Setting contrast using component pads on LCD module - NHD-24064WG-ATMI-VZ

ngatgi

• April 03, 2018 03:07

The contrast of the LCD module referenced in the subject can be adjusted by adding a variable resistor between the VEE (negative supply), GND and Vo (input for contrast adjustment).  When tracing where the interface header pins are connected, there appears to be resistor pads that will allow either a variable or fixed contrast setting directly on the module.  As this display is replacing a different type of display, I don't want to require external components for this purpose.

LCD Module Datasheet

https://newhavendisplay.com/content/specs/NHD-24064WG-ATMI-VZ.pdf

The component locations in question are R6, R7, VR1, R81, R82 and Q2.  The stock configuration has 0Ω jumpers at R6 and R7 and I believe Q2 is a PNP 2SB1198K based on the device markings.  VR1, R81 and R82 are not populated.  I have included pictures of the actual PCB and a hand-drawn schematic as I see it.  Note that there is one node in the schematic that I couldn't determine it's connection, but it would appear to be a supply connection with voltage greater than VEE.

I am sure there must be a configuration that would only require a change in population (no cuts or jumps) otherwise why would they bother to include all these component locations ... ESPECIALLY when you see the pads for the variable resistor (VR1)!

The transistor is what is throwing me.  I would like to know what it's purpose is in the default configuration.  If R6, R7 and Q2 were removed, a fixed voltage divider can be created using R6 and R81 and a 0Ω jumper at R82, just as recommended for the external voltage divider.  Obviously, this could be adjustable by populating a pot for VR1 and not populating R6.

If any of you have been down this road before, I would appreciate you sharing your experience!

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  ngatgi

  • April 04, 2018 15:40

  I think I figured this all out.  Basically the PNP transistor is an emitter follower used to provide a low-source-impedance for the "GND" supply pin of the quad op-amp which buffers a 5-resistor divider for the LCD bias voltages!  It also directly provides one of the bias voltages to the 4 LCD driver ICs. 
  
  As I see it, there is no great re-population option using the existing pads.  I'm not a fan of adding resistance in series with op-amp supply pins, but especially in this case where the op-amp current over temperature may change the static bias condition which may have a noticeable affect on the contrast ... but it might be ok ... I just haven't tested it either way.  Another possibility I've considered is to add a resistor between the PNP base and VEE as there are pads that would allow for easy component placement, however this may have the same bias current issue (maybe worse?).
  
  It seems as though one of the manufacturer's intended options would be to remove the transistor, 0Ω jumper R81, and replace R6 with some resistance that would achieve the desired contrast, again with the op-amp bias current "potential issue".
  
  All of that said, eliminating the op-amp bias current as a source of variability would be as easy as connecting the GND pin directly to VEE after disconnecting it from the resistor divider.  Since the supply delta would be higher than it "needs" to be, I suppose there may be more heat generated, but again, I haven't tested that.  Note that the V5 connections of the LCD drivers still need to maintain this connection for proper LCD bias.
  
  Again, if anyone has any results related to the options above or others, please share!!!
  
  

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