# What voltage Vo does my LCD requrie?

I have a Powertip "PC1602F B" LCD I bought a while ago for a project, and have long lost the design specifications for it. The datasheet is rather confusing, listing several different voltages. Do I need a negative voltage? What should the range of adjustment for contrast be? Is the voltage relative to Vdd? (which will probably be 3.3V.)

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Yes, that datasheet is a mess. If this is a hobby project, experiment carefully. If this is a real commercial project, ditch that LCD and get one that is properly specified. In general, voltages are with respect to ground unless otherwise specified. However, that datasheet leaves it unclear what exactly ground is. It lists Vss, Vee, and a negative supply for the backlight. It also seems to use different names for the pins as in the electrical specs section. No thanks. –  Olin Lathrop Dec 19 '11 at 0:32
Try putting a say 10 k $\Omega$ potentiometer between Vdd and GND and connect the center tap to the contrast and see on which position the contrast looks good. In general it should be between Vdd and Vss. Unfortunately this datasheet doesn't specify limits for that, but on such screens it's usually around Vdd+0.3 V. You don't need negative voltage for this screen from what I see. It definitely looks like any other 2x16 character screen (the standard name for the controller escapes me at this time). –  AndrejaKo Dec 19 '11 at 0:50
I'm looking for the contrast voltage. My LCD doesn't have a backlight. In particular, "LCD driving supply voltage", -0.3V to 13V has me confused; why such a large range? –  Thomas O Dec 19 '11 at 3:16

As Olin says, the datasheet is pretty terrible. The standard of display datasheets seems to be quite low in general IME.

It states Vss = GND, and Vdd - Vss max is 5.5V.

The LCD driving voltage looks to be internal so don't worry about that.

The Backlight doesn't need a negative supply, the A and K are for anode and cathode of the LED. It states either 40mA or 120mA for the BL current consumption, depending on the module type - in the mechanical specs table it specifies either no backlight, an EL backlight or LED backlight. I'd assume the edge and array in the electrical specs refer to the EL and LED respectively.

So if we assume your model is the EL, you need 40mA to drive it. You will need to confirm this and adjust the below workings as necessary if it's the 120mA version - if you don't have the info on the package it came in there is probably a code printed on the PCB somewhere. Either way it's safer to assume it's the lower current version to start with if not entirely sure.

You need to know the Vf of the LED, which is not clearly stated, but VB/L = 4.2V is given which is possible for a white LED.
You could measure the drop if you have a multimeter with diode setting available.
So, assuming the LED Vf is 4.2V, you are running the Vdd from 5V and need 40mA for your edge LED, you get (5V - 4.2V) / 0.04A = 20 ohm resistor needed between Vdd and A (K to ground)

For Vo I can't see anything at all, but I doubt it needs a negative voltage (unless they are unbeliveably sloppy - possible)
I would try setting up an e.g. 1k resistor from Vdd to top of 1k pot, with it's other end to Vss with it's wiper to Vo. IIRC usually the contrast for these LCDs is controlled over a pretty small range around 0 to 1V or so. This setup will give you from 0 to 2.5V, start with wiper to ground and adjust if necessary.

If you're feeling lucky you could just tie Vo to ground (through a smallish resistor if nervous) and see if it works.

In the past with a couple of non critical debugging type setups using any old LCD (usually no datasheet handy) I have just tied the Vo to ground and it worked fine - of course this may not be the case for your particular model but I'd be surprised if it was out of the 0 - 2.5V range.

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Vss is ground, which by definition is 0V. Therefore, Vdd - Vss = Vdd - 0V = Vdd. Vdd must be greater than 2.7V and less than 5.5V.

I'm not sure about the back light voltage, but, that should get you started. If this is for a hobby project that should be sufficient. If you want the backlight then try to power it with an adjustable supply. I think if you keep it under 5V you will not damage the display, but, I am guessing from clues on the data sheet. If you do fry the thing then toss it and buy one that is specified better.

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The real confusion is that left most pin is 14 then count down to pin 1 as you move to the right followed by pin 16 then 15 . Go figure . Caution when creating pcb footprint as you may pop a fe boards.m

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