Need some help with this 20x4 LCD

MarkS

I just ordered this LCD from Digi-key: Optrex 20x4. Data Sheet

I have questions about the back light. I *THINK* that this is covered on page 6 of the data sheet, but I'm not sure. Anyway, what voltage do I drive the back light with? There are two back light connectors, two pins on the main 16-pin connector and then a separate 2-pin connector. I'm assuming that they both need to be connected to power, but it's not mentioned in the data sheet.

Also, what the heck is note 4 on page 7 referring to?

Post Edited (MarkS) : 2/15/2008 6:09:15 PM GMT

Bruce Bates

Mark -

I suspect that the reason why two sets of back light connectors appear may be just for convenience, as they seem to be the same to me. I can't see how it would hurt to connect them both, but I also don't see the necessity of doing so.

The circuit shown on Page 9 will show you what is required for the contrast circuit. I'm not sure what Note 4 on Page 7 is either, but I wouldn't worry about it too much unless the LCD doesn't perform as expected.

Regards,

Bruce Bates

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MikeK

1. Backlight voltage -- The backlight is a white LED(s). It drops 3.5V (typical) at max 30ma. You need to put a current-limiting resistor in series with whatever voltage you drive it with to drop the extra voltage at the current you choose (start with 10-20ma). DON'T drive it directly without a series resistor.

2. According to the spec sheets, the two sets of connectors for the backlight are the same, as Bruce said.

3. Ignore note 4. I think it has to do with the voltages applied to the LCD connections internal to the board.

Mike

MarkS

OK, so I try and drive the back light, which works, but adding a resistor of any size dims the back light far too much. The LED voltage drop is either 3.5v as Mike said, requiring a 120 Ohm resistor for 5v or 1.5v which is what I understood the voltage drop to be, which requires a 51 Ohm resistor. The only way I could get full brightness was to drive it directly from the Prop's 3.3v source. This leads me to believe that the voltage drop from 5v is, in fact, 1.5v (5v - 3.5). If this is the case, couldn't I just drive the back light directly from the 3.3v source? I plan on having a software controller brightness control via PWM (which I also have tried and it works).

Post Edited (MarkS) : 2/21/2008 3:36:58 AM GMT

MikeK

With a 5V supply, a 50 ohm resistor should have been bright enough (but wasn't, see below). Connect it, then measure the voltage across the backlight terminals. The way I read the spec sheet, it should be around 3-3.5V (a white LED I've got measures 2.97V).

I just looked at the spec sheet again, and the drawing that you can get to from Digikey's page. It looks like there are two LED's in parallel, EACH of which can be supplied with 30ma. In that case, you want 60ma (maximum) from your supply, which translates to a 25-33 ohm (minimum) resistor with a 5V supply or a 5 ohm (minimum) resistor with a 3.3V supply (assuming that the backlight voltage is less than 3.3V).

Try 5V with a 33 ohm resistor. That should work if you've got a 3.5V drop across the backlight.

MarkS

OK, I tested it like you stated, and it's showing 3.9v with a 5v supply. With it dropping 1.1v, that comes out to around 37 Ohms.

MikeK said...
It looks like there are two LED's in parallel, EACH of which can be supplied with 30ma.

Where did you find this information in the data sheet? All I can find are two sets of LED connectors on the PCB, but I cannot find a reference as to whether both are needed.

MikeK

Near the bottom, where it says LED Circuit. http://www.optrex.com/SiteImages/PartList/DRAWING/51847add_drawing.pdf
In the various references, the pins are labeled A and K, for LED anode and cathode. All implications are that the two A's and the two K's are the same.