How are you guys powering the LED backlights on your LCDs? The LED Forward Voltage range on my LCD is between 3.8v and 4.4v, with the typical voltage at 4.1v.
Can I use a PWM output to power it, or do I need to come up with a voltage divider circuit?
I think you can just use a current limiting resistor [ fixed typo ]
Good luck 
Do not use a PWM pin for this, use a current-limiting resistor.
I am using a NewHaven LCD. In the data sheet, it says that there is an on board resistor in series with the LED which allows for safe 5v operation. Check your data sheet to see if protection is already in place.
Hey Anachrocomputer, why would you not want to use a PWM pin for this? If you wanted the screen to fade out after a menu selection was made, wouldn't you want a PWM, or were you just speaking about the protection from the 5v?
I was concerned that if you use PWM, you'll be applying the full 5V across a LED, in short pulses. This could damage a LED! If you use PWM, the peak voltage is always 5V, but the average voltage is variable, according to the mark/space ratio.
Using PWM on an LCD with a built-in resistor, or in combination with an external current limiting resistor, is OK and will allow you to fade up/down the backlight brightness.
Thanks! That's what I'll do. Looks like I need about a 2ohm resistor, > 1w.
In my research I also found a schematic for using a mosfet (powered by PWM) to adjust the brightness of the LCD, but I'll stick with the resistor for now.
Looks like I need about a 2ohm resistor, > 1w
NO do your sums again this is way too low.
@Grumpy_Mike does 33 ohms sound right?
This is assuming 40mA, 5VDC input voltage and 3.8VDC LED voltage
assuming 40mA, 5VDC input voltage and 3.8VDC LED voltage
If that is correct so is 33R.
Actually it is 30R but you won't get that value.
However, with Vf at 3.8 V there must be some LEDs in series and 40mA looks a bit on the high side for an LED they usually are at 20mA.
@Grumpy_Mike
40mA was a guess based on the fact some LCD backlights have 2 LEDs instead of just one and others have a slightly more powerful single LED
@20mA the value would be 60 ohms and the common value is 68 ohms I believe
@JustinHoMi It would help greatly if you could link us to the datasheet
Here's the datasheet: http://purdyelectronics.com/pdf/AND721GST.pdf
I was actually looking at the "Absolute Maximum Rating" for the LED Forward Current, which is 480mA. It looks like the recommended current is 230mA.
If the LED Forward Voltage is 4.1, then my math says a 3.9 ohm, 1/4 watt resistor will be fine. Correct me if I'm wrong!
What is the most common resistor similar to 3.9 ohm?
R=5.217 ohms
I=0.230A ( 230mA )
Vs=5VDC
Vf=3.8VDC
common value = 5.6 ohms
1/2W is recommended
R=3.91304347 ohms
I=0.230A ( 230mA )
Vs=5VDC
Vf=4.1VDC
common value = 4.7 ohms
1/2W is recommended
Awesome, thanks. I found of each in an old modem and a broken audio receiver.
But, haha... it appears as if my previous fooling around with it burned out the LED. Oh well, it was a freebie!
If you have $20 - $30 burning a hole in your pocket you can get a fairly nice assortment ( 2000 units 1/4 or 1/2W 1% 50 values ) of the most common values online
maybe you can salvage it
It works! I just got a new breadboard, which doesn't have all of the ground and +5v bars tied together like my old one did.
It actually saved me, because I previously tried placing 5v and gnd directly to the LCD without a resistor
Thanks for the help!