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Back light!!

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Ian Rogers

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I don't normally ask questions but I'm hooking a LCD colour screen to my Pic32!!

All's well but the "recommended" back light circuit looks a bit overkill... Why have they done this..

If I read it correctly there will be @1.25v at the base of the final NPN?? It is still saturated... So why the extra components.
 

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The final transistor is not saturated, it is a current regulator.
If the base-emitter voltage of the final NPN is 0.7V and its base voltage is 1.25V regulated by the two diode-connected transistors then the 12 ohms emitter resistor regulates the collector and LED current at (1.25V - 0.7V)/12 ohms= 46mA.

The circuit is used to regulate the current because different color LEDs have different forward voltages so a simple resistor to limit the current cannot be used.

Wait a minute! Purple is red and blue at the same time. Cyan is green and blue at the same time. Yellow is green and red at the same time. White is all 3 colors at the same time. So all they needed was one current limiting resistor for each of the 3 color groups.
 
I don't normally ask questions but I'm hooking a LCD colour screen to my Pic32!!

All's well but the "recommended" back light circuit looks a bit overkill... Why have they done this..

If I read it correctly there will be @1.25v at the base of the final NPN?? It is still saturated... So why the extra components.

You're mistaken Ian, it's a constant current source, which can be switched ON/OFF by the LH transistor. The two diode connected transistors provide the voltage reference for the constant current source, and the emitter resistor sets the current (to about 60mA).
 
I doubt that an LCD screen with LEDs for its backlight uses white LEDs and color filters because a white LED is actually a blue one with a yellowish phosphor on top and does not produce all the colors that are in real white light.
 
audioguru !! Its just a single colour backlight.. so I would have though a single resistor should suffice.

Nigel Goodwin !! Why on earth would they do this...

Hang on a mo!!! The connection on the board specifically shows three LED + connections and they have been paralleled up.... This is probably why!!!
 
Because it's best to feed LED's with a constant current, and it makes it easily switch-able as well (and presumably PWM could be used to vary the current via the same transistor).
Thanks for your help Nigel.... First I tried a lone 15 ohm resistor... About 91mA..... I quickly disconnected it and did the current sink..... As I said its running on about 34mA.... Three channels... so about 11mA per channel....
 
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