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From: "Nucharin W. Jansen"
Subject: Re: Best way to power array of LEDs?
Date: Thu, 07 Nov 2002 06:10:43 +0700
Organization: C.S. Communications Co. Ltd.
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Typical Graph between V(f) and I(f) of each Ultra LEDs are not the same
that made me very hard to calculate R drop and I think It can't
calculate R on paper.
I have to try adjust the POT in real world one by one with every
And don't swap LEDs between strings any more.
> Yes, I mean in series.
> But two strings can not use the same R values.
> So sad it very hards to calculate the current and R.
> the forward voltage drop does not identical in each LED.
> and it changes when current change too.
> strings 1 : ( 12 volt - ( 3.453 + 3.6324 + 3.833 ) ) / 40 ohm = 27 mA
> Now I want 20 mA , How to calculate the resistor ?
> Since forward voltage change when the current change too.
No, If you answer R = 40 * 0.027 / 0.020 = 54 ohm
It is totally wrong. You can't get 20mA from R = 54 ohm.
Because now at R = 54 ohm, the V(f) of 3 LEDs are change.
headache.. We can't calculate R drop at the exact 20mA in the paper.
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