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Topic: How to control the display's backlight in the sketch? (Read 7 times) previous topic - next topic

floresta

Quote
I've used 2 diodes 1n4007 insthead the resistor ( series ) and it works great.

You don't understand the problem.  This is a different way of reducing the maximum voltage applied to the backlight.  The problem is that it is the current rating of the diode that we don't want to exceed.

Once again:
if you to try to design a circuit to drive an LED with some specific voltage you are doomed to failure.

Don

psteve

:) Hi Don,

I'dont try to design a circuit to drive an LED with some specific voltage, my problem was turn off light (and mA needed).

:) in sainsmart shield with 2 diodes is solved.

Steve

dhenry

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Once again: if you to try to design a circuit to drive an LED with some specific voltage you are doomed to failure.


The story is actually far more complicated than that.

Some data:

For 1n4001 (Vishay, diode), If = 20ma @ Vfwd = 0.65v; If = 50ma @ Vfwd = 0.7v (delta Vfwd = 50mv); dynamic Rd = 50mv / 30ma = 1.6ohm.
For CLA1B (Cree, LED), If = 20ma @ Vfwd = 3.05v; If = 22ma @ Vfwd = 3.10v (delta Vfwd = 50mv); dynamic Rd = 50mv / 2ma = 25ohm.

For the same change in Vfwd, the led's If increased much less than a diode's.

Contrary to conventional "wisdom", high power LEDs behave far more like a resistor than a diode.

guix

#38
Oct 17, 2012, 05:18 pm Last Edit: Oct 17, 2012, 05:30 pm by guix Reason: 1

The current consumption respectively 86-105-118 mA.


:smiley-eek: How is that possible, if the PWM pin should not "give" more than 25mA ? And with LEDs OFF, it still use 86mA current? What I don't understand here... can you explain?

Thanks :)

floresta

Quote
Contrary to conventional "wisdom", high power LEDs behave far more like a resistor than a diode.

Is that what we are talking about here?  I associate 'high power' LEDs with those that are becoming available to replace household incandescent bulbs etc.  The LEDs being used in flat screen TVs could well fit into this category as well. 

I would think that any LED backlight that is being driven by an output pin of the Arduino would be classified as low power, and it would behave much like any diode, not like a resistor.


Don

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