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Topic: Diode at lcd contrast pin (Read 234 times) previous topic - next topic

surepic

I got assigned a project to port code from 32 bit arm to 8bit avr which was done without sacrificing performance. Next step is to customise hardware that's not needed and also switch from 5v to 3.3v.

Lcd that they are using is NHD-C0220AZ-FSW-FTW from Newhaven display. Its chip on glass. Reading the schematic provided for the project I found a diode connected to contrast pin 2 of LCD.

http://www.newhavendisplay.com/specs/NHD-C0220AZ-FSW-FTW.pdf link to datasheet.


Attached is the part of that schematic with diode.

In datasheet I cant find anything about that plus I never ever used a diode with any lcd on contrast pin was either pot or after determining the value either a fixed 1 resistor or a divider with 2 fixed resistors or sometimes directly to GND.

From datasheet:
pin 2 V5 Adj. Power Supply Supply Voltage for Contrast (approx. 0.6V).

Supply for LCD (contrast) (VDD - V5) 4.1(min) 4.4(typ) 4.7(max)V

this is all the information available in datasheet about that pin and sample circuit with pot.

If that pin is connected to 10K pot or fixed resistor or directly to GND nothing is visible on LCD i.e. its impossible to calibrate contrast.

When I`m putting diode (Schottky in my case) everything is working.

Experiment was done to eliminate all circuitry leaving only lcd and supply voltage + Arduino with lcd sample code.

Whats the purpose of that diode?

Thanks.



Grumpy_Mike

If it is in the data sheet I would follow that. However, the schematic does not show a Schottky diode so I would not use one, just a normal junction diode.

It is probably there to drop a bit of voltage and add a bit of temperature compensation as well.

surepic

#2
Feb 13, 2020, 11:03 pm Last Edit: Feb 13, 2020, 11:07 pm by surepic
I meant that its not in the schematic but in the circuit of the project they provided it is.

I contacted newhaven today they said there is no need for any diode just pot as is in datasheet now am preping debug data that they asked to investigate this "problem" because i tried on 3 lcds and none of them is working without that stupid diode.

Schottky i put because didnt have any other type by hand:-)

If its to drop a voltage then pot must do that as well. In datasheet of nt7605 (chip in that lcd) it shows resistor ladder the last pin is v5 which is connected either to gnd directly or to pot for contrast adjustment.

herbschwarz

My guess is that the designer thought the diode
is smaller, easier to connect, cheaper than
a pot and provides "good enough" contrast.
Herb

Grumpy_Mike

Quote
If its to drop a voltage then pot must do that as well.
No it won't. A diode voltage drop is almost independent of current where as a resistor drops voltage directly proportional to current.

surepic

Good point!

I switched to 3.3v lcd from same manufacturer today put pot as usual and everything starts to work.

Investigation for 5v lcd problem is ongoing - sent requested debug information to manufacturer as per their request.

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