Would a LED short itself?

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Mr. Man-wai Chang

I was challenged on this... I have never studied material science,
notably, LED and diodes. SO... :)

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Mr. Man-wai Chang said:
I was challenged on this... I have never studied material science,
notably, LED and diodes. SO... :)
Click to expand...

The only mechanism I was taught, was that "diodes failed short".
It had something to do with a current density hot spot developing
in the silicon die, and the short would happen at that point.

I have no idea, whether all diode types behave the same way.
There are diodes with pretty tiny junctions in them, and
perhaps they don't go the same way.

Once a short forms, it would then depend on the circuit,
as to whether the diode is burned to a crisp, and eventually
goes open circuit because it has been physically destroyed.

A LED circuit typically has a current limiting resistor,
which means the LED probably won't be blown to bits.

So the initial failure might be a short circuit, and what
happens to the device next, will depend on the circuit. If
the diode has mega-amps available to it, then it could
burn off the bond wires, or physically explode due to
thermal effects.

Paul
 
I was challenged on this... I have never studied material science,
notably, LED and diodes. SO... :)
Click to expand...

Of course. . . if it's depleted it's short a card from one full deck.

The p–n junction have useful applications. A p-doped semiconductor is
relatively conductive. The same is true of an n-doped semiconductor,
but the junction between them can become depleted of charge carriers,
and hence non-conductive, depending on the relative voltages of the
two semiconductor regions. -wiki

So, how we interpret this, is that when a LED miraculously depletes
its charge carriers at the manufactures MT of/b Failure-rate, commonly
cited for 50 years [citation needed], actually, it's occuring at 2 or
3 years, or, on parity between production at an optimal reversal over
25ths less useage for 25-times more a price-stipend intent over
conventional tungsten(c)*. Please check for your local regulatory
commission for applicable laws* governing licensing and usage,
penalties and forfeitures, inclusive incarceration when applicable,
responsibly when in compliance to the Global Green Effect.
 
conventional tungsten(c)*. Please check for your local regulatory
commission for applicable laws* governing licensing and usage,
penalties and forfeitures, inclusive incarceration when applicable,
responsibly when in compliance to the Global Green Effect.
Click to expand...

Are there 20mA fuses out there? :)

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The only mechanism I was taught, was that "diodes failed short".
It had something to do with a current density hot spot developing
in the silicon die, and the short would happen at that point.
....
Once a short forms, it would then depend on the circuit,
as to whether the diode is burned to a crisp, and eventually
goes open circuit because it has been physically destroyed.
Click to expand...

So extreme heat could break and melt the diode, which would then become
a conductor?

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Mr. Man-wai Chang said:
So extreme heat could break and melt the diode, which would then become
a conductor?
Click to expand...

If the current flow in a diode is uneven, it creates a hot spot
on the surface, and a burn mark results. And apparently, the
failure mode in that case, is a short circuit.

It doesn't require the whole device to become involved. Just
a pinhole of higher current flow can develop, and lead to a
failure. Even ESD (electrostatic discharge) can cause a
defect that looks like that (black spot on the surface).

Some of the power LEDs I have here, they have an ESD protection
device, integrated right into the package. That helps improve
the device yield at the factory (fewer of them killed by static).
So that at least, is some evidence as to what the manufacturers
consider to be a threat to the power LED. They don't waste
money on adding a device like that to each power LED, unless
there is a damn good reason.

My recent work with LEDs, was making home made bicycle
lights. That's why I started playing with them. I nearly
got run down last year, while riding a bicycle at night,
using a weak incandescent lamp. Now, I have a reasonably
bright LED source, to light the way. (And no, I don't
point the light in car driver's faces - the LEDs point
downward, so I don't get flak from drivers.)

Paul
 
Mr. Man-wai Chang said:
Are there 20mA fuses out there? :)
Click to expand...

Use a current source. That's one solution.

It really depends on the operational voltage range.

My favorite for this, is the LM317. On my LED
test jig, I have it set up for less than 20mA,
using an LM317. (That's for testing 5mm diameter LEDs.)
The LM317 has a lower limit, due to its internal bias
current, so you can't go all the way down to zero this way.
But I have spanned, anywhere from 20mA to 500mA this way.
I have no reason to go to 1 ampere right now. My power
LEDs can take it, but my eyes can't.

http://cs.wikipedia.org/wiki/Soubor:LM317_1A_ConstCurrent.jpg

As that is a linear regulation method, it wastes power, and
is not efficient.

Not all LED light sources are "eye safe", so use care
when fooling with this stuff. This is one of the nice
aspects of the remote phosphor revolution, is the
opportunity to make the devices safer for human eyes.

(Example of remote phosphor - the phosphor is deposited
on the yellow plastic, while inside the lamp, the light source
is some blinding blue LEDs. The only thing wrong with
these bulbs, is the light is too yellow in color for my
tastes. I'm waiting for them to fix this.)

http://www.lighting-gallery.net/gal...10034/normal_LEDi12K58RP-230_B22d_Philips.jpg

The record so far, is some dude put 17 amps through a power
LED, while it was soldered to a copper slug. I hope the
guy had his welder goggles on :-)

Paul
 
The record so far, is some dude put 17 amps through a power
LED, while it was soldered to a copper slug. I hope the
guy had his welder goggles on :-)
Click to expand...

Relax. Found a 60amp fuse in a Fire'n'Storm salvage yard for service
behind the washer. Adapted and extended the cord, including the wires
to and from the service, for both these -

http://www.lincolnelectric.com/en-us/Equipment/Pages/product.aspx?product=K1170

http://lincolnwelder.org/lincoln-weld-pak-100/


https://en.wikipedia.org/wiki/File:Great_Egret_strikes_for_a_Fish_-_crop.JPG

Swap you that in a metal Greater Egret with a mailbox mounted on its
head for this if on credit... (Relax. I already got O/A tanks, so
nothing should explode or knock out any cinderblock walls.)

http://www.weldingsuppliesfromioc.com/lincoln-mig-conversion-kit-k610-1
 
I think they go open more often. I have noticed LED array lights where
4 adjacent LEDs are dead - presumably it has strings of 4 in series.
 
I think they go open more often. I have noticed LED array lights where
4 adjacent LEDs are dead - presumably it has strings of 4 in series.
Click to expand...

That's what I thought. I asked because I was wondering about the use of
current-limiting resistor. If LEDs always just blown, why bothered? :)

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That's what I thought. I asked because I was wondering about the use of
current-limiting resistor. If LEDs always just blown, why bothered? :)
Click to expand...
Because the LED will blow, or soon after power is applied. It needs the
resistor, there's no way around that.

Michael
 
Mr. Man-wai Chang said:
That's what I thought. I asked because I was wondering about the use of
current-limiting resistor. If LEDs always just blown, why bothered? :)
Click to expand...

The current limiting resistor is to set the operating point.

But there are other ways to do that. Such as an LM317 in
constant current mode. Or even a switcher which delivers
a constant current.

*******

There is an article which glosses over LED failure mechanisms,
but it doesn't come right out and say in every case, what
happens to the LED. It also doesn't give any idea how
common the failure modes are. Like, how many LEDs these
days will be so poorly constructed, as to have "whiskers"
as a failure mode ? I'd be more interested in the current
crowding case (because that might be similar to how other
diode types fail).

http://en.wikipedia.org/wiki/List_of_LED_failure_modes

Paul
 
I was challenged on this... I have never studied material science,
notably, LED and diodes. SO... :)
Click to expand...

Thank you all for the replies! Live long and proper! :)

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My favorite for this, is the LM317. On my LED
test jig, I have it set up for less than 20mA,
using an LM317. (That's for testing 5mm diameter LEDs.)
The LM317 has a lower limit, due to its internal bias
current, so you can't go all the way down to zero this way.
But I have spanned, anywhere from 20mA to 500mA this way.
I have no reason to go to 1 ampere right now. My power
LEDs can take it, but my eyes can't.
Click to expand...

This thread mentioned LM317! :)
http://forum.allaboutcircuits.com/blog.php?b=378

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Mr. Man-wai Chang said:
This thread mentioned LM317! :)
http://forum.allaboutcircuits.com/blog.php?b=378
Click to expand...

For me, the price was right, and that's why I used it.

The thing is, my local electronics shop (not a Radio Shack)
is a ripoff. So when they actually offer a component
at a decent price, you try to take advantage of that.

To give you some idea, they had a LED on sale there, a
5mm LED, for $3.00. I found a functionally equivalent
LED on the Internet for $0.20. Which makes a big difference,
when you're buying the LEDs 100 at a time.

Paul
 
To give you some idea, they had a LED on sale there, a
5mm LED, for $3.00. I found a functionally equivalent
LED on the Internet for $0.20. Which makes a big difference,
when you're buying the LEDs 100 at a time.
Click to expand...

US$3 for a LED? That's ridiculously expensive. Does it include shipping?

In Hong Kong, there are shops selling LEDs at about HK$1 each, and you
can test it before buying it.

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For me, the price was right, and that's why I used it.
Click to expand...

Compared to a resistor solution, this method is quite expensive. Note
that you also need to buy some capacitors to use the Volt-Reg....

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Mr. Man-wai Chang said:
Compared to a resistor solution, this method is quite expensive. Note
that you also need to buy some capacitors to use the Volt-Reg....
Click to expand...

Not necessarily.

If I was using the LM317 as a voltage source, I might add
0.1uF to 1.0uF caps to the input and output, for stability
if the wiring is a little long. The 1N4002 here, protects
against the case that the input power has failed (and appears
as a short to ground), and the output capacitor has a fairly
large charge on it. The diode forms a shunt path, to prevent
damage to the LM317. If you knew for certain your output
capacitance was 1uF or less, then you might not need
that diode. The idea in this design, is the "load" for
the circuit, may have its own large capacitor, and the diode
protects against such a large capacitor if it is present.

http://www.bowdenshobbycircuits.info/317.gif

*******

When operated as a current source, I probably would
not bother with the output cap. I could still have a cap
on the input side. That would be OK. Like this example.

http://ts1.mm.bing.net/th?id=H.4982730038313416&pid=15.1

Paul
 
When operated as a current source, I probably would
not bother with the output cap. I could still have a cap
on the input side. That would be OK. Like this example.
http://ts1.mm.bing.net/th?id=H.4982730038313416&pid=15.1
Click to expand...

Let me look up the price of LM317 in Hong Kong shops... :)

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Not necessarily.

If I was using the LM317 as a voltage source, I might add
0.1uF to 1.0uF caps to the input and output, for stability
if the wiring is a little long. The 1N4002 here, protects
against the case that the input power has failed (and appears
as a short to ground), and the output capacitor has a fairly
large charge on it. The diode forms a shunt path, to prevent
damage to the LM317. If you knew for certain your output
capacitance was 1uF or less, then you might not need
that diode. The idea in this design, is the "load" for
the circuit, may have its own large capacitor, and the diode
protects against such a large capacitor if it is present.

http://www.bowdenshobbycircuits.info/317.gif

*******

When operated as a current source, I probably would
not bother with the output cap. I could still have a cap
on the input side. That would be OK. Like this example.

http://ts1.mm.bing.net/th?id=H.4982730038313416&pid=15.1

Paul
Click to expand...
But you don't need a 317 to make a current source. If we're talking
average LEDs, then one can use an FET (depending on the current, sometimes
a resistor isn't even needed) or a small signal bipolar with a few
resistors.

If I needed a higher current source, I'd go with the 317, but if I needed
a small current source, I'd be looking at other things, which I could
throw together with junk parts.

Michael
 
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