Re: gEDA-user: LED in reverse

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Sun, Nov 19, 2006 at 09:20:53PM +0100, wk0 wrote:
> ---- Wiadomość Oryginalna ----
> Od: Darryl Gibson <n2diy@xxxxxxxxx>
> Do: gEDA user mailing list <geda-user@xxxxxxxxxxxxxx>
> Data: 19 listopada 2006 18:22
> Temat: Re: gEDA-user: LED in reverse
> 
> > Karel Kulhavy wrote:
> > 
> > > 
> > > 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.
> > 
> When diode is reverse biased it may be locally overheated since the breakdown may occur only at small amount of the junction surface. This may destroy the diode, especially if it is high efficiency heterojunction/multilayer type. Very old diodes should be more resistant, but they are not designed to replace zener diodes.
> 
> I remember old red LED glowing on breakdown at about 15V. It was some 25 years ago and the diode was old, low efficiency type (~1mcd at 10mA).
> 
> Some time ago i read the warning for RF transistors that reverse biasing into
> breakdown of the E-B junction may significantly increase noise figure.

Does it increase the noise figure permanently or only temporarily?

What if I have a circuit with 2N3904 transistors with 5V break down and it has
12V power supply and on turn-on, a little current at 12V goes through the
junction the wrong way? Do I have to design my circuits to prevent that?

CL<
> 
> Wojciech Kazubski
> 
> 
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