Re: gEDA-user: LED in reverse

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Sun, Nov 19, 2006 at 02:39:14PM +0100, Tomaz Solc wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Hi
> 
> > How is it possible that the LED withstands 20mA in forward and is destroyed by
> > 0.5mA in reverse? If it has say 1.5V forward and 5V reverse then the amount of
> > energy dissipated is greater in the forward case.
> 
> I just tried this in practice. I've connected a 1MOhm resistor in series
> with a LED and a DC voltage source. I've tested some old low-intensity
> 3mm yellow and green LEDs and a new high-intensity blue 5mm LED.
> 
> I could not get yellow and green LEDs to break-down. They conducted a
> negligible current with 250V reverse voltage which is as high as my
> equipment will go.
> 
> The blue LED broke down at approximately 25V. There was no flash and the
> LED didn't light up even when there was 0.2mA flowing through it (which
> is enough to light it up when connected in the forward direction). This
> experiment also destroyed the LED because it won't emit any light now,
> although it still has a I(U) characteristic of a diode.
> 
> Now I'm a bit skeptical about these results. It seems strange that those
> yellow and green LEDs can withstand such reverse voltages. However I've
> used this method before to measure break-down voltages of various
> transistors and I always got good results.
> 
> I also don't know why the blue LED was damaged. Transistors I checked

There is SoC inside the LED which has a bitmap-PDF copy of the datasheet in a
ROM.  It runs in cycles. Every time it measures all electrical values and runs
OCR on the datasheet. If any of the maximum ratings is violated, a
self-destruct fuse is activated.

This also explains why the LED isn't damaged by momentarily ESD discharges.
If the current goes between the measurements (which are given by the speed of
the OCR), it goes unnoticed.

> this way weren't damaged in the process and the maximum power
> dissipation in the LED was something like 10 times lower than in normal
> operation of the LED.

The electrons were accelerated to such overly agressive speeds that the atoms
cracked when hit by the electrons. As the atoms are cracked now, they
malfunction.

CL<
> 
> Best regards
> Tomaz
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.5 (GNU/Linux)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
> 
> iD8DBQFFYF6CsAlAlRhL9q8RAjY5AKCU7rxwtrXRb7e0Q1sklhTEWF+VCwCgtdDP
> k0lmbqfak15HJ01xvS7nW7Y=
> =x5IE
> -----END PGP SIGNATURE-----
> 
> 
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user