using DMM to measure the resistances of resistors

Discussion in 'General Electronics Chat' started by PG1995, Apr 15, 2011.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    Is this good practice to use DMM to measure the resistances of resistors (I mean when not connected to the power source so that I don't need to read the color codes)? Please tell me. Thanks.
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    If they are not connected to anything it's fine. If they are in a circuit then other components around it are likely to give you inaccurate readings. Also note that if you are holding the leads of a loose resistor to the probes with your fingers then the resistance of your body will affect the reading, especially for high value resistors.
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    It would be to your advantage to learn to use the color code, as well. That is very helpful in reverse engineering existing circuits.
     
  4. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi Markd77

    If they are in a circuit then how would it affect the reading. In normal cases, isn't the resistance of a resistor measured this way by connecting the DMM (in resistance mode) parallel to the resistor? Please tell me.

    When I'm holding the leads of a resistors to the probes of DMM then I and the resistor is making a parallel connection to the DMM. Right? My body has a resistance of the mega-ohm level then zero (or extremely low) current would pass through my body. But both I and the resistor will have same voltage across because of parallel connection. Could you please clarify it a little? I'm beginner so please keep things simple. Thanks a lot.

    Hi BillB

    I understand your advice. With some effort I can decode the color coding of a resistor to know the resistance. But the real problem is resistor are so small and perhaps it would take a magnifying glass to see the color codings! :)

    Regards
    PG
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Yes, but in many cases there will be other components and combinations of components that are in parallel with the resistor you are trying to measure. Thus, the reading you get is the combination of the resistance value of the resistor in question plus all the other circuitry that is in parallel with it. In such cases, you must isolate (i.e., unsolder and disconnect) at least one end of the resistor from the rest of the circuit in order to get an accuate measurement

    It seems that you understand perfectly, but to give an example, suppose that your body resistance is 10 million ohms and the resistor you are measuring is also 10 million ohms. The resistance you will read on the DMM is 5 million ohms (two resistances in parallel.) But if the resistor you are measuring is a low value like 10 ohms, then the effect of the 10 million ohm body resistance in parallel with 10 ohms will be negligible. To avoid this issue, be sure not to touch both ends of the resistor with your body while you are measuring it.

    If I am building a circuit or replacing a resistor in a circuit, I use the resistor color code to find the value of resistor that I want to use, but then I (usually) verify the resistance with a DMM before I install it.
     
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  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    For example, if you identify by color code a 1.2KΩ resistor in a circuit, you would expect to reak 1.2kΩ with your DMM, but instead you read 600Ω. This is because there is another 1.2KΩ resistor in parallel with it on the other side of the board that wasn't immediately obvious.
    Ideally you would measure the resistance of the resistor alone, so that would be in series. There's no way to measure the resistance of a single resistor in parallel.

    yes
    Mostly correct; The resistance of your body is not consistent across all parts. There are other factors like how moist your skin is. play around with it. For example, I just measured from my thumb to index finder and got 1.2MΩ; I then measured from left thumb to right thumb and got 550KΩ. If I were measuring a 500KΩ resistor in parallel with the 550KΩ resistance of my body, this would significantly affect my measurement, causing my to read roughly half of the true value. If I were measuring a 1.2KΩ resistor, this would cause me to read 1197Ω; not a huge difference.
    That's correct, same voltage. It's common for a meter to put out 1 or 2 volts, so it's not dangerous.
     
  7. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thanks a lot, tracecom. Your reply was really helpful.

    By the way, don't you need a magnifying glass to read the colors correctly?! :)

    Best regards
    PG
     
  8. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    I also offer my thanks to you, strantor. I saw your post once I had replied to the tracecom's.

    Best regards
    PG
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    One other common problem with trying to measure a resistor while it is in a circuit is that it may measure one value with the leads connected one way and an entirely different resistance with the leads connected the other way (switched from end to end on the resistor). This is usually caused by some semiconductor function in the circuit. Think of a resistor tied between the base and emitter of a transistor. One direction, the base/emitter junction is reverse biased and the resistor will read higher than the other way when the base/emitter junction is forward biased.
     
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  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Yes, I actually have to use a cheap jeweler's loupe to see the color codes on resistors and to read the designations on some capacitors, IC's, and other components.
     
  11. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thanks for confirming this!;)
     
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