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From: email@example.com (Bob Wilson)
Subject: Re: voltage or current regulator for White LEDs ?
Date: Wed, 06 Nov 2002 04:15:08 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article <3DC842BD.9E7CDCA6@fanwap.com>, firstname.lastname@example.org says...
>I have problem to control voltage drop across the White LEDs.
>They doesn't same voltage drop that make the current doesn't flow
>equally. So, the brightness will not same ?
>Control the current at 20mA is better than control the voltage at 3.6v ?
>I try to test at 5v (7805) series with resistor 91 ohm.
>Some of them are:
>3.87v at 15.23mA
>3.61 at 12.7mA
>3.55 at 13.93mA
>3.76 at 14.3mA
>3.02 at 20.5mA
>3.08 at 21.1mA
>Can I set regulator output voltage (LM317) to 3.6v and paraller them
>Set the higher output voltage to 4v and use resistor to matching them
>one by one at 20mA ?
You completely misunderstand the basic principles of LEDs. You cannot
control the voltage of an LED. It is simply a diode (that happens to emit
light). The fact that you are using white leds is irrelivant here. *ALL*
LEDs are current-driven devices.
Measuring the voltage under different currents is a total waste of time, and
means nothing. If you were to change the temperature by a few degrees, or
try LEDs from a different batch, you would get different values.
You CANNOT power LEDs from a voltage source! This is simply very basic
electronics. To do so will cause your LEDs to go up in smoke. Further,
unless you carefully match the leds so they all have identical forward
voltages (and track identically as their voltages change with temperature),
you cannot parallel them either. Paralleling is simply not practical. The
one with a slightly lower voltage will hog most of the current.
The most common way to drive an LED is to put a current control resistor in
series, THEN drive it from a higher voltage. If the voltage is high enough,
you can put the LEDs in series, with one resistor in series to contol the
current through them all. The resistor can be replaced by an LM317 that is
configured as a constant current source (**NOT** a voltage source!).
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