Negative Bias for Graphics Displays

1
2
Page 1 of 2

nwrightson

Benutzer

Gender:
Location: Newcastle
Age: 62
Homepage: nweha.homeserver.c…
Posts: 362
Registered: 08 / 2003

Subject:

Negative Bias for Graphics Displays

· Posted: 29.08.2003 - 14:06 · #1

Hi,
I've started using LCD Graphic displays (240*128) and all is going well except for the contrast. It changes a lot with temperature.
I can easily read the temperature using a DS75 I2C temp IC. Now I need to look up the required voltage and adjust the negative bias supply to the require amount.
So the question is, do you people have a preferance for a processor adjusted negative bias supply.

Thanks .
Neil.

Administrator

Gender:
Location: Germany
Age: 24
Homepage: e-lab.de
Posts: 5558
Registered: 03 / 2002

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 29.08.2003 - 20:50 · #2

Hi,

in the next version upload there will be an hard and software example
about controlling a positive and a negative voltage via the PWM of the
AVR.

rolf

nwrightson

Benutzer

Gender:
Location: Newcastle
Age: 62
Homepage: nweha.homeserver.c…
Posts: 362
Registered: 08 / 2003

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 25.07.2011 - 09:00 · #3

Hi Rolf,

Where is the above information?

Neil.

Administrator

Gender:
Location: Germany
Age: 24
Homepage: e-lab.de
Posts: 5558
Registered: 03 / 2002

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 25.07.2011 - 20:51 · #4

Hello Neil,

sorry. 8 years ago. I can't find any info about this.
But a PWM to voltage converter is not very difficult. Simply integrate the PWM output with an RC filter and then pass it to an OP-Amp.

Negative voltages can be achieved with a C and two diodes inserted before this filter.

Higher voltages can be build with a diode ladder (simple charg pump) like used in old television devices.

rolf

nwrightson

Benutzer

Gender:
Location: Newcastle
Age: 62
Homepage: nweha.homeserver.c…
Posts: 362
Registered: 08 / 2003

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 28.07.2011 - 09:30 · #5

Hi Rolf,

Yes it has been a while. You used to have it on one of your schematics.
Oh well, I'll keep looking.

Neil.

Administrator

Gender:
Location: Germany
Age: 24
Homepage: e-lab.de
Posts: 5558
Registered: 03 / 2002

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 01.08.2011 - 12:28 · #6

Hello Neil,

accidently I found my old test programm for a negative bias converter. But no hardware infos yet.

Code

(*******************************************************)
(* This program uses a PWMport, two diodes and a choke *)
(* to produce a negative voltage out of the 5V VCC     *)
(*******************************************************)

program Inverter;

{ $BOOTRST $01C00}         {Reset Jump to $01C00}
{$NOSHADOW}
{ $W+ Warnings}            {Warnings off}

Device = mega644, VCC=5;

Define_Fuses
//  Override_Fuses;
  NoteBook  = C;
  COMport   = USB;
  Supply    = 5, 200;
  LockBits0 = [];
  FuseBits0 = [SUT0];
  FuseBits1 = [SPIEN, JTAGEN, OCDEN];
  FuseBits2 = [BODLEVEL0, BODLEVEL1];
  ProgMode  = JTAG;

Import SysTick, SerPort, PWMport1A, PWMport1B, ADCPort;

From System Import ;

Define
  ProcClock      = 16000000;       {Hertz}
  SysTick        = 10;             {msec}
  StackSize      = $0032, iData;
  FrameSize      = $0032, iData;
  SerPort        = 19200, Stop2;    {Baud, StopBits|Parity}
  RxBuffer       = 20, iData;
  TxBuffer       = 20, iData;
  ADCchans       = 3, iData, Int2;
  ADCpresc       = 128;
  PWMres1        = 8;              {bits}
  PWMpresc1      = 1;
  PWMmode1       = fast, negative;

Implementation

{$IDATA}
const

{--------------------------------------------------------------}
{ Main Program }
{$IDATA}

begin
  EnableInts;
  loop
    //  Load = 60mA     coil   47uH
    PWMport1B:= 90;     // ->  -2.4V an ADC @60mA

    //  Load = 10k      coil    220uH    >470uH<    1mH
    PWMport1A:= 6;      // ->  -3.1V      -2.5V     -2.5V
    mDelay(4000);
    PWMport1A:= 13;     // ->  -5.0V      -4.2V     -3.1V
    mDelay(4000);
    PWMport1A:= 25;     // ->  -8.5V      -6.1V     -3.8V
    mDelay(4000);
    PWMport1A:= 50;     // -> -17.0V     -12.4V
    mDelay(4000);
    PWMport1A:= 75;     // ->            -16.2V
    mDelay(4000);
  endloop;
end Inverter.

For heavy loads maybe it is necessary to use a small MOSfet driving transistor.
But for these very low currents drawn by LCDs this maybe not necessary.

rolf

nwrightson

Benutzer

Gender:
Location: Newcastle
Age: 62
Homepage: nweha.homeserver.c…
Posts: 362
Registered: 08 / 2003

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 03.08.2011 - 03:43 · #7

Hi Rolf,

How were the diodes and inductor connected?
Was a capacitor used for filtering/charge storage?

Neil.

P.S. I'm much better with digital and programming. I've never created a negative charge pump before.

miparo

Administrator

Gender:
Location: Germany
Age: 59
Posts: 956
Registered: 09 / 2007

Subject:

Re: Negative Bias for Graphics Displays

· Posted: 03.08.2011 - 09:51 · #8

Hi nwrightson,

http://birkler.se/www/Projects…e_Download

1
2
Page 1 of 2

Negative Bias for Graphics Displays

Registered users in this topic