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


1 comment

  • ngatgi

    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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