Re: gEDA-user: LED in reverse

[Tomaz Solc <tomaz.solc@xxxxxxxxxx>]

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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
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.

Best regards
Tomaz
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