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While looking into the schematic of the Nucleo STM32F767ZI, I saw a blue led driven by 3.3V, which is the GPIO voltage of the STM32. The datasheet of this led specifies a typical voltage drop of 3.3V, so how does this configuration work with that 680 limiting resistor, as almost all voltage drops on the led?

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On page 5 of the LED datasheet, you'll see a graph plotting forward current vs forward voltage. For the small currents that you need to work with here, it's pretty hard to read, but at very low currents (<1mA) it has roughly a voltage drop of 2.8V. This leaves 0.5V available for R31, so the current will be 0.5V/680R = about 0.7mA. The LED will be pretty faint at that kind of current, but it will work (the luminous intensity vs current graph suggests it will be a little under 2% of full intensity).

Personally, I'd swap that 680R for something a little lower. 470R will bring current over 1mA, which will at least put it actually onto the graphs, and 330R would make significant improvements in the visibility of the indicator.

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  • \$\begingroup\$ "The LED will be pretty faint at that kind of current" - not really. Luminous output of this LED is around 40-50 mcd, from datasheet, at nominal 20 mA. Light output is proportional to current. At 0.7 mA the output will be about 1.5 mcd. Just for reference, Digi-Key lists more than 200 different LEDs with output less than 1.6 mcd. This is plenty of light for logic state indication. \$\endgroup\$ Jul 24, 2018 at 1:25
  • \$\begingroup\$ @AliChen - the datasheet suggests otherwise. It gives a curve for light output that drops very rapidly below approximately 2mA, and like I say suggests that the output intensity will be less than 2% of full, so around 0.8mcd. \$\endgroup\$
    – Jules
    Jul 24, 2018 at 1:27
  • \$\begingroup\$ Okay,okay, I estimated at about 3%, you and datasheet say <2%, 0.8 mcd is still a lot for a human eye, even under sunlight. \$\endgroup\$ Jul 24, 2018 at 1:35