Diode forward/reverse resistance measurement

Discussion in 'General Electronics Chat' started by vinodquilon, Oct 11, 2013.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    See the diagram attached herewith.

    What would the multimeter shows if I connect a multimeter across
    A & B in resistance mode ?

    DFR- Diode Forward Resistance
    DRR- Diode Reverse Resistance

    Both are identical diodes and the multimeter is set in auto range.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    My fluke meter would say OL.
     
  3. #12

    Expert

    Nov 30, 2010
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    Very high resistance.
     
  4. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    My Keithley meter shows resistance in MOhm range !:confused:
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Very high resistance. Meaning the meter basically reading open circuit.
     
  6. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    But D2 is in reverse open condition, so why the meter shows MOhm instead of OL. I think reverse leakage of D2 plays the trick for the above anomaly!
     
  7. #12

    Expert

    Nov 30, 2010
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    So? Are you saying millions of ohms is not a very high resistance, or wondering if your meter is broken, or wondering if the diodes are bad, or something else?
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Look at the datasheet for your diode. If it is a zener, then you should get current flow. If not a Zener, check your meter, maybe it is measuring the unbelievably low current flow of a diode.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Most VOM's will not read a diode on the resistance scale, there is not enough current provided to forward bias the diode.
    This is why meters have a diode test position, this actually provides a bias current and indicates volt drop rather than resistance, this is how my Fluke works.
    Max.
     
  10. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    I am checking the above circuit with reference to the check list specification of OPEN. (we consider resistances above 100MOhms as OPEN condition). Here I reads around 42MOhms!
     
  11. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    As per your quote meter should reads different DFR/DRR values at different RANGE settings of MM in resistance mode.
     
  12. MaxHeadRoom

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    Jul 18, 2013
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    The digital meters I have and use, none will read any useful Diode or semiconductor values on resistance range due to the lack of sufficient current to forward bias.
    The moving coil analogue meters will however, as they supply a bias current in the resistance range.
    Max.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Setting the multimeter to autorange when dealing with nonlinear components can cause you all kinds of grief. The meter has to decide when to switch to a higher or a lower range. It is very easy to end up in situations in which it decides it should be in a higher range and switches, but under the bias conditions of the higher range the response changes enough that the meter then decides it should be in a lower range and switches, and back and forth we go.
     
  14. WBahn

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    Mar 31, 2012
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    What check list specification?

    Who considers resistances above 100MΩ open?

    Why is there any reason to believe that your meter uses those same specs (unless they are the "we" you are refering to)?

    At those resistance levels, is a difference of 2.5x that big a deal? (I have no idea, since it depends on application).

    And, again, the resistance meter is assuming that you are measuring the resistance of a resistor. Gee, imagine that! If you throw a nonlinear component at it, the resistance measurement results are largely meaningless except in very big picture ways.
     
  15. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    To measure DFR, what range should I prefer ?
    To measure DRR, what range should I prefer ?
    In the attached diagram by connecting MM positive lead to A and negative lead to B, D1 in forward and D2 in reverse. Which range should I prefer ?
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The question I have is Why are you posing this question in the first place? The readings are going to be meaningless anyway as already pointed out.
    It seems like an exercise in futility?
    Max.
     
  17. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Look at another arrangement

    Here connecting the MM in forward mode reads 6KOhm against expected value of 1K+DFR

    In reverse mode shows OPEN as expected.
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    And pick another dozen arrangements and the readings won't match what is expected. Not a big surprise since the readings are meaningless! You are trying to measure DYNAMIC resistance with an instrument that is only intended to measure STATIC resistance.
     
  19. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Dfr and drr are not values that can be measured as a single value. The reason is, a diode is a non-linear device. Non-linear is the whole point of a diode. If they provided a constant resistance at all amperages (current flows), then everyone would use resistors instead.

    A standard, useful, graph of measurements for a diode is "voltage drop vs. current" for forward voltage. There are also current leakage equations for reverse current. If I remember correctly though, reverse current is a combination of temperature AND applied voltage.

    In any case, read a datasheet and a good manufacturer will post all of the important data. The information that is not on the datasheet is also noteworthy (eg dfr, DDR - because it is meaningless except at one temperature and one applied voltage AND because nobody needs it).

    If you already understood all of this and needed the measurement anyway, could you please tell us your goal nd why this is important.
     
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  20. ian field

    Distinguished Member

    Oct 27, 2012
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    Unless its a Shottky-barrier diode - they usually have some leakage, for regular silicon diodes you'd pay an awful lot for a meter that can see any reverse reading.

    Most DMMs have a diode check function with direct reading Vf - the value of which can hint at what type of diode.
     
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