EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Mp3 on February 09, 2019, 07:30:15 am
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Hi everyone,
I decided to swap some old black on green LCDs with white on black LEDs from china ebay sellers.
I soldered pin headers on and it all works, but, i think i need a current limiting resistor. The backlight is so bright, it almost looks blue and looks light-grey in some parts.
When i google about the current limiting resistor, i see some suggestions to put a resistor between VSS and K, is this what i should do? I would have to solder it to the other side of the pin header i think.
This is exactly the display i have:
(https://p.globalsources.com/IMAGES/PDT/BIG/961/B1162242961.jpg)
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Backlight is a completely separate circuit. It is connected only to A and K pins. And there is usually a place on the module itself to install the resistors, and they usually are installed. In case of this picture It looks like R8 and R9 are related, but the picture is too low res to tell what is going on exactly.
Look at your display near A and K pins, or take a good picture.
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Like ataradov says, the backlight is separate. In the picture you linked, R8 is a current limiting resistor, but it may be a 0 ohm resistor or a very low value by default, so it would allow too much current to go through the backlight.
You can add a resistor in series, on either the anode pin or the cathode pin, doesn't really matter which one as long as there's a resistor in series.
You have the formula V=I/R so ... the resistance value is basically [Input voltage - Forward voltage of led(s) ] = I * R .... with a 3.2v led and 5v input and 20mA (0.02A) ....you have R = (5-3.2) / 0.02 = 1.8 / 0.02 = 90 ohm, so a 100 ohm resistor would be good enough, or a 82 ohm resistor would also work if there's already another small resistor on the back of that display.
You could also have a 47 ohm resistor in series with a 100 ohm trimpot (tiny potentiometer) which would allow you to adjust "brightness" between 47 ohm and 147 ohm, so you have super bright or dimmer. Values are random, you can pick whatever values you want.