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Explain why DMM reads 00.0 on negative side of circuit

Discussion in 'General Electronics Discussion' started by stspringer, Jul 11, 2019.

  1. Martaine2005

    Martaine2005

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    Yes , exactly.
    Edit, in post 11 it would be the bottom picture
    Martin
     
  2. stspringer

    stspringer

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    So Martin
    If someone were to hand you this breadboard, with this circuit would you be able to tell them if the circuit would work, using just a DMM wouldent the 00.0 reading confuse you?
     
  3. Martaine2005

    Martaine2005

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    Absolutely, of course. Also tell if the resistor was shorted or open circuit. Likewise if the LED was working or not.

    Martin
     
  4. stspringer

    stspringer

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    What reading would you expect to see if the resistor was shorted?
     
  5. Martaine2005

    Martaine2005

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    0;00 or up to 0.03. Remember that a short is electrically connected. Like a direct link.

    Martin
     
  6. stspringer

    stspringer

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    Ok, I understand the short has continuity, but a good reading also has continuity and low ohms. So how do you tell when it is a short just using a dmm. Sorry if I am confused
     
  7. Martaine2005

    Martaine2005

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    Both are short circuit. A direct link. Think of putting the probes together. Albeit, the probe leads will have some resistance in the wire.
    I think the best way to explain is a short is not wanted. A component that should not read such a low resistance and has failed. Components can fail short circuit or open circuit.
    Think about your resistor, take it out and replace with a solid wire link. Now you have 0 Ohms, a short or continuity. YOU know that is not right. It should read the resistor value.

    Martin
     
  8. stspringer

    stspringer

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    Ok, I see what you are saying. Thank you very much. :)
     
  9. Martaine2005

    Martaine2005

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    Glad you might understand. I am terrible at explaining things, so sorry.
    Others will likely enlighten you with better explanations.

    Martin
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    If the resistance is a LOT less than expected, then suspect a short or (in your case) misplaced probes.

    Martin, I think your explanation was just fine. What we have here with @stspringer is a demonstration of the GIGO principle: Garbage In, Garbage Out. And it has been going on for quite some time now with no apparent understanding on his part of what a "short circuit" is, or how it differs (if and when it does) from "continuity," or what resistance means, or what resistance to expect when measuring resistance in connecting wires and dmm probes. Or even where to position the probes to measure resistance, hence his misunderstanding of the dmm reading.

    In other words, to date @stspringer exhibits a total lack of understanding of electricity on his part. Add to this the poor pictures he posted that do not show clearly what is allegedly being "measured" with the dmm, or even what components are allegedly being measured (resistor, LED, wires?), and you have a nice recipe for continued misunderstanding and a total waste of our time. How can anyone advise him about what he is doing wrong when we cannot even see what he is doing? A real schematic diagram would help, along with better pictures that show everything. Otherwise, fuggeddabout it (an old Italian expression, partially translated here into English).

    I suggest we here on EP just give it a rest and let him discover everything he "needs to know" using the time-honored smoke-test procedure, aided perhaps by his dmm if he ever learns how to use it properly. To get sufficient smoke he might need to put together a set of six series-connected D-size cells to provide nine volts (initially) with a source capacity of several amperes. That should be more than enough power to smoke most quarter-watt sized resistors of sufficiently low ohms value and/or almost any 3-mm LED.

    It won't be enough power to melt the connecting wires, which would demonstrate how a real short circuit connected to real power really works. But he probably doesn't "need to know" that right now. After all, the LED-and-resistor circuit does light up the LED when appropriate power is supplied! What more can one expect (or learn) from a battery, a resistor, an LED, and a couple of jumper wires?
    Oh, wait! We can confuse ourselves by making continuity and resistance measurements, using random probe positions, and then post questions about it here in the forums. Yeah, that will work.
     
  11. stspringer

    stspringer

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    hello hevans1944
    I guess I am a slow learner when it comes to electricity. But my question was not about a short circuit it was about the reading of the resistor on the negative side of the circuit.
    I took the best pictures I could with the probes showing where I was testing. Sorry if you didn't see them clear enough I know the resistor was hard to see in the pic.

    You are making me feel like I should be afraid to ask questions on this forum because I am so ignorant of electronics. This is not a good feeling. I was looking for helpful explanations no mater how stupid I may sound asking these questions. I liked Martins response.

    Thanks
     
    Last edited: Jul 12, 2019
  12. davenn

    davenn Moderator

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    don't be afraid to ask and no it isn't a good feeling
    unfortunately hevans1944 can be somewhat terse at times, don't let him put you off :)


    Yes, Martin gave you good answers
    Hang in there, it will sink in with time. We all learn at a different pace :)

    Dave
     
    stspringer likes this.
  13. stspringer

    stspringer

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    Thanks Dave
    I will do my best to grasp all this stuff :)
     
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  14. hevans1944

    hevans1944 Hop - AC8NS

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    @stspringer perhaps my response was a little too harsh, but from my point of view we have been working with you for weeks without seeing any progress in your understanding of electricity. I hope that EP is not your only resource for the study of electricity, because this is not a teaching forum. Some of us may have experience as teachers or instructors (not me), and some who may lack that experience nevertheless do a pretty decent job of explaining things (@Martaine2005 and @BobK, for example), but you need to widen your choices. If you could find a mentor in your area, someone of similar interests who would be willing to sit down with you for a few hours so the two of you could explore those "mysterious" dmm readings in a "hands on" environment, I am sure immediate progress would be made. An amateur (Ham) radio club is a good place to meet such people.

    The main point of my criticism was your inability to provide a clear picture of what you were trying to measure. Excuses do not help. If you cannot hand-draw, photograph or scan, and upload a schematic... or take a clear photograph... that is a problem YOU will have to solve. None of us here have crystal balls that will part the mists to allow us to see what is really going on. If you cannot do that, the task of educating you is hopeless.

    I am sorry if my criticism came across that way. Never be afraid to ask questions here. There are no stupid questions, but you may find lots of stupid answers, or answers that are just plain wrong. That is why it is important to have more than one source before forming an opinion or accepting any answers you receive from the the World Wide Web.

    For example, there is some controversy in another thread about whether diodes, specifically light-emitting diodes or LEDs, have terminals identified by positive and negative polarity. They do not. They have terminals identified by anode and cathode, to which you are free to apply an external voltage of whatever polarity you desire. This is a general statement that applies to ALL diodes, not just LEDs. Diodes have a cathode terminal and an anode terminal. Period. Any Internet reference or citation contrary to this is just plain wrong, although misuse of the term polarity lends to the confusion because diodes are polarized.

    I like the discussion provided by the Kahn Acadamy, which you can read about here. I like this because it does not conflict with what I learned about diodes early in the 20th Century, nor does it go out of its way to identify the anode as positive and the cathode as negative except when discussing how to forward bias the diode. A small point, perhaps, but it will carry through your understanding of diodes for the rest of your life and maybe someday help you with a circuit design.
     
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  15. Martaine2005

    Martaine2005

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    Erm, I think that is this thread....:p

    Martin
     
  16. hevans1944

    hevans1944 Hop - AC8NS

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    Ooops. So it is.
     
  17. Martaine2005

    Martaine2005

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    I couldn't agree more. Watch lots of Youtube videos too. You will soon see who knows what they are talking about and those that don't.

    Martin
     
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  18. stspringer

    stspringer

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    I am learning a little and I like to watch

    hevans1944 Here are some videos that explain things well

     
    Last edited by a moderator: Jul 14, 2019
  19. stspringer

    stspringer

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    This is the best video explanation I ever found



    Tell me what you think
     
    Last edited by a moderator: Jul 14, 2019
  20. hevans1944

    hevans1944 Hop - AC8NS

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    Those videos are better than nothing. I didn't spot any errors in the presentation, but the explanations seemed too long and too complicated. However, if it helps you to understand electricity, the videos will have served their purpose. I would keep looking though. I am sure there are better videos available.
     
    stspringer likes this.
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