Author Topic: Do LEDs fail open or short?  (Read 697 times)

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Offline hamiltont

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Do LEDs fail open or short?
« on: January 06, 2019, 05:47:26 pm »
I'm playing with red LEDs (https://www.sparkfun.com/products/9590) to learn more about failure modes. Using a very simple arrangement - connecting one LED directly to a current-limiting power supply and catastrophically destroying it by raising the max allowed current. Oddly the IF is 20mA (IPF is 30mA) but I can supply these continuously with much more (typically about 120mA before they start to "dim" and 330 mA before they are failed short and acting as a resistor)

Some of the LEDs fail open dramatically e.g. bright flash then black burned spot on the LED. However, most of them fail short much less dramatically - slowly get dimmer and dimmer as the current increases and then eventually offer only a minor resistance e.g. 300  \$\Omega\$ .

Is there a clear reason why some LEDs fail open and others fail short? In a "good" design, would you need to plan for both types of failure?
« Last Edit: January 06, 2019, 05:50:05 pm by hamiltont »
 

Offline chemelec

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Re: Do LEDs fail open or short?
« Reply #1 on: January 06, 2019, 05:55:17 pm »
If an LED is supplied with the Correct Current, It should Last a LONG TIME.
Can't remember when I have Ever had an LED Fail.
 

Online Tom45

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Re: Do LEDs fail open or short?
« Reply #2 on: January 06, 2019, 06:02:06 pm »
I don't know about the open vs. short failure modes.

But a "good design" will limit the current through the LED anyway, so as chemelec points out, the LED is unlikely to fail at all. And if it does, the current limiting of the circuit will protect against bad things happening with a failing short LED.

Failing open obviously isn't going to be a problem either. Although if the compliance voltage of the current source is really high, I suppose failing open could expose other circuit elements to damage.
 

Offline MrAl

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Re: Do LEDs fail open or short?
« Reply #3 on: January 06, 2019, 06:14:20 pm »
Hi,

I think it depends highly on the method of destruction.  You could buy a pack of cheap LEDs and do some destructive tests.
Supply one with larger current, supply another with high reverse voltage, etc.  Test the outcomes.

I had one 'blow' but it didnt really blow it just got very dark purple.  It was a white LED that got too much current.  It still produced light but ti was very dim and purple in color.
 

Offline David Hess

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Re: Do LEDs fail open or short?
« Reply #4 on: January 06, 2019, 06:51:52 pm »
At lower currents, diodes fail with a short due to overheating of the semiconductor.  At higher currents, especially with LEDs which use a bond wire for the top connection, they fail open.
 

Offline Zero999

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Re: Do LEDs fail open or short?
« Reply #5 on: January 06, 2019, 07:24:13 pm »
At lower currents, diodes fail with a short due to overheating of the semiconductor.  At higher currents, especially with LEDs which use a bond wire for the top connection, they fail open.
That's true and in some cases, the bond wire can fail leaving the semiconductor intact.

I don't think I've ever seen an LED fail a very low resistance though. Normally they still have a significant resistance, compared to the PCB track and will limit the current enough to stop it or the wiring from melting.
 


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