Powering a TFT LCD

**claylong** · 03-07-2002, 11:09 AM

I have a few questions on powering a tft. First, I plan on using a keypower DC-DC psu for the computer and components, but I do not think i want to use it for the display, heres why.

From what I can figure out, the only component of a tft screen that needs power is the inverter. I have not found the pinouts for the LCD itself yet, but am I to believe that it does not need a power source?

The second issue is powering the setup. Even if the inverter is the only component this setup looks like it might be a bit difficult to get running. Now i have a TDK inverter for a LQ10D421 Sharp 10.4 LCD, and the inverter is rated 7W dimming (dual output). I also have the specs sheet on the inverter. The input voltage is approx 9-14V (12v nom), and pulls at most 1.2 A. That is not the hard part, and in fact I might be able to pull this from the PSU. The hard part are the other two voltages needed to run this thing. Here's what i mean....

The dimming funtion can be preformed with a potentiometer, where 0v = Max brightness and 3.4v = Min brightness. The 3.4 volts has to be supplied, and the only place I can think to get this voltage (short of some sort of built supply) is the 3.3v line of the PSU.

Also, there is an Enable/Disable line that turns the backlight ON and OFF. That line needs anwhere from 5v to 7v for ON, and 0v for off. Again, could the 5v line be used from the PSU?

Best i can tell from the pdf, the two functions above pull minimal current. I am not opposed to builing a small psu just for this screen, though I think it can be done w/o going through the trouble. Power gurus, lemme know what you think.

**zootjeff** · 03-07-2002, 11:49 AM

TFT displays require a power source. Probably 3.3 volts. It isn't very much power compared to the inverter.

If you spent the money on the keypower, I would just use it. It is probably going to have more than enough power for your setup.

You can run all those controls off of the 12 volt line if you use resistor dividers and pots. What do they require? microamps?

IE if you use two 1k resistors in series between 12 volts and ground, you have 6 volts. If you use a 1k and a 3k in the middle of them you have 3 volts. if you use a 3k resistor and a 1k pot, you can create your contrast voltage or whatever, and do the dimming voltage in the same manner. If you build a 5 amp supply to supply the 50 microamps that the display requires, don't you think that it is overkill?

**claylong** · 03-07-2002, 01:52 PM

Good point.

While i understand the concept of using resistors to reduce voltage, i do not however understand how you come up with those res values. What kind of eqn do you use? I need a refresher.

Thanks for the info!

**zootjeff** · 03-07-2002, 03:33 PM

5 seconds on hotbot searching for "resistor divider calculator" yeilded:

http://www.eworld.contactbox.co.uk/calc/calcdiv.htm

It is simple ohms law calculations, but if you don't want to do the math, this one does it for you.

**claylong** · 03-07-2002, 03:42 PM

Thank you for the info! I do not want to seem dumb, but I thought the V=IR calculation needed a current value, and i do not know what current it pulls. Thats why i ask.

Thanks again