About LCD bias in hd44780

[Buriedcode]

hd44780 contrast

Hi,

I've got several LCD character modules, varying in size and price, none of which are 'wide temperature range', so they don't need a negative bias. I've used the usual voltage divider on the VO/VEE pin, to give 0.1-0.9V. But..

I read somewhere on google (EDN magazine?) about supplying a negative voltage to VEE to improve contrast, even on a 'standard' temp module. What I'm after is an LCD module with exceptional viewing angle, and the best contrast possible, but I can't find character modules (16x2 - 4x20) that have exceptional contrast, Ive got a few FSTN modules, but its contrast is nothing compared a TFT mointor.

I understand that these modules are entirely different (STN/FSTN) to TFT's, but I'm sure that the contrast for a cheap module can be improved. Hopefully so that when its negative, the characters are really bright (with a huge backlight) and the background jet black .

So really, I'm asking, has anyone provided a negative bias voltage (-5 to -0.1v) to a standard LCD module that DOESN'T actually require a negative bias? I did read on a german website that the 'wide temperature' modules, when driven properly with a negative voltage, have brilliant contrast and viewing angle - so why don't the standard temperature ones?

Hope someone can help, I don't like asking silly questions, and I'm fully aware that I can't change the optical properties of an LCD, but I'm sure some 'tweaking' is possible. Also, if anyone knows of some cheap, graphic (or character) monochrome LCD's with excellent contrast, please tell me. The best I have is a nokia TFT lcd, but thats only 1.5" :/

regards,

BuriedCode.

 

[philba]

lcd bias

I don't believe that only wide temp LCDs need neg bias voltage. I have an older module that didn't say wide temp but the spec calls out -V bias.

do you have specs for these? probably not...

You seem to imply that you only took bias up to .9V. Most spec's I've seen call for it going all the way up to +5. I'd try that first before applying negative V to the pin.

 

[Buriedcode]

hd44780 negative voltage

Hi, thanks for your response.

You seem to imply that you only took bias up to .9V

Well, using a potentiometer, (10k) I've varied it over the entire voltage range, 0.1-0.9v was that range in which the pixels are fully on, to full off. Over about 1v the display is blank.

I have also read a few datasheets for the modules, and some say 'negative voltage provided on board'. I have a 320x240 STN module that requires something silly like -12v, but I assumed that was because the module was so large.

I'll do some experiments I have a few cheapo ones, so if they go south its not all bad. I'm mainly concerned with character modules, 2x16 - 2x20. The negative (transmissive) displays especially. I guess I'm trying to find an LCD substitute for an OLED display, but I doubt an LCD could ever reach the contrast ratio of one of those.

Thankyou,

BuriedCode.