feature of your excellent work. Oh, yes… signature! A valued colleague here at TI insists that you should put your name on your schematic. (Thanks, Jim.) You may find that this simple act will cause you to recheck your work one last time. ;-)

Schematics… what’s your pet peeve? What’s your rant? Leave your comments.

Thanks for reading,

Bruce       email:  thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

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  • You can learn a lot about an analog circuit only by looking at well drawn schematcs

    Sadly, that is not the case for today digital circuits, that usually include huge behemoth with hundreds and even more than a thousand of pins each. But even  for these complex devices, many things can be done to improve the schematics

    Give meaningful names to signals and buses

    Divide the devices in smaller blocks, group functional signals of the same kind on each block (power, memory bus, interfaces, etc.)

    do not heasitate to redraw the symbols if that helps to straight the connections between two symbols

    If possible and if it doesnt affect clarity, prefer real nets to named nets connections

    Avoid using names like Fpga-tx and  fpga-rx. I prefer fpga-to-pci and pci-to-fpga.

  • Today, many schematics are automatically generated from libraries. The software generating them may put everyhting together nicely, and the router will be able to do its work, but the result is lacking human readability.

    However, once more the things said about schematics also apply to software.

    While high-level programming languages give you, and even force you to use structured elements, peopl emanage to mangle their code until nobody understands what's going on. And of course helpful annotations are scarce. (the ones that are present often only state the obvious, or, worse, contradict the code). Too bad that some of the most popular layout programs do not really support free text input. (possible, but uncomfortable to handle)

    Thanks again for some valuable gems of wisdom about hardware design.

  • Bruce...you are ranting "to the choir".  I nag my students endlessly about drawing schematics.  It wastes my time to have to figure out what each of them is doing when they don't follow a basic set of rules, as you mentioned.  I finally wrote a guideline...not perfect perhaps, but a starting point. www.ksu.edu/.../Technote%208%20-%20Guidelines%20for%20Drawing%20Schematics.pdf

  • My main gripe is inconsistent circuit symbols. There are standards, you know, but how come people don't follow them? Also, what about the resistor symbol? Should it be a rectangle or sharp, wavy lines?

  • I too strongly agree with the thrust of these comments. A detail I didn't notice in Tim Sobering's document is the desirability of minimising the number of corners, crossovers and joggles in the connections. Each one adds, if only slightly, to the effort of reading the diagram, and its removal can make the result fit more comfortably on a given sheet size. But again, it takes more time to organise the layout of the diagram - well worth it, in my opinion.

  • The schematic on page 37 of the datasheet for the ADS1258 EVM is an excellent example of how NOT to draw a schematic.  It is a cramped, confusing jumble of jiggles and jaggles.  I can only say two things positive about it.  1) it is correct, and 2) there are no 4-way junctions.  www.ti.com/.../sbau126c.pdf

  • I agree 100% about your comments regarding the importance of a well-drawn schematic.  A schematic should be like reading a well written book.  It should flow from start to finish - a schematic should flow from left to right; it should tell a story so that 10 years later someone can pick up the schematic and understand it.

    As a former field application engineer I was asked to review many customers’ schematics.  The worse was from a major networking company; it was for a standard synchronous buck regulator with a controller, external FETs, inductor, input/output capacitors, and a discrete resistors and capacitors.  The schematic was spread over four sheets!   Controller on one, FETs and inductor on another, all the discretes for the board on another, and all the board's bypass capacitors and input/output capacitors on a fourth sheet!  The connectivity for the net list was correct, but the schematic was impossible to read.  

    Another company's schematic had many missing visual connections on the drawing, but the net names were included and correct so that the PCB layout was correct according to the net list!   But, unless you very carefully looked at the visual schematic, it was wrong.

    When I am asked to review a schematic, often the first thing that I do is redraw the schematic with my own well trusted schematic capture program.  It is amazing how many mistakes I find in the original schematic, and how much I learn by this process.

  • Good suggestion Mike.  I'll have to revise that into the next version of the doc.  But I have to say I think I'm swimming against the tide with the students.  

  • OK, I'm my soapbox too.   Schematics are convey information on functionality, not physical representation.  A proper schematic will reveal how the signal flows (analog or digital) through the circuit and what happens to it.  This is invaluable for debug and test.  It irks me when a see a IC on a transistor, directly copied from the pin diagram.  Either automated form a CAD package, lazy or the person just don't understand electronics!

  • I always thought the old-style transistor drawings (often seen in IC datasheets) were drawn pretty poorly. They had a line for V+ and one for ground and everything was attached from top to bottom… Unless you redraw, you have no clue about how all the transistors interact.

    Also, a good schematic can (and should) lead to a good layout, with the signal flow and current paths matching what’s on the schematic. If your symbols are drawn right, it helps to prevent signals crossing over each other and minimizes trace lengths and layer jumps. Sometimes it's worth redrawing the stock library symbol to make the layout easier, and sometimes it also includes the choice of pins on your microcontroller. You can also add labels and boxes to the slikscreen art. A layout that matches the schematic flow is invaluable for debugging and repair.

    Great article!

  • Whitham, the problem with symbol standards is that there are so may of them. It starts with simple digital gates. The typical American dsymbol is totally different from the Eurpean ones. And I don't mean that the American ones are drawn in inches and the European in centimeters :) Both types are official standards.

    It continues with more complex things like D-latches or so simple things like resistors. Eagle, the layot program we use, comes with separate libraries for both "worlds".

    And when it comes to special ICs, the usual way is to draw a rectangle and arrange all signals in the order of the pinout. Standard OpAmps have teh typical trianle and if a chip contains more than one, they are separate elements (and the supply is separate too), but that's the end of the line. Everything above is just a rectangle with many pins.

    I try to maintain our own internal symbol library, ut it is not easy at all and takes quite some time. So if a new part is required for circuit, there's usually not the time to think about a proper symbol first: back to the rectangle. To be fixed later (often means: never).

    Alan: we once had outsourced doing the layout of our PCBs. And we had schematics as you described, with all the blocking caps on one sheet. And the one who did the layout did put all caps together in one corner of the PCB.

    Of course this layout was totally useless.

    Now we have the blocking caps directly (and visually) connected to the explicit (rather than implicit by pin/net name matching) supply symbol for each part and on the same sheet as the functional unit of the part. They are still out of the way of the functional signals,  but at least it's obvious now where they belong.

  • Nothing makes a circuit more understandable than a well-drawn schematic. Here are my suggestions:

    for circuit blocks that are specific, more or less textbook circuits, use the canonical form of the circuit, such as found in the textbooks.

    avoid 4-way junctions. from a style standpoint, I don't use jumpovers; I draw thru, and use dots.

    V+ should be up, V- should be down.

    grounding should flow from the diagram, unless you are deliberately trying to obfuscate some circuit detail that is the difference between someone copying your circuit successfully and someone having difficulty. I belive in the free exchange of schematics, but there are companies out there now who will copy your work and sell it as their own, so you might as well not make it easy for them.

    circuits should be drawn in blocks. Grouping the reference designator numbers by block also helps.

    Large numbers of parallel lines that join blocks together should have a line identifier placed on them every so often, as well as at the source and destination. This helps prevent blindness on the part of someone trying to figure out where things go.

    Off-page symbols should ident the page where they reappear, as well as where they originated.

    Some draftspeople may not have circuit-sense, in that case, you may have to at least block the circuit out, or maybe you must do the schematic capture yourself to make the circuit the most readable.