Super Regulator, Need sub for P309 LED

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Hello,

I want to build a few improved Super regulators as per Walt Jung's AE article. Problem is the P309 LED is no longer available from Digikey. Does anyone have any ideas as to where I can get some? Or offer an alternative LED as a substitute. Any help would be appreciated.

Thanks.
 
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what is that LED used for? Most likely as a reference voltage source for a constant current source or as an indicator. In the first, you just need to readjust the associated resistor(s) and in the 2nd case, it most likely will work without any modification.
 
I use ALW's schematic as a reference:

Uled = voltage across the LED (1.6V red, 1.8V green, 3 V blue, value may vary)

Vbe = 0.7 V voltage between base and emitter

R1, prefered current 10-25 mA, I'll take 10 mA as an example.

R1 = (Uled-Ube)/I = (1.6-0.7)/0.01 = 90 ohms Red LED

R1 = (Uled-Ube)/I = (3.0-0.7)/0.01 = 240 ohms Blue LED


You must also change the voltage of the preregulator. Higher voltage of the LED => more voltage across the Q1.

Needed voltage is 0.8 + Uled (roughly) = 4-5 volts for blue LED

Preregulator
R2 = 1 k
R3 = 2.7k => 4.6 volts across Q1, collector-emitter.

As you may notice it's good to have a low voltage as reference, your losses will be less if you use a preregulator. OK, the coolness factor will be higher with a blue LED but with the sacriface of higher losses.
 
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