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I'm trying to power on a LCD on a breadboard. For some reason, the LCD is not turning on (No backlight). Here's how I wired it and I'd like to know if I messed up on anything:

Datasheet: http://www.lcd-module.de/fileadmin/eng/pdf/doma/dip203-4e.pdf

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I've checked the voltages on VDD (Pin 2) and LED Backlight (Pin 17) and they are getting 3.3V. Measuring voltage on VEE (Pin 3) gets about 2V, which I'm assuming is the voltage being dropped from the pot and internal resistor inside VEE.

Before connecting the LCD, I've also measured the current I'm driving on pin 17 using an ammeter on LED B/L+ (17) and it's at 33mA. The datasheet says the maximum current on LED Backlight pin is 45mA so I think I'm safe here. Connecting the LCD drops that current to 0.7mA though.

I haven't wired the data lines yet because I can't even get this LCD to power up.

The LCD was bought 2 days ago, so while not impossible, it's very unlikely this is defective.

Is there anything I've messed up in my wiring?

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    \$\begingroup\$ While that's a lower LED backlight voltage than I'd expect (more typically they are a series string of LEDs), your current suggests perhaps it is actually on. Apart from seeing leakage through corners of the housing, it's not clear that the backlight would be evident through the front glass if you are not yet driving the LCD itself to make it less than opaque. Measuring the current over a range of voltage would let you see if you are in fact crossing the knee in a classic diode response, indicating it is on. \$\endgroup\$
    – Chris Stratton
    Oct 2, 2016 at 1:19
  • \$\begingroup\$ @ChrisStratton Sorry, I've worded what I meant by that current wrongly. 33mA is the current I'm driving that pin 17 with (Before connecting LCD). After connecting the LCD, the current drops to 0.7mA. Increasing the voltage from 3.3v to 4.2v also increases the current to 2.3mA . \$\endgroup\$
    – Xiagua
    Oct 2, 2016 at 1:51
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    \$\begingroup\$ That statement makes no sense. You cannot drive current to a circuit that is unconnected. Did you try to measure "the current of" your power supply by shorting it out with a meter? That would be meaningless and erroneous. Depending on the model (color) you have you may not have supplied sufficient voltage for the backlight, but again do not necessarily expect to see anything (apart from current draw) without driving the LCD itself. \$\endgroup\$
    – Chris Stratton
    Oct 2, 2016 at 1:56

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Based on your diagram and the datasheet, you have confused where the datasheet lists the LED backlight voltage & current requirements for the LCD module with the yellow/green backlight (which is the part number on your diagram) and the requirements for the module with the white backlight (which are the numbers you quoted in the question).

If you have any doubts about the datasheet, I recommend that you double-check with the manufacturer. However based on the information you provided, I have listed the problems that I found.

Your diagram shows the part number as:

EA DIP203G-4NLED

From the datasheet, that G means it is the version with the "Y/G" (yellow/green) colour LED backlight.

You said:

I've checked the voltages on VDD (Pin 2) and LED Backlight (Pin 17) and they are getting 3.3V.

However the datasheet says that for the green LED backlight:

Forward voltage for yellow/green backlight is 3.9~4.2V

Therefore one problem is that you are not supplying the correct voltage to the yellow/green backlight.

Also, you have got the current calculation wrong, because the datasheet says:

LED BACKLIGHT Y/G max. 150mA@+25°C

But you were quoting the 45mA LED current for the white LED backlight (which is a different LCD module part number):

The datasheet says the maximum current on LED Backlight pin is 45mA

Therefore if you have listed the correct part number EA DIP203G-4NLED on your diagram, you need a 5V or greater power supply to the LED backlight, and then calculate the series resistor to pin 17 for an appropriate current (I would not aim for the maximum 150mA current, and note that the datasheet mentions de-rating the maximum current above 25°C, although it doesn't give further details). I hope you have a 5V (or greater) power supply, which can supply the necessary current!

For example, with a 5V backlight supply, depending on what resistor values you have available, I would start with around a 20 Ω resistor, aiming for around 50mA current and a 1V drop across the resistor, to give 4V at pin 17 - and tune the value from there, after you see how bright the backlight is.

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  • \$\begingroup\$ My god, you're right! Thankfully, I have 5V power supply. So can I just wire a 20 ohm resistor to 5V power supply, then the LED backlight pin to the resistor? Or do I need to drop the voltage to near 4v using diodes, then 20 ohm resistor, then to LED backlight pin? \$\endgroup\$
    – Xiagua
    Oct 2, 2016 at 4:02
  • \$\begingroup\$ @Xiagua - "My god, you're right!" - lol, sometimes I am :-) "I just wire a 20 ohm resistor to 5V power supply, then the LED backlight pin to the resistor?" - Yes. You disconnect only pin 17 from the 3.3V supply on your diagram, and connect it via a 20 ohm resistor to 5V and test. I hope you would see a visible light from the backlight. You should measure around 4V at pin 17. \$\endgroup\$
    – SamGibson ♦
    Oct 2, 2016 at 4:13

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