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Backlight Issue

deranged

New member
I read here that an lcd backlight requires 4.2v and I was driving it with 5v, after putting in a 22k resistor it dropped it down to 4.2v but now the lcd backlight doesn't power up now, it does if I cannot 5v straight to it. I am not using a crystalfontz lcd but I didn't know where to ask, if anyone could help me that'd be great. It is a dv-16100 lcd with an led backlight.

Thanks
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X7JAY7X

New member
22k is wayyyy to much resistor. There is a tolerance on the backlights. 5v should be fine. There is a formula on here somewhere to figure the resistor out, it shoudld be below a 100 ohm depending on what power supply you have.

I think I am using a 10ohm resistor on mine.
 

deranged

New member
also I won't be using a potentiometer, I have no idea how many mA it draws to figure out the formula. I just want to make sure 5v won't blow the backlight on the lcd. I just ordered a varitronix here is the link to their backlight guide http://www.varitronix.com/catalog/BkLgGuide.html it is a led backlight for the one I ordered. It says 4.5v max for voltage. I don't want to blow the lcd because it is covered by warranty if too much voltage is applied. Thanks
 

CF Tech

Administrator
5v can damage the backlight if it is applied without a dropping resistor.

You need to find out the rated current and voltage for the backlight, then we can help you calculate the resistor.
 

X7JAY7X

New member
CF some cheaper lcds specs do not give a backlight power, they are assumed at 5v. I have been running mine for a while now with a 10 ohm resistor and a 5v power supply, no problems at all.

If this lcd cost more then I would be more specific in what power is applied.
 

CF Tech

Administrator
No specific currents are given in that link, except a 10mA and 15mA typical. The 565 is a wavelength (nm, not mA).

If you assume 10mA (to be on the safe side):

(5v - 4.2v) / 0.010 A = 80 ohms

(pretty close to the 100 ohms X7JAY7X gave as "safe")
 

deranged

New member
but when I use a 100 ohm it doesn't drop the voltage down to 4.2v it doesn't even drop it down to 4.5v is it still okay to power the lcd with it? Thanks you have been a big help. I couldn't figure it out without you guys.
 

CF Tech

Administrator
When you hooked up the 5v, the LEDs were damaged, and now their forward voltage is 4.5v instead of 4.2v.

They often shift color, become dimmer and sometimes become intermittent after having too high of a voltage applied across them (which forces too much current through them).
 

deranged

New member
ok, but I am getting my new display soon should be here in a few days and the specs are 4.2v with the 100 ohm resistor in place when I read the voltage it shows a slight drop in voltage from 5v but it doesn't drop it all the way down to 4.2v or 4.5v if I connect my new lcd with the 100 ohm resistor in place even though the voltage is still higher than 4.5v will it damage the leds? the old one I have is functioning with the 100 ohm lcd, it is just dimmer. Just don't want to ruin the new lcd.
 

deranged

New member
will the resistor prevent my lcd backlight from blowing if 5v is going to it or should I use something to limit the voltage, if I do how abouts do I do that? thanks.
 

deranged

New member
was reading on the manufacturers website and it says not to drive the lcd with dc voltage. Should I be driving it with ac? also will the led baclight drive off of ac if that is the case? thanks
 

X7JAY7X

New member
I have never seen an AC LCD, CF would know more about this.

I thought your LCD worked fine before, you just had a problem with the backlight??
 

CF Tech

Administrator
If they are saying AC, then it pretty much has to be an EL or CCFL backlight--both of which would need to be driven by an inverter made for that purpose.
 

deranged

New member
No it is definitely an LED backlight I made sure, I didn't want an el or a ccfl, also it turns out the current is 240ma or 480ma max at 4.1v. so that should equal a 40ohm resistor right? or there abouts? also now I cannot find where it says about the AC, so if it LED backlight then it should be DC for it right? thanks.
 

deranged

New member
also does it matter if the multimeter still shows 5v instead of 4.1v with the resistor installed? Just want to make sure that the voltage will not destroy the backlight. Thanks
 

Ifyh

New member
LCD's require an AC drive voltage with virtually no DC component. Segments are controlled by the magnitude of the AC voltage across the LCD segment, but there must always be AC voltage across ALL segments of the LCD.

Prolonged DC operation may cause electrochemical reactions inside the displays which will cause significantly reduced life. The initial indications of display degradation because of excessive DC current is a loss of alignment along the edges of some of the characters. The visual indication will be a "fuzzy" appearance of some of the characters.
 
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