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?
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?
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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.
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.
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.
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.
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.
The fact is that the super bright LED is different in construction and based on the produced curve must have a different equivalent circuit.
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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.
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.
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.
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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.
Curious. First generation LED datasheets from HP etc, specify LED breakdown voltage backwards as 7 v. I don't have datasheets on anything newer, as LEDs with leads come from surplus houses now without a part number. Radio Shack stocks no electronic components smaller than a cell phone in this market.
First gen Optoisolator datasheets from HP also have a 7v backwards breakdown, whereas the toshiba optoisolator datasheet I looked up had no breakdown voltage specified except across. So I have been carefully putting 1n4007 in series with any optoisolators I use that might be subjected to back voltage. Is this silly?
First gen Optoisolator datasheets from HP also have a 7v backwards breakdown, whereas the toshiba optoisolator datasheet I looked up had no breakdown voltage specified except across. So I have been carefully putting 1n4007 in series with any optoisolators I use that might be subjected to back voltage. Is this silly?
No, I don't think it's silly. I have blown newer generation leds (white blue etc) from reverse voltage by simply driving it on AC with just a resistor. A diode connected antiparallel with the LED stopped the popping LEDs. Red ones seem to be a bit more robust when reverse voltage is applied.
ALL diodes - LED, zener, rectifier or otherwise have a similar reverse characteristic. The differences are in how sharp it turns on in reverse and at what voltage. The limit to how much current it can take in reverse is limited only by temperature rise - hence, the power dissipation rating of a zener.
LEDs tend to have low power dissipation capability and low reverse breakdown voltage (a few times the forward voltage). They get hot when you crank up the forward current, don't they? This means they're relatively easy to overload in reverse - the current capability is several times less than in forward mode just due to power dissipation. And since they break down at less than 10V so they're not very good at blocking, either.
High brightness LEDs vary in construction. Ones that have a high Vf either use a SiC die like the white or blue ones, or possibly several AlGaAs dies in series/parallel to get more light output in the red and green. I've seen Vf's at 5V, which is 3 in series. If a zener is used for protection, it probably has a higher Pdiss capability than the LED itself and the reverse voltage is always known.
LEDs tend to have low power dissipation capability and low reverse breakdown voltage (a few times the forward voltage). They get hot when you crank up the forward current, don't they? This means they're relatively easy to overload in reverse - the current capability is several times less than in forward mode just due to power dissipation. And since they break down at less than 10V so they're not very good at blocking, either.
High brightness LEDs vary in construction. Ones that have a high Vf either use a SiC die like the white or blue ones, or possibly several AlGaAs dies in series/parallel to get more light output in the red and green. I've seen Vf's at 5V, which is 3 in series. If a zener is used for protection, it probably has a higher Pdiss capability than the LED itself and the reverse voltage is always known.
I ran the LED in reverse with 100ma (.73W) running thru it for around 4 hours. No apparent issues, barely warm to the touch. The curiosity is, if this protection diode is designed for ESD protection, would that be of benefit over a regular zener? And make it useable in audio circuits such as for biasing small tube driver and power stages?
Found a picture that shows the internal wiring I had mentioned previously.
Found a picture that shows the internal wiring I had mentioned previously.
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There's a lot of variety available. Some high brightness are sold as "Triple Chip" but only 2 pins and those handle about 20ma+10ma+15ma at most. The 5050 SMD LED is a bit different "triple chip) with 6 individualized pins, since it can have 3 resistors connected for 20ma+20ma+20ma, acting like three "single chip" -or- it can have the pins bridged to act just like a "triple chip" led at somewhat lesser maximum current tolerance.
Most LED datasheet figures are peak tolerance not particularly conducive to long term use. If using AC signal, I usually will connect a series schottky to guarantee that the LED won't see reverse voltage. A typical LED is very fragile in reverse and easily broken. The weird/rarer LED that is durable in reverse is a special AC rated LED, and with so much variety it is hard to re-stock those reliably (the vendor probably won't send you the same thing twice, so there's risk). A different option for protecting normal LED's is to parallel a Zener so that if one LED in a series array fails, the rest stay lit.
If I remember right, the LED is more current dependent than Zener, and that difference is potentially useful, especially to the compressor in my signature below.
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A LED is used as a light emitter.
It can also be used as a voltage reference.
Both these uses allow current in the forward direction.
Dan has explained how the LED can be protected from damage if accidental or incidental reverse current is passed.
Is there any use that requires the LED to be reverse biased?
It can also be used as a voltage reference.
Both these uses allow current in the forward direction.
Dan has explained how the LED can be protected from damage if accidental or incidental reverse current is passed.
Is there any use that requires the LED to be reverse biased?
As I recall you can use LED in reverse fed with very small current to act as a light sensor. This is the way optocouplers work. One LED emitts light and the other catches this light.
LED circuit - Wikipedia, the free encyclopedia
I even once tried use LED as light sensor.
Naturally it is better use devices that are made for it.
Can you test to see if the avalanche knee voltage is more current dependent than a regular zener?
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