voltage drop on a multi meter

Discussion in 'General Electronics Chat' started by chrissyp, Sep 29, 2012.

  1. chrissyp

    Thread Starter Active Member

    Aug 25, 2008
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    10
    Hi
    When using a handheld digital multimeter in current measuring mode is there a voltage drop across the meter when taking a current measurement.I am measuring around 850ma to 1A. the meter is a Tenma 72-7730.I cannot find anything in the user guide.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,785
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    The 'ammeter' portion of a handheld multimeter is a short thick piece of wire with a know resistance value. The 'ammeter' actually reads the voltage drop and displays it as a 'current' value. The meter is still measuring voltage and not current directly
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    Since you insert a very small resistor in the current path. You will have a very small voltage drop also Ohms law will apply. However this voltage drop is neglectable
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    To be accurate t06, rather than saying "However this voltage drop is neglectable " you should say "...may be negligible." To determine if a quantity is negligible you must take into account other circuit values and the magnitude of changes to all circuit parameters. You hope the insertion of test equipment for measurement purposes will have negligible effect on the circuit under test but you cannot be certain until verified that that is in fact the case. TEST & VERIFY!

    False assumptions can lead to wrong conclusions, and wrong conclusions can lead to wrong actions. In some cases, we can laugh at the outcome of our false assumptions. In other cases, false assumptions can be deadly.
     
  5. colinb

    Active Member

    Jun 15, 2011
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  6. chrissyp

    Thread Starter Active Member

    Aug 25, 2008
    82
    10
    Hi
    Thank you all for your input.The problem that sparked the question is this-I bought a dimming control circuit for a luxeon led ,at maximum its output should be 850ma,however when I measured the output with my meter the output maxed at 780ma, when i complained the reply i got was Quote -the input voltage is 5v to the +v of the luxeon the -v is connected to a ifrz24n mosfet drain,which is connected via the source of the mosfet to a 0.33Ω resistor,which is connected to 0v.Because I connected the meter in this chain the voltage drop across the meter plus the voltage drop across the luxeon+mosfet+resistor+the meterthe voltage drop will have exeded 5v ,therefore my meter was giving a false reading.After reading this reply I thought I was being fed bulls--t,but having read the replies on the post i am not sure.
    PS
    The gate of the mosfet is connected to their dimming circuit.I would appreciate you input.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You can measure the voltage across the .33Ω resistor, and then divide by .33 (I=V/R).
    Your accuracy will be no better than the tolerance of the resistor.
     
  8. colinb

    Active Member

    Jun 15, 2011
    351
    35
    It can be better than the resistor tolerance if you first measure its resistance with a multimeter, but this is difficult since most handheld multimeters won't measure sub-ohm resistances accurately, even the industry-standard Fluke 87. You'd really need a 4-wire kelvin measurement of the .33 ohm resistor.
     
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