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Thread: LED Help! Which is positive and which is negative?

  1. #1
    Join Date
    Sep 2013

    Question LED Help! Which is positive and which is negative?

    I ordered a few LED's to experiment with and got them today. Unfortunately I can't tell which lead is positive and which one is negative? The + and - sign on the plastic are kinda equidistant from the leads. One lead has one hole and another lead has two holes if that helps.

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  2. #2
    Join Date
    Feb 2011
    St. Louis


    Looks like this to me. The base may also be positive or negative.

  3. #3
    Join Date
    Jun 2010


    Test them with a 9V battery and say 470 ohms current limit resistor.... I have a LED tester here, very handy as I can test at different currents easily. Its one of those must have tool box testers. 2nd must have is a 2.50000000V voltage reference, handy for checking multimeter calibration plus other things.
    This space for rent.

  4. #4
    Join Date
    Oct 2013


    You can test with a VOM in the ohms measuring mode. The red lead is 2 volts and that is enough to energize even blue and white LEDs. You will easily see them come on faintly, even in a moderately lit room. The current is in microamps, so the resistance will show off scale high. The meter is just a handy, safe, current limited power supply.

    And of course, test in both directions. The direction that lights is the forward direction.

  5. #5
    Join Date
    Nov 2013
    State of Shock USA


    You do know the last post was almost 4 months ago?
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  6. #6
    Join Date
    Mar 2006


    Quote Originally Posted by TechJunkie View Post
    You do know the last post was almost 4 months ago?
    His two other posts show he's (she's?) very interested in diodes and LED's specifically. I test LED's with a multimeter like that too, it's usually faster than grubbing around for a battery and an appropriate resistor. If the ohms tester won't do it, try its lowest range. The diode test mode definitely will, if the meter has one. NSFLD! (Not Safe For Laser Diodes).

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