multimeter

Discussion in 'General Electronics Chat' started by matty204359, Jul 13, 2013.

  1. matty204359

    Thread Starter Member

    Apr 6, 2011
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    3
    I'm curious about the behavior of common digital multimeters. I understand the concept of meter loading on a circuit, but I never noticed before now. my multimeter seems to be creating a current path between the cathode and anode of my circuit when it is reading voltage. Enough current that it will visibly dim an led when measuring the voltage drop across the diode. it will also even cause enough current to light up an led. I noticed this when I removed a wire connecting the cathode of the led I was examining to my surprise it stayed lit albeit very dim, the only current path was the meter.

    This isn't a high quality fluke meter or anything, just a hardware store branded multimeter. is this normal behavior for cheapo meters?

    edit: I never noticed before because I will commonly use a series resistor(10-15 ohm) and measure the voltage drop across the resistor to determine current in a circuit. for some reason I never trusted its ammeter functionality. now I'm questioning if it is even useful as a paper weight.
     
    Last edited: Jul 13, 2013
  2. WBahn

    Moderator

    Mar 31, 2012
    17,757
    4,800
    What scale are you using?

    Do you have access to a good meter?
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Perhaps you would describe the connection more fully?

    You don't actually need a good meter, just a high value resistance, the 1M - 10M range.
    Substitute this for the meter and see if you get the same illumination.
    Better, a resitance box would tell you at what resistance the illumination begins to occur and give you an estimate of the input resistance of the meter.
    The meter's input resistance is normally set by a resistor so if the one in your meter has gone low for some reason this test will show it up.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Typical meters have a input resistance of 10 megaohms on the DC scale, even the cheap ones, so the behavior of yours seems odd.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What are you using the ammeter function for? If you are trying to measure current, you must break the circuit and insert the ammeter in series.
     
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