🚀 go-pugleaf

In article <3DC86222.BD639CE8@fanwap.com>, tong@fanwap.com talked about...
> Is it easy to buy "current limiting diode" there ?

Honestly, I've never seen a circuit on a PC board that uses one.  They're 
just not found, perhaps because it's easy to make a current limiting 
circuit with a few standard parts.  One can even be made from a JFET with 
the gate tied to the source, as has already been mentioned.  

Here is a constant current circuit using the TL431, which can be found in 
just about every PC switching power supply.  Just take one out of a defunct 
AT supply.  Check out figure 14 on page 6.
http://www.fairchildsemi.com/ds/TL/TL431CP.pdf

One problem with this is that the transistor has about 2.5V at the emitter, 
so the current setting resistor, which for 30 mA is 83 ohms, has to drop a 
relatively large amount of voltage and dissipates 75 milliwatts of power.  

But, by changing the transistor to a 2N7000 enhancement mode MOSFET, the 
voltage across the resistor goes down to about .4V because the gate turnon 
voltage of the 2N7000 is about 2.1V.  So I think that I can get away with 
using the 10 to 15 ohm resistors with a lot less power loss.  And since the 
gate of the 2N7000s draw no current, I can parallel a dozen or more of them 
across the TL-431 to get as many of the LEDs as I want regulated to 30 mA.  
All I need add is another MOSFET, another 10 ohm resistor and another LED.

Come to think of it, a TL431 is not needed for the 2.5V reference.  A green 
LED has about 2.4V across it, so it would make a cheap and dirty voltage 
regulator.  And a power on light as well.  

> I using 8mA current limiting diode CCLHM080

> Just connect it series with LEDs.
> 

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