I have a LCD module . when I connect the arduino mega r3 on pin 5v . arduino LEDs are turned off , it turns off completely. it could be ?.
It has not happened with other modules. must be energized separately?
Hi and welcome.
First of all, check for short circuits and reversed polarity.
You can't just connect anything to anything and expect it to work.
You need to find out some specifications before you do anything like that.
Your Arduino is either powered through USB (typically 500 mA allowed), or by an external power supply and the "barrel jack".
If you power it externally, then the on-board regulator will be set to work.
It will try to convert whatever higher voltage (7 - 12 volts preferred) to 5 volts so your Arduino can run within specification.
That regulator can handle the Arduino itself, and has a small reserve which can be used for some small peripherals.
The datasheet (click !) tells about the power consumption.
If you are using the backlight, that will take 210 mA at 4.2 volts.
That's probably too much for the on board regulator to handle.
There's no indication in the datasheet about a current limiting resistor for the backlight.
So add that.
If you're still powering it form 5 volts, you need the resistor to drop 0.8 volts.
And if you want to reduce the current to 100 mA, that would result in a 8 Ohms resistor (you'll end up with 8.2 Ohms).
The backlight will be a lot less bright, but the current will be limited to a safe value.
An other standard value is 15 Ohms, which would make 53 mA and barely visible backlight i guess.
To the OP, snakeve:
What is the value printed on resistor R8 on the back of the module? Can you cite the website from where you purchased it - a picture would be useful for reference.
MAS3:
An other standard value is 15 Ohms, which would make 53 mA and barely visible backlight i guess.
A quarter of the nominal backlight intensity would be perfectly visible.