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From: Sylvan Butler
Subject: Re: voltage or current regulator for White LEDs ?
References: <3DC842BD.9E7CDCA6@fanwap.com> <3DC9926A.A7B5B2D@fanwap.com>
User-Agent: slrn/0.9.7.4 (Linux)
Date: Wed, 6 Nov 2002 17:07:31 -0700
On Thu, 07 Nov 2002 05:06:35 +0700, Nucharin W. Jansen wrote:
> How does to calculate the current of *unknown spec* white LED ?
Can't calculate the current.
> Since we don't know their forward voltage drop.
Can figure the Vf.
> I have power supply 5V. I would love to run china white LED at 20mA.
> Can I measuring the voltage drop under 20 mA then put in the ohm
> formular ?
I=E/R, .02=5/R, R=5/.02=250
Use a 250ohm or higher (eg 270r) in series with your 5v supply and
the unknown 20ma+ LED. Measure the Vf drop across the LED.
Recalculate the resistor with (5-Vf).
> When LM317 or LM334 act like a current regulator,
> It still requires 5V overhead voltage ?
2v to 3v is usually sufficient. But you are going to have a hard
time running a 3.6v LED on 5v supply with a 2v overhead regulator.
> Which one require a minimum overhead voltage ?
Look for "low dropout".
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