I'm trying to intuit how pull-up and pull-down resistors work

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Discussion Overview

The discussion revolves around understanding the function and implications of pull-up and pull-down resistors in digital circuits, particularly in relation to switches and microcontrollers. Participants explore the behavior of these resistors in different configurations and their impact on logic levels when switches are engaged or disengaged.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant seeks clarification on how pull-up and pull-down resistors function when a switch is on or off, questioning the stability of voltage levels in these scenarios.
  • Another participant warns about the complexities of understanding resistors in digital circuits, mentioning the role of termination resistors in preventing signal reflections.
  • Some participants explain that pull-up and pull-down resistors are used to establish defined logic levels (high or low) for digital inputs, particularly in relation to CMOS and TTL technologies.
  • There is a discussion about the implications of connecting a microcontroller directly to ground or VCC, with some participants suggesting that pull-up resistors prevent potential damage to inputs by avoiding floating states.
  • Concerns are raised about the behavior of logic gates when inputs are left floating, with one participant noting that this can lead to oscillations and increased current consumption.
  • Participants discuss the differences between using SPST and SPDT switches in relation to pull-up resistors, highlighting the potential issues with floating connections during switch transitions.
  • Some participants mention that certain microcontrollers have internal pull-up resistors that can simplify circuit design.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the concepts discussed. While there are shared insights about the roles of pull-up and pull-down resistors, no consensus is reached on all aspects, particularly regarding the implications of floating inputs and the specific configurations of switches.

Contextual Notes

Limitations include the varying levels of knowledge among participants, which affects the depth of discussion. Some technical details about resistor values and their effects on circuit behavior remain unresolved, particularly in relation to different logic families and their characteristics.

Who May Find This Useful

This discussion may be useful for individuals learning about digital electronics, particularly those interested in the practical applications of pull-up and pull-down resistors in circuit design.

  • #31
Khan academy has some really good tutorials about basic circuit analysis. You may find studying that more productive than this approach.
 
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  • #32
Averagesupernova said:
It's interesting you go 'there'. Nowhere have I said or implied you are faking not understanding this material. It is my opinion that you don't understand circuit analysis (kirchoff, ohms law, etc). I don't know whether you realize you don't understand it and are not willing to admit this or you just don't want to admit it to us because you believe it's not necessary or are simply ashamed to admit you don't understand it.
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You post the text that describes kirchoff and ohms law. That's nice. You may be able to do that and recite those definitions. Could be helpful on Jeopardy. But it's obvious you don't know how to apply that information to real circuits. I've tried to get some of the basics across. Don't expect to get a passing grade or a break because you show up for class brain-dead tired or whatever.
DaveE said:
Khan academy has some really good tutorials about basic circuit analysis. You may find studying that more productive than this approach.
Let's restart. I did a lot of problems for those (Kirchoff's Laws, and Ohm's Laws too, but that's kind of trivial) and my answers matched the answer keys. (For what it's worth, my studying wasn't using Khan Academy, but still.) If I noticed I didn't understand something, I would just say it.

I think my struggle is with making the Wikipedia line circuits (and also the other image(s)) be loops on which I can use those theories, as opposed to lines like what Wikipedia (and other sources) show(s). I don't know if that makes sense. All the problems I've done were loop-y, not line-y. In other words, this doesn't look like the circuits in textbook problems to me.

I think it's just the beginning, of setting up the problem, that I don't get here, when it comes to applying Kirchoff's Laws. Plus, I'm not entirely sure why that's desired. I know Kirchoff's Current Laws are used for nodal analysis to get nodal voltages (by using nodal voltages across resistors to determine currents flowing into or out of any given node and then solving the system of equations obtained). I'm guessing that's what the attempt to get me to see is, specifically since logic is voltage-controlled, but the switches being opened further complicate that for me and pretending that that's an infinite resistance can yield a wacky result?
 
  • #33
s3a said:
Let's restart. I did a lot of problems for those (Kirchoff's Laws, and Ohm's Laws too, but that's kind of trivial) and my answers matched the answer keys. (For what it's worth, my studying wasn't using Khan Academy, but still.) If I noticed I didn't understand something, I would just say it.

I think my struggle is with making the Wikipedia line circuits (and also the other image(s)) be loops on which I can use those theories, as opposed to lines like what Wikipedia (and other sources) show(s). I don't know if that makes sense. All the problems I've done were loop-y, not line-y. In other words, this doesn't look like the circuits in textbook problems to me.

I think it's just the beginning, of setting up the problem, that I don't get here, when it comes to applying Kirchoff's Laws. Plus, I'm not entirely sure why that's desired. I know Kirchoff's Current Laws are used for nodal analysis to get nodal voltages (by using nodal voltages across resistors to determine currents flowing into or out of any given node and then solving the system of equations obtained). I'm guessing that's what the attempt to get me to see is, specifically since logic is voltage-controlled, but the switches being opened further complicate that for me and pretending that that's an infinite resistance can yield a wacky result?
Your post ends with a question mark, but I'm not seeing the question.
Keep studying, you'll get it. Otherwise I'm not sure how we can help.

IDK where the infinite resistor thing came from (that's a rhetorical comment, I don't care). Switched networks are typically analyzed as two different simple networks, one with the switch open, another with the switch closed. The transient solutions during the change are way outside of the scope of your questions and understanding, let's ignore those.
 
  • #34
So what's stopping you from redrawing the circuit? Draw a battery and connect the positive to Vcc and negative to the ground(s) or Vss(s). If one style of a drawing doesn't suit you, switch it to one that does. If you truly understand this you should know redrawing doesn't really change anything. @berkeman could draw the schematics posted here and tell me to build them. I could then hand the physical wired circuit that I have built to you and tell you to draw a schematic based on how I've wired it. You could then draw it the way you understand it and it would be acceptable even though it may not look like the schematic @berkeman drew.
 

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