LED going nearly short cct

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LED going nearly short cct

Postby Viewmaster » Fri Jan 25, 2013 12:38 am

I have 2 ultra bright LEDs recently go short cct or nearly so after running a few hours.
On went down to about 200 ohms and the other to about 5 ohms.
They were running well under their max rating so I wonder what may cause this? Probably just duff !
I always thought that LEDs went open cct, not nearly shorting out.
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Postby Steve Anderson » Fri Jan 25, 2013 12:50 pm

I'm not sure about LEDs, but most semis generally go short or low-resistance when they fail...that is when the failure isn't the result of an over-volts/current/power.

If these two are from amongst the 1500+ in the Niptrix it's simply a case of Infant Mortality, a failure rate of 0.13% (so far).

The failure curve of most electronic components looks much like a cross-section through a bath-tub...for that reason it's called the 'Bath-tub Curve'. At first there's some failures but that drops off rapidly (the end of the bath-tub where the taps would be),

After about 300 hours you're at the bottom of the bath and failures are much reduced. After 10+ years the curve rises gently due to age...now at the end of the bath where one reclines to have a good soak. The numbers are very approximate.

Any good manufacturer soak-tests his products for some time to catch the most likely failures, but not the full 300 hours, that would simply be too costly. Military and aerospace products are soak-tested and environmentally tested for 1000 hours or more, one of the reasons they're so expensive.

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Postby Viewmaster » Fri Jan 25, 2013 7:12 pm

Steve Anderson wrote:I'm not sure about LEDs, but most semis generally go short or low-resistance when they fail...that is when the failure isn't the result of an over-volts/current/power.

If these two are from amongst the 1500+ in the Niptrix it's simply a case of Infant Mortality, a failure rate of 0.13% (so far).
Steve A.


Thanks for that info Steve and yes, they are in the Niptrix.
Fortunately the layout I settled on is such that replacing any LED, although a bit fiddly is quickly done........So I daresay I may have some more violin playing to do from time to time. :lol:

The puzzle for me was that a partially shorted LED caused ALL the other LEDs on that line to go partially ON too !!
It has taken me ages to get yet another Niptrix penny to drop. :)
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Postby Steve Anderson » Fri Jan 25, 2013 7:49 pm

Grant Dixon did mention somewhere that he did have problems due to leakage currents which didn't allow some LED rows/columns to fully extinguish. Thus far it appears that this problem hasn't reared its ugly head for you. But a very leaky LED will act as a huge leakage path, hence what you observed.

If it happens again you'll know what to do. When confident with the display, burn it in for a few hours every day on real program material not just a test-card so all parts get 'exercised'. Prod and push it to find any duff joints (it happens). Turn it off, give a good shaking to simulate at a simple level a car journey...or even take out for a joy-ride in a real car...."Officer, I'm just taking my Niptrix for a bit of sight-seeing." "Sergent, we've got a right Cadbury's here."

Steve A.

It reminds me of sometime in the late 70s or early 80s when the Yorkshire Police decided to replace their aging fleet of patrol cars with Skodas. One officer was interviewed on TV, "16 valves? Yes. Eight in the engine and eight in the radio."
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LED gooing nearly short cct

Postby rommert » Fri Jan 25, 2013 8:24 pm

Good morning Albert.
On my Nipkow monitor i have had twice a shorted led in the cluster with six bright (white) leds. The problem occured after switching on/off the drive motor(s). The video circuit is on a seperate voltage stabiliser IC.
Ofcourse one defective leds protects the others when they are all in parallel.
( each one had their own series resistor ) After installing a protection diode anti parallel to the led cluster the fault did not happen again. I had the impression that the bright white leds where more sensitive for spikes (or wrong polarity) then conventional red leds.
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Postby M3DVQ » Sat Jan 26, 2013 10:47 am

Ok, I soaked all my LEDs in the bathtub but they keep blowing :shock:
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Postby Viewmaster » Sat Jan 26, 2013 7:14 pm

M3DVQ wrote:Ok, I soaked all my LEDs in the bathtub but they keep blowing :shock:

Maybe you have the water too hot?
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Postby Klaas Robers » Sun Feb 17, 2013 12:56 am

LEDs are diodes, but they are not meant to be connected to a reverse voltage of more than say 5 volts. I always try to avoid any reverse voltage.

If you have a LED cluster it is worthfull to place a normal diode anti parallel to the cluster, so any reverse voltage will be shorted automatically.

If such is possible for a LED array, I don't know. That depends on the design of the circuitry.
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Postby Steve Anderson » Tue Feb 19, 2013 1:51 pm

Indeed, most opto-electronic devices do have a low reverse-bias breakdown voltage, this is always specified in the datasheet for the device and is usually 5V or less.

But some are not rated for ANY reverse bias...as I recall some Luxeons are rated for ZERO reverse bias, but oddly this depends on the colour, I think it's the blue ones that are fussy about this, maybe the white ones too. It also seems to vary from one manufactuer to another.

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