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Old 25th October 2012, 12:53 PM   #11
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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?
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Old 25th October 2012, 01:45 PM   #12
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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.
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Old 25th October 2012, 03:10 PM   #13
wg_ski is offline wg_ski  United States
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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.
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Old 25th October 2012, 05:13 PM   #14
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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.
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Old 26th October 2012, 03:30 AM   #15
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Quote:
Originally Posted by avincenty View Post
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.
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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Last edited by danielwritesbac; 26th October 2012 at 03:34 AM.
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Old 26th October 2012, 10:28 AM   #16
AndrewT is offline AndrewT  Scotland
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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?
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Old 26th October 2012, 10:38 AM   #17
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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.
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Old 26th October 2012, 11:27 AM   #18
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My understanding is that LEDs are horribly inefficient as light sensors. I also guess that the difference between zener breakdown and avalanche breakdown is relevant to this thread, but I'm not sure where, exactly. AndrewT asks a good question above.
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Old 26th October 2012, 02:59 PM   #19
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The curiosity is not about using traditional LED's reverses biased, is about the radioshack bright LED that has a built in protection zener. Does this built in ESD zener provide any performance benefits over a regular zener at 7.3 volts?
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Old 26th October 2012, 03:12 PM   #20
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Quote:
Originally Posted by avincenty View Post
The curiosity is not about using traditional LED's reverses biased, is about the radioshack bright LED that has a built in protection zener. Does this built in ESD zener provide any performance benefits over a regular zener at 7.3 volts?
Can you test to see if the avalanche knee voltage is more current dependent than a regular zener?
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