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Old 01-03-2013, 07:44 PM   #75
ProDigit
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Quote:
Originally Posted by geekmaster View Post
Enough unsubstantiated claims. Here is some ACCURATE information about LEDs:
http://www.maximintegrated.com/app-n...ex.mvp/id/3070
FYI, a charge pump is a voltage boost convertor that steps the output voltage UP, so it is higher than the supply voltage. It is common practice to do this when driving LEDs from a lithium battery. The kindles also use even higher voltages internally, such as the high-voltage waveform used to electrostatically control the eink display. The battery is not the highest voltage (even for the LEDs).

Note that the referenced LED documentation mentions that LED brightness may be controlled by simple hardware or software-controlled PWM as I mentioned, or it may be controlled by varying the supply CURRENT (not voltage) at the expense of inconsistent emitted light color and more complex control circuitry. PWM results in a more consistent white color at all brightness levels, and does not require extra current-control circuitry for software control.

EDIT: If you refer to ANY spec sheets for LEDs, you will see that NONE of the commonly available colors use 3V forward voltage as you claim. In fact, blue LEDs typically use about 4V, and other colors typically use about 2V. Some super-bright LEDs may have lower reverse breakdown voltages, such as a super-bright blue LED that uses 3.3V. You can measure that voltage drop across the LED while lighting it using a common button cell battery such as a CR2032. The actual battery supply voltage depends on the load, and the internal resistance of the battery will drop the measured voltage to what the LED actually uses. Supplying more current than the LED can dissipate (such as using a benchtop power supply) will burn out the LED if it gets hot enough to initiate thermal runaway. At a minimum, you need a series current limiting resistor to prevent overdriving the LED to destruction. But with small batteries, the internal resistance (and lack of high current capacity) is enough to protect the LED.

You can drive a blue or white LED from a lower voltage battery (such as 3V, or even 1.5V) using a simple boost converter such as a "Joule Thief" (a fun little project in itself):
http://en.wikipedia.org/wiki/Joule_thief

EDIT2: The "tens of volts" reverse breakdown voltage of which you speak applies only to small signal diodes and power rectifier diodes, not LEDs (which typically use 5V reverse breakdown). Zener diodes may have even smaller reverse breakdown (such as 3.3V) depending on how they are doped. Reverse bias beyond breakdown is only destructive with insufficient current limiting (and in fact is the NORMAL mode of operation for Zener diodes).
Forward voltage is different from backward voltage.
There are diodes that have a backward voltage of 97V as well.
I'm not saying that these are the leds installed in the kindle, all I'm saying is that the backward voltage with 'device -1' is not more than a few milli-volts, and would not harm the leds, neither would reversing polarity of full battery voltages cause any damage to the leds.
More than likely battery voltage is around 3.8V; but some e-devices have 2 batteries in series having 7.4V.
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