12VDC to 3.3VDC LED Metered after resistor

[Sunnysky]

try using base emitter as a Zener. The current is very limited before failure unlike collector emitter breakdown

 

[(*steve*)]

Yeah we had a discussion a while back about this. I was of the opinion that there would be some minimum current to cause damage to the BE junction.

After some experiments and some additional reading I changed my mind (and learned something in the process).

In the case of the reverse biased BE junction, the damage may not be immediately apparent, but it is measurable.

The same is true for LEDs.

 

[Sunnysky]

Thanks Steve for affirming my past experience. 10 yrs ago my LED client in NZ was having 10% Field,returns with at least 1 in 20 per unit bad LEDs and one symptom was increased capacitance causing a loop on VI sweep on bad LEDs and some still working also had similar peculiarity. I decided to fly down to NZ to see the problems first hand and discovered all,sorts of ESD handling issues including storage bins and injection molded plastic conveyor. So I changed the LED orders after that to all include Zener protection and no more failures for ESD occurred in over 1 million shipped.

Wounded LED's can cause high infant mortality not always when they are stressed.

 

[CDRIVE]

This thread is an exemplary example of never judging a book by its cover. In this case it would be never judge a topic by its first post. When I read that post and viewed the schematic by Stevelin9 (welcome to EP Steve) I smiled and nearly clicked my mouse to move on to the next topic. Who wudda thunk that such a basic circuit would be a catalyst for a discussion that's 'Electronics Steak & Potatoes' for us but obviously far beyond the comprehension of Stevelin9.

Chris

 

[BobK]

He measured the voltage across the LED at 0.45V. That is far below the typical absolute max for reverse voltage, which always seems to be 5V. How does that damage the LED?

Bob

 

[CDRIVE]

He also said that his supply is 12V but the LED is powered by a GPIO pin. This doesn't compute either. If the uC is being powered by 12V its left this world and is destined for the recycling pile.

Chris

 

[Herschel Peeler]

510 Ohms. Green Brown Brown

About 500 mV just sounded like it was getting microamps of current.

 

[CDRIVE]

About 500 mV just sounded like it was getting microamps of current.

It's also possible that the LED has been fried and there's nothing left but resistive impurities but if true I would expect the same voltage drop with either polarity. On that note why guess? He should test the LED + limiting resistor with 9V battery and be done with it.

Chris