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From: Kevin McMurtrie
Subject: Re: voltage or current regulator for White LEDs ?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Wed, 06 Nov 2002 00:05:50 GMT
NNTP-Posting-Date: Tue, 05 Nov 2002 16:05:50 PST
In article <3DC842BD.9E7CDCA6@fanwap.com>,
"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
>Set the higher output voltage to 4v and use resistor to matching them
>one by one at 20mA ?
LEDs should by driven by current, not voltage. The usual way to drive
them is to start out with a voltage that's much higher than the typical
LED voltage drop then use a resistor to limit the current.
Driving them from only 5V at an even brightness will need more than a
resistor. The simple current mirror has a low enough voltage drop
(barely) to work. They can all share the same bias voltage.
Base ------> to bias voltage
~10 Ohm resistor
Another option is to string them all in series and use a current
regulated switching power supply that increases the voltage. It's much
more efficient if you have many LEDs. Such chips exist specifically for
this. You can also hack out such a circuit using a 555 timer chip and
other common Radio Shack parts. I have one on my bicycle that powers 10
amber LEDs in series from a 12V battery.
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