Sort of going off on a tangent here.  FWIW, the max brightness
I want is what is recommended for the LCD.  At night, I need
less current going thru it, so no chance of blowing it due to
overcurrent (beyond rated).

-Neil.




Wagner Lipnharski wrote:
>
> >
> > That doesn't sound right, something else must have been going on.
> > LED backlighting is simply that - LEDs, and they should be able to
> > take all the punishment we dish out to normal LEDs from DC to MHz
> > PWM. It is very important to have a current limiting resistor though
> > because the LEDs aren't designed to run off 5V directly
> >
>
> It is very difficult to visually notice an overcurrent on LEDs.
>
> Increasing from 15mA to 30mA you will see a little increase of brightness,
> not a visual indication of overcurrent.
>
> Increasing more, to 50mA or even 80mA you will see the LED shinning more,
> but going to 100mA it will start to reduce brightness due the heat
> generated until it blows up the junction.
>
> Some backlights are made with a parallel / serial combination of LEDs, but
> without even looking for the datasheet, you can guess some numbers;
>
> Suppose the whole backlight uses 8 LEDs, each led 1.7V @ 15mA
>
> Option 1)
> =========
> 8 LEDs in parallel, it would drain 1.7V @ 120mA.
> Your 5.1 Ohms resistor would hold 5.1 x 0.12 = 0.6V, what is wrong, since
> if the leds would hold only 1.7V, R5.1 should hold 3.7V at least.
>
>
> Option 2)
> =========
> 2 blocks of 4 leds, connected in serial.
> It would hold 3.4V @ 60mA, then R5.1 would hold 5.1 x 0.06 = 0.36V, and it
> should do 1.6V at least.
>
> Option 3)
> =========
> 3 blocks of 3 leds (suppose it uses 9 leds), connected in serial.
> It would hold 5.1V (5V is ok) @ 45mA, then R5.1 would hold 5.1 x 0.045 =
> 225mV
> What seems to be the correct answer.
>
> But if 3 Leds in Series needs 5V, why use the resistor?  safety!
> The Voltage across the LED junction grows much slower than current.
> Applying 1.8V across a LED, the current would go almost twice of would be
> if applying only 1.7V.
> So, imagine something going wrong with the voltage regulator, and it
> applies 5.25V instead 5.0V, that is the stated +-2.5% error from LM340T
> datasheet.  With this 5.25V applied directly to the LEDs, they could
> increase the current in 50%, what would destroy the LEDS in few months of
> use.
>
> The 5.1 ohms resistor, even holding just 225mV when Vcc=5V, would increase
> substantially this Vdrop if the LEDs current increase from 45mA to 70mA.
> 5.1 x 0.07 = 0.357V, it means, the Resistor VDrop went from 225mV
> to 350mV,
> what would create a "automatic" strong current limiter without interfering
> much when VCC is 5V.  This extra Vdrop of 125mV will pull down the voltage
> applied to the LEDs and they goes back to a safe current consume region.
>
> When you use a R10 ohms, the 45mA would generate a RVdrop of 450mV, what
> would reduce VLEDs to around 4.55V, current will drop substantially to
> reduce RVdrop and create an intermediate LEDsCurrent and bright.
>
> At this very short range of Voltage (less than 500mV) is somehow more
> difficult to control the brightness you want (day/night).
> What I would do it exactly keeping two 5 ohms resistor in series,
> and short
> circuiting one during day-time.  The resistors are important to
> the current
> limiting job.
>
>
>
>                  +5V
>                   o
>                   |
>     .-------o-----o
>     |       |     |
>    R1k      |     |
>     |     |<E     |
>     o----B|       R5
>     |     |\C     |
>     |       |     |
>    R2k      '-----o
>     |             |
>     |             R5
>    LDR            |
>     |       9 LEDS BACKLIGHT
>     |             |
>    _|_           _|_
>
>
> Transistor PNP 2N4403
> R2k could be a 10k+ Trimpot to Adjust light sensitivity.
>
> All of this, of course, based on a possible 3 series LEDs in a total of 9
> Leds.
>
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