LCD Display Backlight drive question

[Mark_R]

Hi,

Intigrating a 20x4 LCD display into a project, hoping someone could take a look at the attached schematic and make sure I'm not doing something stupid with the transistor for the backlight drive....

**broken link removed**

Cant get a definitive answer from the datasheet, but I believe that a current limiting resistor is not needed (provided on the LCD), correct?

datasheet lists LCD max current at 60mA.
Hfe of transistor is 100

Thanks.

 

[Boncuk]

Hi Mark,

connecting the backlight without current limiting resistor it will fry very quickly.

You didn't post the LED forward voltage. It should be somewhere close to 3.5V.

Use a 47Ω resistor and if the backlight is too dim reduce the value to 27Ω.

Boncuk

 

[Gobbledok]

Most of the character LCD's I've used do have internal resistors, but connect with a resistor first and measure the current just to be sure.

In fact a couple I've used have the anode and cathode of the backlight connected to the supply for the LCD = can't do backlight dimming.

 

[Mark_R]

Found the app note at the manufacturers website;

https://dl.dropbox.com/u/23138954/LCD%20app%20note.pdf

Resistor is required.
Vf=3.5V
Inom=60mA

Rl =25Ω

 

[Mr RB]

Check the back of the LCD near the backlight LED (which is normally quite easy to find). See if there is a little SMD resistor attached to the LED.

It's very rare for new LCD these days to have no resistor! Unless you bought an old "surplus" LCD or something.

 

[Mark_R]

Check the back of the LCD near the backlight LED (which is normally quite easy to find). See if there is a little SMD resistor attached to the LED.

It's very rare for new LCD these days to have no resistor! Unless you bought an old "surplus" LCD or something.

I don't have one yet, I'm designing off the datasheet. This one seems to be very popular based upon stock levels at DigiKey.

 

[Mr RB]

You may find when it arrives that it includes a resistor, datasheets are often old and if they have specs for the backlight LED it may be an old spec or the spec might just be for the LED, and ignores the fact that someone has since added a resistor on the PCB in one of the more recent production batches.

 

[Mark_R]

You may find when it arrives that it includes a resistor, datasheets are often old and if they have specs for the backlight LED it may be an old spec or the spec might just be for the LED, and ignores the fact that someone has since added a resistor on the PCB in one of the more recent production batches.

Just received email from factory, unit has no on board resistors. Needs 27 OHM external @ 5v.
Optrex model # C-51847NFQJ-LW-AAN

 

[Mr RB]

Interesting! Thanks for the info. I've got LCD samples here from quite a few manufacturers and it's been a long time since I have seen one with no resistor.