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it's necessary to put a resistor between pin 15 and +5V?

[Reply #1 on:]

it's necessary to put a resistor between pin 15 and +5V?

Short answer: Sometimes.

Long Answer: The LCD has a 14 pin interface.  The additional pins are for the LED backlight which is a different issue entirely.  LEDs requires some sort of current limiting and this is typically implemented with a series resistor.  Some display manufacturers put this resistor on the pc board and some require you to provide an external resistor.

If you provide us with a link to the display documentation we may be able to determine which situation applies to you.  Otherwise try a 150 ohm resistor and see what happens.

Don

[Reply #2 on:]

Thank you
the model is qc1602a, I think it's a quite common type of display

I found this:

https://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDQQFjAA&url=http%3A%2F%2Fe-licktronic.com%2Fattachment.php%3Fid_attachment%3D20&ei=pg9CUan8M4uSswb-6IHADA&usg=AFQjCNFVaU0zbOkEn1MlN_ZnUQt3snl2DA&sig2=_zdNpGa3avgkf33jRJISsg

[Reply #3 on:]

The datasheet suggests that the backlight connections are driven from 5V. However, I suggest you check. You can normally find the connections to the LED itself on one side of the display. Either trace the connections from them to pins 15 and 16 looking for series resistors, or use a multimeter to see of these is any resistance between the LED connections and pins 15/16.

[Reply #4 on:]

My comments are essentially the same as those of dc above, I started them a while ago and got interrupted before I could finish.

In my opinion the documentation that you have is ambiguous as far as the backlight is concerned. 

If you look at section 6 it says that pin 15 should be connected to +5 V implying that no resistor is required.

On the other hand, if you look at section 5 they say that the typical* LED current is to 120 mA but they do not say what limits the current to this value and they show no resistor in the diagram of section 4. 

The best thing to do is measure the backlight current and make sure that it is below 120 mA.  You might also find that the backlight will be sufficiently illuminated with less current which will make things easier on your power supply.

Don

* The column heading 'TYPE' is most likely the Chinglish abbreviation for 'typical'.

[Reply #5 on:]

Thank you very much!