LCD - Positive or Negative?

[Valant]

So I happened to find a cheap DMF5005N recently. I spent a few days getting it to work and it looks great. Despite what the datasheet said (or so I think it says), this is a negative contrast voltage display.

Datasheet Here

Notice that in section 3.1 of the datasheet that the "Recommended LCD Driving Voltage" has no negative signs anywhere! So, I assumed it needed a positive voltage for contrast. My power supply disagreed. Lo and behold, with a negative voltage, the LCD works!

What gives? Is there a surefire way to determine what an LCD needs?

Furthermore, I am playing around with another LCD, the DMC-50448N.

Another Datasheet

This datasheet, again under section 3.1, shows positive values for the voltages. But now I'm skeptical. Perhaps this one requires a negative voltage as well?

[Imp]

In both datasheets, there is a schematic under 4.2 which shows how to connect the driving voltage.

It's quite hidden...I'd missed that one too..

The 2nd display needs positive voltage, as Vo is connected to a voltage dividing pot between Vdd(+) and ground.

[julienvoirin]

1st LCD,

section 2.1

Vcc-Vee = 23

if Vcc = +5V then that means that Vee is negative (remember some maths)

Vee sets the contrast

Section 3.2 : typical LCD Driving voltage : Vcc-Vee = +13V

I let you solve this very complicated equation at 3rd degree.

Your contrast is driven by a negative voltage. provide a source of -18V, DC

2nd LCD

use the circuitry of the core to control it. it is a classical CLCD (toshiba driver). just use a 20K trimmer for contrast instead of 10K

Edited August 10, 2010 by julienvoirin

[Valant]

In both datasheets, there is a schematic under 4.2 which shows how to connect the driving voltage.

It's quite hidden...I'd missed that one too..

The 2nd display needs positive voltage, as Vo is connected to a voltage dividing pot between Vdd(+) and ground.

Aha, I missed that too. Thank you! I will read datasheets more carefully.

1st LCD,

section 2.1

Vcc-Vee = 23

if Vcc = +5V then that means that Vee is negative (remember some maths)

Vee sets the contrast

Section 3.2 : typical LCD Driving voltage : Vcc-Vee = +13V

I let you solve this very complicated equation at 3rd degree.

Your contrast is driven by a negative voltage. provide a source of -18V, DC

2nd LCD

use the circuitry of the core to control it. it is a classical CLCD (toshiba driver). just use a 20K trimmer for contrast instead of 10K

The maths - I recall!

Vee = Vcc - 13

I read section 3.2 prior, but I misunderstood it as "Vcc-Vee", like it were some special symbol, but it really is a minus sign.

Many thanks. This makes a lot more sense.

[julienvoirin]

you are welcome :thumbsup: