Backlightb LED got dimmed Issue!

RAGHUL

• November 14, 2019 10:07

Hi,
We are using NHD-1.8-128160EF-CTXI#-T display module. In the data sheet it is mentioned as Backlight Supply Voltage = 3.3 V (Max) and Backlight Supply Current = 30 mA (Typ) and 40mA (Max). We made a custom pcb to place the display module and provided direct 3.3V supply to the pin 17 of the display  module (Backlight Anode) . We used a TI microcontroller to communicate with the display module which share the same 3.3V as that of the display module. It was working fine for some days. But suddenly the backlight blinks for two to three times and the display went off. It also affected the TI microcontroller which was used to communicate with the display module and the micro controller went dead. We had to replace the microcontroller with a new one. And after replacing, the backlight is not glowing as it used to. The light intersity got very dimmed and as the result the display is also dimmed. However the display is working fine. Only problem is that backlight LED illumination got dimmed.
My doubt is,
Is there any resistance for the backlight LED placed internally inside the module to limit the current or should we have to do it externally?

Since in the datasheet, the backlight voltage is mentioned as 3.3V, can we give a direct 3.3V to the Backlight Anode pin without any resistance?

Is there any other reference schematic design for the display for reference?

Thank you for your time!

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  Paul_B

  • November 15, 2019 10:07

  Hi RAGHUK,
  
  The NHD-1.8-128160EF-CTXI#-T does not contain a current limiting resistor, please see page 3 in the displays datasheet for the backlight circuit.

  https://newhavendisplay.com/content/specs/NHD-1.8-128160EF-CTXI-T.pdf
  
  Unfortunately, it would appear that your TFT is toast  
  
  Listed below are two options for driving your Backlight:
  
  

  • Place current-limiting resistors in series with the LEDs to prevent them from blowing out.
  • Use a constant current LED driver (Schematic Attached).

  

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  RAGHUL

  • November 18, 2019 10:07

  Thanks, Paul.
  From your two suggestions, placing a current-limiting resistors in series with the LEDs seems to be the simpler one, as it requires a simple modification in the existing schematic and gerber.
  Since the Backlight Supply Voltage = 3.3 V (Max) and 2.8 V (Min) and Backlight Supply Current = 30 mA (Typ), can we provide a 3.3V through a current limiting resistor to the LEDs? Whether it will cause a voltage drop across the resistor resulting in less voltage (less than 2.8V) to the LEDs?
  Could you pls suggest a resistor value for this case?
  
  Thank you for your time.

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  Sergio_M

  • November 19, 2019 10:07

  Hi RAGHUL,
  
  Using a 10 ohm resistor in series with the LED backlight and a 3.3V source voltage will provide the 30mA at 3.0V to the LEDs.
  Formula: R = (Vs - Vf) / If
  
  Hope this helps!

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