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I have some doubts regarding 2.4 LCD TFT ( NHD-2.4-240320CF-CSXN ) backlight current limit resistor value.

Datasheet note is not clear

6. Backlight: White LED / 3.1V / 100 mA ( Typ )

3.1V means:

a) backlight LEDs forward voltage ? or b) expected backlight source voltage ?

I assume this means LED forward voltage. Then, when powering backlight from 5V source the current limiting resistor should be (5V-3.1V) / 100mA = 19 ohms Is that right ?

Thank you

2.4" LCD TFT

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Yes, you are correct, however it might give you more even backlighting if you used 4 resistors that equal ~19 ohms in parallel. Eg. 50, 100, 100, 100 for K1..K4.

I would also check to see if the display is adequately bright at 50 or even 33mA. At lower currents, any imbalance in the white LEDs may appear worse because the resistive term in the Vf of each LED contributes less drop

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It's highly likely it means a) backlight LEDs forward voltage

Then, when powering backlight from 5V source the current limiting resistor should be (5V-3.1V) / 100mA = 19 ohms Is that right ?

Yes it is (in the absence of a data sheet).

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  • \$\begingroup\$ I assume the cathodes will be connected to one 20 ohm 0.5W resistor \$\endgroup\$
    – Tomasz Dzikuć
    Commented Feb 11, 2021 at 13:11
  • \$\begingroup\$ @TomaszDzikuć you might find that with individual resistors you can balance the light output across the display more evenly. \$\endgroup\$
    – Andy aka
    Commented Feb 11, 2021 at 13:16
  • \$\begingroup\$ Thank you. It's very valuable for me ! \$\endgroup\$
    – Tomasz Dzikuć
    Commented Feb 11, 2021 at 15:10
  • \$\begingroup\$ @TomaszDzikuć if we're done then you should select my answer or Spehro's answer for formal acceptance. If you need more information just ask. \$\endgroup\$
    – Andy aka
    Commented Feb 11, 2021 at 15:15

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