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Author Topic: EL Backlight inverter  (Read 2064 times)
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metal
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« on: February 27, 2013, 09:39:21 21:39 »

Hi,

I was thinking about buying some LCDs with EL backlight, but then I must to consider building the inverters for the backlight.

Is it possible to re-use the boards inside energy saving bulbs? I don't know how these boards work and what they actually do inside the energy saving bulbs, no idea what voltage they output or what frequency, it was just a thought, more info are appreciated. If not, can I re-use the transformer at least, how then?

I don't want to use the 555 and domestic transformers to build the EL backlight inverter, I simply want to use a small transformer, a transistor and few passive components. I sought on the internet, but amazingly there is little info about such topics.

thanks
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pickit2
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« Reply #1 on: February 27, 2013, 09:56:28 21:56 »

how about the inverter from scrap laptop screens, a lot easier than hacking ES bulbs.
most have 12V input and 240V output,also most have logic on/off input.
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zab
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« Reply #2 on: February 28, 2013, 06:00:04 06:00 »

The spec. are required for that ,any how the basic idea of design can be taken from this site.http://www.electronicproducts.com/Optoelectronics/Designing_an_EL_backlight_system_for_an_LCD.aspx
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Mega32
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« Reply #3 on: March 09, 2013, 09:43:34 09:43 »

Metal Be lazy  :-)

http://www.reichelt.de/Leuchtfolien/EL-INVERTER-08/3/index.html?;ACTION=3;LA=446;ARTICLE=60205;GROUPID=3033;artnr=EL-INVERTER+08

/Mega32
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« Reply #4 on: March 09, 2013, 11:28:28 11:28 »



I think this design with your criteria.


http://www.simplecircuitdiagram.com/2010/01/14/super-simple-inverter/


Transformer in circuit characteristics:
http://datasheet.octopart.com/P-6134-Stancor-datasheet-599.pdf


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titi
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« Reply #5 on: March 09, 2013, 11:57:18 11:57 »

Hi Metal,

Here is some explications to drive EL Backlight: http://www.nerdkits.com/videos/backlight/

a video : http://www.youtube.com/watch?v=NRnqGBisKY8

the schematic :

This allows to dim the light by software.

May be this helps.

Best regards.
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« Reply #6 on: April 08, 2013, 01:28:59 13:28 »

I developed a commercial project some years back that used el backlit lcd character displays. The prototypes worked fine but the customer vanished into the ether so I guess I can do what I like with the design and code.
I used a simple linear tech switcher driving a two diode multiplier and managed by a part of the isr in the pic code. This allowed software control over the el.
You need some form of control. The performance of els deteriorates over time and it is fairly rapid. I maintained a performance parameter in eeprom that could be user adjusted from time to time. Eventually you run out of adjustment then you replace the el. Not hard - its just a sheet of plastic tucked in behind the display.
Note you need to ensure you have displays with semi transparent backplanes.
I still have the code, written in hitech picc and the schematics. Your welcome to it. Just ask.
cheers
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metal
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« Reply #7 on: April 08, 2013, 07:16:54 19:16 »

How can I wind that inductor?

Yes please john.
« Last Edit: April 08, 2013, 07:31:03 19:31 by metal » Logged

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« Reply #8 on: April 09, 2013, 12:57:29 00:57 »

Here is a few code snippets. This was done a long time ago and I am better at it now so no critics please... The code works and it works well with the hardware, see attached schematic. This was all done in smt and uses very little pcb space. Today of course you just buy a chip that does it all but thats now and this was then!

Note that I didn't have an rtos when this was written so it was done the hard way. The el bl isr runs the buzzer too, and some other things.
(The lines of dots are the code I left out)

Note also, the code controls the bl frequency which sets the brightness of the bl. The voltage is set in hardware by a pot and influences the life expectancy of the bl. The el bl is just a light emitting capacitor. Stuff it with ac and it glows - simple.

Hope this is of some use.


Code:
//    ......................
// LCD backlight defines
#define LCDBLPORT  PORTC                                // port for backlight control
#define LCDBLDIR   TRISC                                // direction reg for backlight
#define LCDBLPIN   1                                    // port pin for backlight control
//#define LCDBLPLOG                                     // backlight logic (positive logic)
#define DEFBLFREQ  40                                   // default backlight frequency (Hz/10)
#define MINBLFREQ  10                                   // minimum backlight frequency
#define MAXBLFREQ  100                                  // maximum backlight frequency
//    ......................
// LCD Backlight macros
#define enable_lcd_bl(state)    flg_lcd_bl_en=!!state
#define lcd_bl_ison()           flg_lcd_bl_hi
#define lcd_bl_isenabled()      flg_lcd_bl_en
#define init_lcd_bl(freq)       lcd_bl_dir=B_OUT;\
                                setup_timer_2(T2_DIV_BY_16,5);\
                                set_bl_freq(freq);\
                                enable_interrupts(INT_TIMER2);\
                                enable_lcd_bl(true)
#ifdef LCDBLPLOG
#define set_lcd_bl(state)       lcd_bl_drive=flg_lcd_bl_en&state;flg_lcd_bl_hi=!!state
#else
#define set_lcd_bl(state)       lcd_bl_drive=!(flg_lcd_bl_en&state);flg_lcd_bl_hi=!!state
#endif // LCDBLPLOG                   
#ifdef _FAST_CLOCK
#define set_bl_freq(freq)       bl_freq=freq;\
                                PR2=2500L/bl_freq-1
#else
#define set_bl_freq(freq)       bl_freq=freq;\
                                PR2=1250L/bl_freq-1
#endif // _FAST_CLOCK
//    ......................

void init_proc() {
//    ......................
    // Initialize peripheral hardware.
    init_buzzer();                                      // enable and turn off buzzer
    lcd_init();                                         // initialize the lcd
    init_lcd_bl(DEFBLFREQ);                             // initialize the lcd backlight to default
    enable_global_interrupts();                         // set it all in motion !!
    init_switches();                                    // get initial keyboard switch states
    setup_spi(SPI_MASTER | SPI_H_TO_L | SPI_CLK_DIV_4); // setup spi - must be before init_eeprom
//    ......................

main() {
//    ......................
    while(true) {
//    ......................
        if(read_switches()) {                           // read and process keypad switches
            switch (swt) {
                case 2  : lcd_brighten_bl(TRUE); break; // for testing
                case 10 : lcd_brighten_bl(FALSE); break;// for testing
            }
        }
        enable_lcd_bl(sys_flags.sys.awake);             // set the backlight
//    ......................
}

// Backlight interrupt handler - default 400 Hz for electro luminescent display.
//    ......................
    else if(TMR2IF) {                                       // toggle the backlight drive
        if(lcd_bl_ison()){set_lcd_bl(false);}               // is backlight on, yes, turn it off
        else {set_lcd_bl(true);}                            // no, turn it on
        if(!--tmr_10ms) {                                   // decrement the 10mS timer
            tmr_10ms = bl_freq/4;                           // if zero, set timer to 10ms
            if(!!buz_elaps) --buz_elaps;                    // decrement the buzzer timer to zero   
            if(!!dbc_elaps) --dbc_elaps;                    // decrement the debounce timer to zero
            if(!!auto_rep)  --auto_rep;                     // decrement autorepeat timer to zero
        }
        clrwdt();                                           // good place to tie up the dog
        TMR2 = 0;
        TMR2IF = false;                                     // clear the interrupt flag
    }
//    ......................

// Alter the brightness level of lcd backlight.
void lcd_brighten_bl(unsigned char brighten) {

    if(brighten) {
        if(bl_freq < MAXBLFREQ) {
            set_bl_freq(++bl_freq);
        }
    } else if(bl_freq > MINBLFREQ) {
        set_bl_freq(--bl_freq);
    }
}
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