1. I had made a project in which 16x2 lcd is interfaced with couple of other functions.

2. After some hours(varies) of functions correctly i had seen that lcd backlight starts going bit dim. Ten it fluctuates. It goes high & low repeatedly.
However text on it is printed correctly.

i assume:
1. Since text is printed correctly so my lcd code is fine. & it has nothing to do with lcd backlight going dim.
2. I checked 15 pin of lcd on CRO it shows correct +5V all the time.

What could be problem??

 

LED or electroluminescent?

 

Without a schematic and parts list it is hard to check.

The alphanumeric LCDs I use (and typically what you get with these) bring the LED directly out onto two pins. It is the connecting circuits job to supply proper power, meaning yuo need a dropping resistor to limit the current. While I would imagine 5V on a LED would blow it immediately you may just be overpopwering it, and after some time it heats up and acts flacky.

 

yes I haven't connected any current limitor.
My ckt is attached

 

Add a 220 ohm resistor in series with the Led pins. You need to limit the current or you need to use a constant current circuit to supply power to the LED. About 20 mA.

 

Almost all LCDs have resistors on the PCB, and I never saw one that needs 220 Ohms.
Usually its a blue light or 2 green LEDs in series.
There are some which only have one high efficiency LED too.

Is it unusally bright? Can you measure how much current it draws? How is it composed? whats the color? Have you checked the connections?

 

@takao21203 ,
I goes dim after some time & then fluctuates i.e light goes dim.
Haven't measured its current.
Connections diagram are ok, checked multiple times.
It has yellow light.

@GopherT , I will put a 220 ohm resistor & will see

 

Almost all LCDs have resistors on the PCB, and I never saw one that needs 220 Ohms.
Usually its a blue light or 2 green LEDs in series.
There are some which only have one high efficiency LED too.

Is it unusally bright? Can you measure how much current it draws? How is it composed? whats the color? Have you checked the connections?

It is not common to need one but some do. 220 is likely too big but a place to start. At least tithe forward voltage can be measured and you will know if one (about 2 volts) or two (about 4 volts) LEDs are in series. Or if a white LEd is used (about 3 to 3.3 volt forward voltage).

See middle of second page on this datasheet - says a resistor or current source is required.
http://www.lcd-module.com/fileadmin/eng/pdf/doma/dip162-de.pdf

 

power supply bad?