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Bad Circuits - Test Your Knowledge

  1. Jul 27, 2007 #1

    berkeman

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    Bad Circuits -- Test Your Knowledge

    We were having a discussion in a different thread about the fun sections at the end of each chapter in "The Art of Electronics" by Horowitz and Hill, where they show simple schematics as examples of Bad Circuits, without any explanation of what is bad or wrong about them.

    It was mentioned in that thread that sometimes it can be hard for students learning about circuits to determine what exactly is "bad" about the circuits shown, so this thread is meant to provide a place where we can discuss those circuits, and also to post other problematic circuits that folks have run across in their work or studies.

    So if one of H&H's "Bad Circuits" is bothering you, or if you have run across any examples of errors in schematics that others can learn by, please post them here for others to see and discuss.

    I'd like to keep this thread as tutorial as possible, so here are a couple requests:

    -1- Please do not post homework problems in this thread. You all know where homework questions should be posted for tutorial help.

    -2- If you can see the problem with a circuit easily, please hang back for a bit to let others discuss it, and maybe offer a small clue or two. Students will learn better if there is a discussion about the circuit, rather than hearing the answer early and outright.

    That's it for now. I'll pull out my H&H copy and post something on Monday, unless somebody beats me to it.


    EDIT -- BTW, we are posting small parts of Horowitz and Hill's textbook under the Fair Use portions of copyright law. They are being used for educational purposes only. Many of us here are fans of this book because of its practical approach to basic electronics, and use of real-world components throughout the course of the book. I encourage you to take a look at the book at your local technical library, or at your bookstore. It is now in it's 2nd edition:

    http://www.amazon.com/Art-Electroni...5879301?ie=UTF8&s=books&qid=1185840378&sr=8-1

    .
     
    Last edited: Jul 30, 2007
  2. jcsd
  3. Jul 27, 2007 #2
    What an awesome idea.
     
  4. Jul 30, 2007 #3

    berkeman

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    Yeah, we can thank ranger for the idea. It came up in a homework help thread last week.

    I'll post something here in a few minutes to kick things off.
     
  5. Jul 30, 2007 #4
    This is a really good idea. Can't wait for it to start.
     
  6. Jul 30, 2007 #5

    berkeman

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    Okay, here are a couple easy ones to start off with. These are from Chapter 9 ("Digital Meets Analog") of the first edition of H&H, page 450. There are at least two things wrong with each circuit. What are they, and what would be the best way to make things right?

    Remember, if it's easy for you, hang back for a bit. We'll get to some harder ones soon enough.


    EDIT -- BTW, let me know if this scan is hard to read. If it is, I'll figure out some way to enlarge the attachments in the future.
     

    Attached Files:

    Last edited: Jul 30, 2007
  7. Jul 30, 2007 #6
    Could you please enlarge the picture? I can't make out the numbers.
     
  8. Jul 30, 2007 #7

    berkeman

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    Okay, I'm still figuring out how best to scan and post these. Here are (a) and (b) enlarged. How's that?
     

    Attached Files:

  9. Jul 30, 2007 #8
    Well for (b) you need a resistor in front of the LED.

    For (a), should the resistor be in series with the cap? I think so.
     
  10. Jul 30, 2007 #9
    Hello there,
    First of i would like to appriciate you for this wonderful idea.
    And i think i can answer the second one..
    I am not sure, but i think i am right, the two errors are:
    1. There is always a resistor connected in serires with LED's because if the applied voltage is more than 1.5V than it get burnt.
    2. There is no need to ground a LED.
    please dont get angry if i am wrong, i am not sure about this.
     
  11. Jul 30, 2007 #10

    berkeman

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    Maxwell and goldy are correct that at least a resistor is missing in (b), but that's about all that's correct so far. Keep on thinking them through....

    Remember, H&H emphasizes real devices with real datasheet specifications and real-world signals....
     
  12. Jul 30, 2007 #11
    and in (a)
    there is no need to ground a capacitor.
     
  13. Jul 30, 2007 #12
    Perfect, thanks I appreciate it.
    I have to be honest, I'm not very familiar with digital logic. I know the basics, but I don't have an in-depth knowledge of digital logic in real life problems and use in circuits.

    If I may make a suggestion, I think that if someone posts an answer as to what is wrong with a circuit, they should explain why, just so the people who don't understand (like me :smile: ) could learn.
     
  14. Jul 30, 2007 #13
    Hey EugP,
    I am agree with you and I already have explained my first answer.
    I am not quite sure with my second answer...
     
  15. Jul 30, 2007 #14

    ranger

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    Berkeman, I'll give a crack at a) My reasoning is as follows:

    As the circuit currently is, it has a RC time delay of 1s. This means that whatever was inputed will show up at the output with a time delay. If a high was the output of the first gate, the cap would take a great deal of time to charge up to that value (5RC). Will the signal of the first gate last that long? Then to discharge, will also take a long time. The issue with large time constants makes a difference because TTL logic uses a fairly narrow range for HIGH and LOW. What if only a short pulse was present at the the output of first gate? Then the final output could more that likely be in the "forbidden region". For cases with large time delays I'd use CMOS logic because of a more forgiving range of HIGH and LOW.
     
  16. Jul 30, 2007 #15
    Yeah, I have a feeling that the RC time constant is too large.
     
  17. Jul 30, 2007 #16

    ranger

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    Hi goldy,

    I'm curious as to why you stated this. And also the same for the logic indicator circuit.
     
  18. Jul 30, 2007 #17
    Oh i am really very sorry that i have mentioned above, its just a misconception but i am still quite confused with the circuit (b), what is the need to ground the LED.
     
  19. Jul 30, 2007 #18

    ranger

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    [Circuit B]
    What would happen if you left it floating?

    Also if you didnt ground it, but connected to a voltage terminal of some odd polarity (+6V), would the circuit still function as it should?
     
  20. Jul 30, 2007 #19
    For circuit B;

    i think for circuit b is should be +5V -> LED -> resistor -> output of logic. the way it has it configured now works (need resistor of course) but just bad practice because the output of the logic is driving the current to power the led.

    I'll rather have a source driving the led, and grounding the other side. Much like a ucontroller, make a port be open drain and have the led connect to the source. toggle the port low to turn on led.

    Is this right?
     
  21. Jul 30, 2007 #20

    chroot

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    There are definitely some superficial lessons to be learned here. The second circuit (the one with the LED) will indeed fail catastrophically if you do not put a current-limiting resistor in series with the LED. As soon as you apply power to it, it'll burn itself up.

    However, there are actually some much deeper lessons to be learned here, too. Slapping a resistor into the second circuit will prevent it from blowing itself up, but the circuit still won't work well at all. Anyone know why?

    - Warren
     
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