Thread: LCD Backlight.

Okay, So I finally got an LCD Module (JHD 162A) ... It has 16 connections, the last two of which is LED+ and LED-. I guess it is for the back light... However, I have not yet found any datasheet for it. Do I have to put a current limiting resistor to these to use the LED or is it buit in?

Well you can look at the module's traces to see for sure, but I think it's safe to assume that it does not. The module mfg would not know what source voltage you planned to use thus would not know the ideal resistance. Too low and you still require the user to add more resistance anyways or it'll blow, too high and it won't work adequately on the available voltage.

Looking at the traces, I see a chip resistor designated as 2R2. Will any additional resistor cause a very dim light or so? Is the backlight independent of the other electonics? I mean, can I use the back light without any other connections?

I believe 2R2 means 200 ohms. You can guess the LED voltage from the color, but the LED current rating is going to be a mystery unless you find the spec. I suspect the LED resistor is sized for proper operation at 5v anyways.

LED is going to be completely independent of other electronics, yes.

Originally Posted by Oznog

I believe 2R2 means 200 ohms.

2R2 is 2.2 ohms, this is the world standard for labelling resistor values!, by removing the decimal point it makes it far more clear what the value is.


Likewise 2K2 would be 2200 ohms.

Connect a 220ohm resistor in series with it and measure the voltage across the LED display, if it's near a multiple of 2.2 then it probably doesn't have a a seires resistor, if it's neard the VCC then it probably does.

The datasheet mention the max current to provide to your Backlight
datasheet here : http://www.sunrom.com/media/datasheet/2000-1588.pdf

They mention 100mA as maximum section 4.0.

Usually, when hardware permit, i don't use any external resistor but a Transistor (or else driver) connected to a spare PWM channel.

Originally Posted by mister_e

The datasheet mention the max current to provide to your Backlight
datasheet here : http://www.sunrom.com/media/datasheet/2000-1588.pdf

They mention 100mA as maximum section 4.0.

Usually, when hardware permit, i don't use any external resistor but a Transistor (or else driver) connected to a spare PWM channel.

OK don't listen to me on reading SMD R-vals. Listen to Nigel.
Ah:
LED power supply current ILED VLED=5V, R=8.2ohm - 100 - mA

Sounds like you're supposed to connect an 8.2 ohm resistor in series with 5v. That's a really, really bad idea.

Do NOT PWM a switching transistor instead of a ballast resistor. That's

i used a 100R and it works ok .

Assuming 3v(ish) for the LED array and 5V supply then thats 2V drop across a 100R resistor (plus the 2R2 on the PCB!) which would give a current of 2(V)/100(R) = 20MA. a nice safe current for your LCD.

Most LED's are ok up to about 30MA, so you may be able to "ramp it up" a bit so using ohms law (again?!) this give us R(esistance)=V(oltage)/I(Curent) or 2(V)/30(MA) = 66.666666(etc) Ohm's so about 66R

You could use PWM to control the brightness from this max value of 30MA, this will allow you to fade from max to no light.

Oh and no come backs if it goes BOOM!

Here is a link that may help you/others
http://ourworld.compuserve.com/homep...Bowden/led.htm

Hope this helps.

I powered the display without the resistors for a few seconds from a +5v source. The backlight lit brightly. Can't say if it was unsually bright or no....

Originally Posted by Oznog

Do NOT PWM a switching transistor instead of a ballast resistor. That's

Do you mean to suggest that the PWM is not a substitute for the current limiting resistor?

Anyway, I guess a resistor won't harm.... But I thought inbuilt resistors would be most handy.

No PWM is not!! think about if, if the pic locks up, for any reason, boom/fizz, goodbye backlight PWM is used to control between max and min, that's all.

I hope this helps!

Well if you only have High or low then it's 50%, if you have High, low or highZ then it's 33%, too close for comfort for me!

The other problem is that I am assuming that the code is not all written, and 100% bug free (is there such a thing?) and so during development and beta-testing, aka product roll-out The chances of anything going wrong goes up considerably.

I suppose an AC coupled circuit could be constructed to trap a "stuck" I/O line and fail-safe in the off postion, but this is going to be more complicated than a resistor!

One other thing If you want to drive the backlight from the PIC the the current availible is only going to be 25MA per I/O, so you would need at least 2, but when these switch on, the peak current may excede the 50MA from 2 I/O ports, this could latch the chip and then destroy the LED backlight and probably the PIC, all to try and save the cost of 1 resistor......