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I am testing the green LED of my RGB LED, and I am confused about its behavior on my DMM.

In the following picture, I connect the black lead to LED's cathode and the red lead to LED's anode, and get an open circuit reading. However, I was expecting something that tells the forward voltage.

enter image description here

In the following picture, I connect the red lead to LED's cathode and the black lead to LED's anode, and get 750mV reading. I was expecting an open circuit though. enter image description here I wonder what the implications are from my DMM readings. Why 750mV? Thanks so much everyone!

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  • \$\begingroup\$ If you turn off the lights do you see any illumination from the LED? Please identify or link the data sheet of the LED and show how you have connected to the package. \$\endgroup\$
    – Chris Stratton
    Oct 5, 2020 at 15:23

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Older meters (especially analog ones) typically had the negative lead as the positive polarity in resistance-like modes where the meter provides power.

It's also possible you have mixed something up with the target LED connections, particularly where a multicolor LED is concerned.

For your purposes, do a cross check with an ordinary silicon rectifier diode of clearly marked polarity, single reputable factory fresh red LED with a longer anode lead, or similar. That will lead you to identifying the actual source of unexpected results and determine how your meter behaves with a known diode connected in an unambiguous way.

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  • \$\begingroup\$ So what exactly was the issue? I'm glad if this helped you understand the problem, but on sheer curiosity if nothing else, I'd really like to know what you found out. \$\endgroup\$
    – Chris Stratton
    Oct 5, 2020 at 16:03
  • \$\begingroup\$ Moving coil multimeters usually had black positive on resistance mode so that the meter coil is driven the right direction. Digital meters wouldn't need that and it would mess with the minus sign. \$\endgroup\$
    – Transistor
    Oct 5, 2020 at 17:16
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It would help to give a 3-LED part #.
Some 3-LED chips include diodes to divert reverse-current through the LEDs. This provides some versatility in their use, allowing polarity reversal to turn on one of them while turning off others.
For example a RGB LED type VLMRGB6112 looks like this:
VLMRGB6112
In this case, your multimeter may be measuring voltage drop not of the LED, but of the parallel diode.

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    \$\begingroup\$ +1 Interesting. I presume those diodes are intended for ESD protection. They would preclude driving in a matrix, rather inconvenient. If the Vf of the green LED exceeds 1.999V the OP's DMM will display 1--- overrange, which would explain the observed display. \$\endgroup\$
    – Spehro Pefhany
    Oct 5, 2020 at 15:52
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I suspect there's something funky with your multimeter. 750mV is too low to be the forward voltage of common LEDs.

It looks like your multimeter is set to diode/continuity mode, are you sure that it's in diode measurement and not continuity sensing mode?

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  • \$\begingroup\$ Hi @syntax, thanks for your quick reply. It is set to diode and continuity test mode. However I do not know if my multimeter can separate out the diode vs continuity modes. Not able to find the manual on the internet. \$\endgroup\$
    – YumekaMengjiaLYU
    Oct 5, 2020 at 15:22
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The limit of DMM output voltage for diode test functionality might be lower than actual forward voltage drop of the LED (although it is hard to say with no specs). That explains open circuit reading.

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