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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: voltage or current regulator for White LEDs ?
Date: Tue, 05 Nov 2002 23:02:03 GMT
NNTP-Posting-Date: Tue, 05 Nov 2002 18:02:03 EST
"Nucharin W. Jansen" wrote:
> 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
> directly ?
> Set the higher output voltage to 4v and use resistor to matching them
> one by one at 20mA ?
The variations you measured do not include the change in forward
voltage with temperature change. At the very least, use a resistor in
series with each to swamp out the variations in forward voltage before
paralleling. At best, connect them in series, and build a current
regulating supply for them. Flyback or boost switching regulators
work efficiently, but the added cost of the inductor is often
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