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-   -   High Brightness LED Zener Like Curve (http://www.diyaudio.com/forums/solid-state/222179-high-brightness-led-zener-like-curve.html)

avincenty 24th October 2012 10:15 PM

High Brightness LED Zener Like Curve
 
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I was testing a small semiconductor tester purchase very cheap at my local surplus and I came across a Zener like curve on a Radio Shack 5mm High Brightness White LED (276-0017)

It shows a 2.5V forward voltage and a 7V reverse voltage. I tested a bunch of other generic LED's and those all show a diode looking curve with each LED's respective forward voltage.

I also plotted a small signal NTE159M transistor and it displays similarly to a zener (no surprise).

This High Brightness LED must be built differently. Is this common with the newer, brighter LEDs? Could the LED be used in reverse voltage as a 7V reference?

DUG 24th October 2012 10:49 PM

Try Vbe of any BJT...often used for a cheap zener.

This characteristic can be used to differentiate the collector from the emitter when an unlabeled BJT shows up.

avincenty 24th October 2012 11:47 PM

Quote:

Originally Posted by DUG (Post 3214253)
Try Vbe of any BJT...often used for a cheap zener.

This characteristic can be used to differentiate the collector from the emitter when an unlabeled BJT shows up.


Yes, that's what I meant with no surprise for the NTE159M.

I took out a small 100x microscope and I was able to see he structure from the side (top is very difficult due to the lense). The high brightness LED is definitely different, it has 3 wires vs the one wire on the regular LED's. I have been doing some searches but haven't really found anything describing what I am seeing.

AndrewT 24th October 2012 11:56 PM

you have misread/misinterpreted the results.
At low current the forward voltage is lower than Vf@ rated current.
The device behaves a bit like a very high resistance with Vf rising rapidly as If starts from zero nA to say 1mA
Then the device starts to pass significant current as Vf nears the knee in it's Vf vs If curve.
Now the diode starts to behave as a small value resistor in series with a fixed voltage or back emf.

This looks very similar to a Zener and to a normal diode. The difference between them, is at what voltage the knee occurs.

This applies to all LEDs, not just white, or blue, or red and not just to high brightness but to all brightness/light output.

It's why LEDs are used as cheap voltage references. Once Vf is above the knee the effective resistance is low and that back emf is the voltage reference. Just like Zeners.

macboy 25th October 2012 12:24 AM

Based on your comments, you clearly do not understand Zeners. Vf is practically immaterial for a Zener, it is the reverse breakdown voltage that matters. This has a very sharp knee and very high slope compared to the forward voltage curve. What avincenty observed is a very low reverse breakdown voltage on the super bright LED, comparable to a Zener, and in contrast to a normal diode. I think that normal LEDs typically have a low reverse breakdown voltage, but maybe not as low as 7 V.

avincenty 25th October 2012 12:35 AM

The pictures are all under the same drive conditions. On the scope the horizontal is accurate at 5V/div. Vertical center is 0V Current is not calibrated as I have not looked at the value of the current sensing network.

The fact is that the super bright LED is different in construction and based on the produced curve must have a different equivalent circuit.

avincenty 25th October 2012 01:11 AM

I am currently running a test in the bench. Variable Power Supply + to 1K resistor to LED to ground. The regular LED, reversed biased did not go into breakdown until it hit aprox 100V on the PS, it then killed the LED.

The Bright LED has been running as a zener of 7.3 volts passing a current of 25ma. I am going to leave it overnight, then raise the current until I blow it.

avincenty 25th October 2012 01:37 AM

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Well...... I did a search on the part number and found a thread that mentions that the extra internal connections is for a ESD protection diode. It t is a built in protection zener after all. Also found a structure diagram for a different product that must be similar in construction.

AndrewT 25th October 2012 08:52 AM

Quote:

Originally Posted by macboy (Post 3214336)
Based on your comments, you clearly do not understand Zeners.

OK, I forgot to describe that I need to turn the zener around to be able to plot the If vs Vz curve. So what, the rest applies.

LEDS Zeners and diodes all show the knee in the If vs V curve.

cliffforrest 25th October 2012 10:29 AM

Andrew, I don't think anyone else here is talking about the FORWARD curve, but the REVERSE bias situation!

Used as a zener, a LED, BJT-Veb etc is never forward biased so that part of the characteristic is not relevant.


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