LCD backlight connection

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I just ordered an LCD for a project to test with and am having a little trouble designing the back light circuit. The datasheet says the backlight has a Vf of 4.2v with 160mA current. If I run this off 5vdc, this means I need an 5 ohm resistor in series? Maybe it has a built in current limiting resistor? Something doesn't seem quite right, but then again I have never used an LCD before.

Link to datasheet: http://www.sparkfun.com/datashee...

Chief Tinkerer

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Internally, they appear to be a series-parallel combination of LEDs. No limiting resistor. You RARELY need full current - that is very bright.

They are easy to PWM for variable lighting. You can put your PWM switch in either lead of the backlight. Negative end works very nicely with an NPN or N-Channel device.

But, yes, a series current limiting resistor is appropriate. I would use a 10-12 ohm resistor for starters - maybe even as big as 25 ohms. Would have to be rated at 1W or better. Then, maybe add the PWM after you have things basically working.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I used the following on one project. PWM the input to the transistor, as Jim mentioned. Some projects have Full On, Full Off, and ~ 40% for a Dimmed value.

Others use a photo-transistor and auto-adjust the LCD backlight based upon the ambient lighting.

JC

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FWIW, I've got a Chinese 20x4 LCD with built-in backlight resistors (2x0805). One resistor is 0, the other is 15. Backlight draws 30mA and is plenty bright. I was pleasantly surprised at how low the current was.

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thanks. I left a pad for a 1 watt smt resistor. I will try a couple until I get a good brightness, otherwise I have the OC1A pin free so might do PWM if I feel like modifying my current board layout. Hmmm :)
Chris

Chief Tinkerer