Testing LED w/ Digital Multimeter

Discussion in 'General Electronics Chat' started by Lightningbolt, Mar 1, 2013.

  1. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
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    0
    I was working on a project that involved LED’s. I tried to check them with my digital multimeter (DMM) set to the Diode function. No mater which way I connected the test probes, the DMM read "OL". Yet, when I tested a normal diode, the DMM read "OL" when the probes were connected the wrong way, and the correct Voltage drop when connected the correct way. The LED’s were typical with 20mA, 2-4V ratings.

    I know the LED’s I was trying to test were good, because they worked just fine when installed in my project.

    Why did my DMM read "OL" testing the LED and the voltage drop when testing the normal diode?

    Thanks in advance for your help!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    LEDs especially are non-linear devices. With a diode, you can get 0.2VDC to 0.7VDC forward voltage drop, which is what a meter is looking for. With a LED you can get 2.0VDC (really old, obsolete LEDs) up to 3.8VDC drop, which is a lot different.

    One of the devices in my toolbox is a really simple LED tester, which is a battery and a resistor (more or less). With modern LEDs this can still go wrong, due to the fact some LEDs have a PIV of 5V! In other words, if you connect more than 5V backwards to a LED it can blow.

    Then add the ESD sensitivity to some LEDs (part of the PIV I mentioned) and you get the idea.

    Here is a commercial tester I bought a long time ago, but it is easy enough to roll your own.

    [​IMG]

    http://www.bgmicro.com/LED1063.aspx

    A really simple LED tester
     
  3. Lightningbolt

    Thread Starter New Member

    Feb 4, 2013
    9
    0
    I am unfamiliar with the acronyms "ESD" & "PIV"; would you please provide an explanation?

    If I understand you correctly, the reason my procedure failed for the LED test is because the DMM did not supply a great enough voltage (2-4 VDC) to overcome the forward voltage of the LED (to open the gates of the LED using the hydraulic analogy); therefore, it determined there was no continuity between the test leads which resulted in the "OL" display.

    While for the normal diode test procedure, the DMM was able to supply enough voltage to overcome its forward voltage (~ < 1 VDC); therefore it determined the forward voltage drop and displayed that result.

    Is this a correct interpretation?
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Electrostatic discharge and peak inverse voltage.

    That's correct. Some meters will supply enough voltage to dimly light some LEDs and some won't. I like to use a constant current supply made from an LM317 voltage regulator and a 9V battery to test and identify LEDs since it works with all colors equally well.
     
    Last edited: Mar 6, 2013
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