[sdiy] LEDs as Diodes

[Harry Bissell Jr]

Inline snipped


> Then again when I tried to make a diode filter using
> LEDs what bit me was
> the bad matching of the LEDs.

So match them.  I did this myself. It worked well...
otoh I cannot say its any improvement over a matched
diode ladder... except in the dark :^P
> 
> Then again again, I love LED as signal limiters and
> prefer them over a stack
> of silicon diodes :) However in this application
> usually the LED don't light
> unless there is massive clipping.

I was using an MV5094AQP  (Quality Tech) which was
a red/red bipolar LED.  Why red/red ???  One, you can
run them on AC power and get fullwave
response...two...the back to back LEDs protect each
other against reverse voltages.  I used them in
clipper
circuits a LOT...and now they are GONE (damn!)
 
> And a bi-color LED makes a nice -10...0...10 volt
> indicator.
> 
> Last but not least this trick let you use one LED as
> if it is two
> anti-parallel LEDs, instant matching :)
> Fixed font fixes the garbage
> 
>   +--->|--+--|<---+
>   |       |       |
> --+      LED      +--
>   |       |       |
>   +---|<--+-->|---+

This trick works better at higher voltages... The
diodes cause 1.4V drop... the LED maybe 2.1V. If those
diodes are not matched, they can contribute as much
error as the LED itself. You might have an advantage
with thermal issues.  This trick DOES work very good
with a higher voltage zener diode, where the LED
forward drop forms a small fraction of the total drop.