[sdiy] LEDs as Diodes

[Theo]

Not only a difference in voltage drop but also in maximum continues forward
current , maximum forward voltage, maximum reverse blocking voltage,
switching speed, matching and noise level.
I would say the maximum current, forward voltage and blocking voltage are
the most important factors to consider cause these can be fatal to the LED.
Then again when I tried to make a diode filter using LEDs what bit me was
the bad matching of the LEDs.

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.

No doubt there are applications where you can use a LED instead of 4148 and
get a free level indicator. Especially in low voltage digital circuits this
can be a nice debugging feature.
But to monitor a 0..10V control voltage it would be better to hook up a LED
to GND though a 5K...15K series resistor.
For extra points put the LED in the feedback loop of a opamp.
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      +--
  |       |       |
  +---|<--+-->|---+


Have fun,
Theo



----- Original Message -----
From: Pat Kammerer <spivkurl at wearerecords.com>
To: sdiy <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, December 21, 2004 10:42 AM
Subject: [sdiy] LEDs as Diodes


> Hi all,
>
> I'm working on an envelope follower design right now, and had a question
for
> someone who might know better.  I really like the trend of LEDs being used
> in the place of usual silicon diodes for circuits, and I would like to use
> this technique in my circuit.  I would hope that the light emition would
> respond to signal/voltage levels present.
>
> So the question...  Are there circumstances where a silicon diode (such as
> 1N4148) absolutely cannot be replaced with an LED?  Are there some basic
> guidelines to modifying a circuit to accept an LED instead?
>
> I'm guessing the big difference would be voltage drop right?  Hmmm...
> something to ponder whilst I sleep.
>
> Thanks,
> Pat
>