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CFAX12864CP1 and backlight

geckosenator

New member
Hi, I just got a CFAX12864CP1 It has a backlight. I assumed the backlight was powered by the two tabs on the side, however, when I apply power to them, no current is drawn and the lcd does not light up.

Do I have to communicate with it to turn on the backlight, or is my lcd broken?

According to the datasheet for the CFAX12864CP1-WGH the Drive Voltage for the backlight is from 78 to 85 Vrms, am I confused about this, I only put 5v across it in both polarities with no luck. It also states the Supply Current as 80mA, is this supposed to be uA?

Thanks
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geckosenator

New member
Ok, this explains my problems

If I get the version with the led edge backlight, I won't need a fancy driver? What is the advantage of electroluminescent?

Thanks,
Sean
 

CF Tech

Administrator
The advantage of EL is that it is very thin, and very uniform. As LEDs progress, this advantage becomes smaller.

The LED versions need some care to drive them. At a minimum, there needs to be a current limiting resistor. Connecting 5v across the EL did no damage, but the same thing across an LED would have damaged it.
 

geckosenator

New member
The advantage of EL is that it is very thin, and very uniform. As LEDs progress, this advantage becomes smaller.

The LED versions need some care to drive them. At a minimum, there needs to be a current limiting resistor. Connecting 5v across the EL did no damage, but the same thing across an LED would have damaged it.
I guess it's nice to know I didn't damage it. Does the EL use less power? I couldn't make sense of the datasheet.

What about driving the LED at 3.1v? I am thinking about going with the LED version.
 

CF Tech

Administrator
I think there was a time when EL was the preferred low power approach. It is more complicated though (switching supply, etc) and LEDs (especially white) are more popular for low power applications now.
 
What about driving the LED at 3.1v? I am thinking about going with the LED version.
You would need a current limiting resistor to use the LED backlight, and a voltage source of more than 3.5 volts. From a 5v supply, a 47 ohm 1/4 W resistor would give about 40 mA of LED current.
 

geckosenator

New member
5v is sort of annoying when you don't have it around. I am running everything off a lithium ion battery.

Instead of resistors, why not just drive the led directly with 3.3v? Or maybe put a very low resistance resistor in series at 3.3v. I don't care if the backlight is at 100% brightness anyway. I typically wire up 3 leds in series across a 9v battery, and I have had this setup run for many battery cycles with no problems.

To make the character LCD work I used a single diode and capacitor off the pwm output of my uC to drive VO. Even though the part claims it is 5v, it worked flawlessly with 3v, where VO was pulsed but average -2 to -3 volts.
 

CF Tech

Administrator
LEDs are current devices. Ideally you would control the current that goes through them. The "proper" way to do it is to use an LED driver, but those drivers typically assume that the LEDs are in series, which they are not in this display. The big scare about connecting them directly to the LION cell is that if the cell is on the high side, and the LED's Vf is on the low side, a lot of current could flow. So, you might be able to "get away" with connecting it directly, but it would not be a solid design.

At a minimum, use cosmicvoid's 47 ohm to be safe. The LEDs will still come on and you will not destroy them.
 
Actually, for the listed forward voltage spec of 3.1 v, the LED BL would work from a 3.3 v Li battery by using a 5 ohm resistor (0.2v / 0.04A = 5 ohms). The trouble with this scenario is there is so little headroom. When the battery voltage drops toward 3.1 volts, your backlight will get dimmer, and finally go out when the voltage goes below 3.1 volts.
 
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