View Single Post
Old 01-02-2013, 10:09 AM   #61
geekmaster
Всё гениальное просто.
geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.geekmaster ought to be getting tired of karma fortunes by now.
 
geekmaster's Avatar
 
Posts: 5,069
Karma: 6789001
Join Date: Nov 2011
Location: Щедрость не имеет пределов.
Device: *.*
Quote:
Originally Posted by ProDigit View Post
Their forward voltage is 3V, their backwards voltage could sometimes range in the tens of Volts. ...
Enough unsubstantiated claims. Here is some ACCURATE information about LEDs:
http://www.maximintegrated.com/app-n...ex.mvp/id/3070
Quote:
...
Standard red, green, and yellow LEDs have forward voltages in the range 1.4V to 2.6V, depending on the desired brightness and the choice of forward current. For forward currents below 10mA, the forward voltage varies only a few hundreds of millivolts.
...
A trick is employed to make white LEDs. Blue-emitting InGaN base material is covered with a converter material that emits yellow light when stimulated by the blue light. The result is a mixture of blue and yellow light that is perceived by the eye as white.
...
Many portable and battery-operated devices use white LEDs for background illumination. In particular, the colored displays of PDAs need a white backlight to obtain color reproduction that is close to the original.
...
In most cases a single white LED is not sufficient, so several must be operated together. Special steps must be taken to make sure their intensity and color is matched, even as battery charge and other conditions vary.
...
At first, when the battery is fully charged, all diodes are illuminated but with different shades of light intensity and color. As battery voltage drops to its nominal level, the light intensities decrease and the differences in white become stronger. The designer must therefore consider the value of battery voltage and diode forward voltage for which the series resistor is calculated.
...
The goal of an LED power supply is to provide a sufficiently high output voltage, and to force the same current through all LEDs connected in parallel. Note (Figure 5) that if all the white LEDs of a parallel configuration have identical currents, all will have the same chromacity coordinates. Maxim offers a charge pump with current control for that purpose (MAX1912).
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).

Last edited by geekmaster; 01-02-2013 at 10:32 AM.
geekmaster is offline   Reply With Quote