Connect with us

Explain why DMM reads 00.0 on negative side of circuit

Discussion in 'General Electronics Discussion' started by stspringer, Jul 11, 2019 at 6:27 PM.

  1. stspringer

    stspringer

    89
    5
    May 10, 2019
    Hello All,
    Another question any help appreciated. Thanks in advance
     

    Attached Files:

  2. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    That's actually really confusing.
    Can you explain that again please?
    All I can think of is the LED is acting as it should, a diode.
    But as I couldn't decipher your question, or where you are probing, it's just a guess.

    EDIT: LEDs don't have positive and negative legs.
    EDIT2: Put the resistor before the LED and try your probing again. See if you get the opposite results on your DMM.
    Martin
     
    Last edited: Jul 11, 2019 at 7:17 PM
  3. stspringer

    stspringer

    89
    5
    May 10, 2019
    Thanks for your reply

    "LEDs don't have positive and negative legs."
    Yes they do. Positive is the anode negative is the cathode. https://www.westfloridacomponents.c...w-to-tell-which-lead-is-positive-or-negative/

    My picture shows the negative leg of the led. I put the black probe from my meter in the negative power rail. I put the red probe in the hole on the negative led leg. My meter reads .474 which is the resistance of the 470 ohm resistor. This is all done on the negative side of the circuit as shown in the picture.

    Now with the black lead still on the negative power rail if I place the red probe on the positive side of the circuit on the other leg of the resistor my meter reads 00.0 which means there is continuity, but I was wondering why it would not read .474 ohms like the positive side did.

    I hope you are clear on what I am saying about the more positive side or more negative side of the circuit, which I point out in the picture.

    "EDIT2: Put the resistor before the LED and try your probing again. See if you get the opposite results on your DMM.
    Martin"

    The resistor is bridging the negative and positive sides of the circuit on the breadboard see the picture.
     
    Last edited: Jul 11, 2019 at 7:34 PM
  4. BobK

    BobK

    7,592
    1,636
    Jan 5, 2010
    Are you measuring Ohms on a powered circuit? That is a no-no.

    Bob
     
    Martaine2005 likes this.
  5. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    That will measure the resistor only.
    Sorry you lost me. Please edit the circuit picture with connections from your DMM.
    Also, LEDS as you already stated have anodes and cathodes. Which tells you which leg to connect to the positive and negative of a power source.
    Martin
     
  6. stspringer

    stspringer

    89
    5
    May 10, 2019

    No, it is not Powered Bob I learned that from you and others :)
     
  7. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    That never even crossed my mind. Well spotted.
     
  8. stspringer

    stspringer

    89
    5
    May 10, 2019
    No, it is not Powered :)
     
  9. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    Can you clarify post 5 please.

    Martin
     
  10. stspringer

    stspringer

    89
    5
    May 10, 2019
    The circuit is not powered. I will try to explain it better.

    My circuit, the side with the short red jumper wire, going to the anode leg of the led, I am calling this the more positive side.
    My circuit, the side with the small black jumper wire feeding the other leg of the resistor to complete the circuit, I am calling the more negative side.

    The resistor is the bridging the breadboard center channel to complete the circuit.

    If I keep my black meter probe on any negative power rail, and then I place my red meter probe on the more "positive side" on the "negative cathode side" of the LED "the side with the short red jumper wire" my meter reads .474 ohms.

    If I keep my black meter probe on any negative power rail, and I then place my red meter probe on the more "negative side" the side with the short black jumper wire" on the other resistor leg my meter reads 00.0 to 00.3

    I will upload more pics
     

    Attached Files:

    Last edited: Jul 11, 2019 at 8:10 PM
  11. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    upload_2019-7-11_20-8-35.png Is this what you are doing? Excuse the terrible pic

    Martin
     
  12. stspringer

    stspringer

    89
    5
    May 10, 2019
    Pretty much please see my new pics. The resistor is hard to see in my new pics but it is in line with the negative leg of the led across the breadboard gap and fed by the small black jumper wire.
     
  13. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    Ok we got there in the end.
    What you are reading is perfectly normal. You are measuring the resistor. Then you are just probing a wire between the resistor and power source, it's a dead short for the DMM.

    Martin
     
  14. stspringer

    stspringer

    89
    5
    May 10, 2019
    Why do you say it is a dead short? Isn't 00.0 on the ohm setting meaning continuity? Also there is no power going to this breadboard.
     
  15. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    Yes, continuity means a loop without a break.
    What you are doing *is* just putting the meter probes together.

    Martin
     
  16. stspringer

    stspringer

    89
    5
    May 10, 2019
    Ok so here is where I am confused. Why am I getting a good resistor reading .474 ohms only on one side of the resistor, if the resistor is the bridge to complete the circuit?
     
  17. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    [​IMG]
    This Is continuity.

    Martin
     
  18. stspringer

    stspringer

    89
    5
    May 10, 2019
    Yes but I don't see "understand" where I have a break. Why is the resistor braking my path?
     
  19. Martaine2005

    Martaine2005

    2,310
    616
    May 12, 2015
    You don't have a break. You have very low Ohms reading. A short Or a link or electrically connected.

    Martin
     
  20. stspringer

    stspringer

    89
    5
    May 10, 2019
    So in my case it would be electrically connected, right? I mean the circuit works when I apply power
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-