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On a simple circuit containing a tri color LED (Part Number: 5988740302F) when power is applied the LED only lights up Green but should be Blue when 5VDC is applied. To explain the troubleshooting, 4.4V was measured across the +5V node and ground. On another circuit (the good circuit) the LED is lighting correctly (Blue) and the measured voltage is 4.6V.

More troubleshooting: On both circuits When the cathode of the Yellow LED is shorted to ground, the LED lights up Yellow.

Is it possible that something in the circuit is driving the voltage down? There are other parts of the circuit that driven by +5V.

Forward voltage (from datasheet): 3.2V Blue, 3.2V Green, 2.2V Yellow

enter image description here

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  • \$\begingroup\$ why is green connected to the circuit? \$\endgroup\$
    – jsotola
    Nov 13, 2020 at 2:15
  • \$\begingroup\$ Study the datasheet further, those are TYPICAL forward voltages for a current of 20mA which is more than you'll be getting with a 450 Ohm resistor. Blue and green can range from 2.8V to 3.5V at 20mA and lower still at less. \$\endgroup\$
    – Finbarr
    Nov 13, 2020 at 15:42

1 Answer 1

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You need to use a distinct resistor for each LED.

The lower forward voltage of the green LED is basically clamping the joined anodes at a voltage lower than that where any meaningful current will flow through the higher forward voltage of the blue LED's junction.

In fact not only do you need individual resistors, you probably want unique resistor values to take into account each LED's specific forward voltage and efficiency.

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  • \$\begingroup\$ Thanks for the input but why does the other circuit light up Blue, actually have 3 more circuits that are blue with the same part number LED. I also checked the resistor values. \$\endgroup\$
    – iomskii11
    Nov 13, 2020 at 1:55
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    \$\begingroup\$ Who knows, could be manufacturing tolerance. But that's irrelevant because this is simply not a valid design; you must use an individual resistor per LED. Really you should do that even if all the LEDs were the same color. Individual resistors would also make it easier to tell if you had something like a soldering issue. \$\endgroup\$ Nov 13, 2020 at 1:56
  • \$\begingroup\$ As Chris says the different results for the same setup are probably small variations in the manufacture. Bear in mind although the quoted datasheet voltages are the same for Blue and Green (3.2V) small variations in voltage for a forward biased diode give rise to large changes in current and dissipated power so the Blue LED for example could dominate (perhaps as in your second circuit). \$\endgroup\$
    – mhaselup
    Nov 13, 2020 at 2:03
  • \$\begingroup\$ The device has a common anode for the 3 LEDs so the resistors need to be on the cathode ends. Having worked with a lot of different coloured LEDs, I don't trust the voltage for blue; I would expect it to be much closer to 4V or more. \$\endgroup\$ Nov 13, 2020 at 12:02
  • \$\begingroup\$ The schematic in the question shows a six pin device with individually numbered anode and cathode leads, which is indeed unusual. If that is not the actual device (the stated part number is a drastic mismatch!) then this is an error in crafting the question. Indeed the resistors would need to go on a side which is not tied to the other LEDs. \$\endgroup\$ Nov 13, 2020 at 14:22

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