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Using MIL-HDBK 217F to determine FAILURE RATE OF led

jacksona

Nov 8, 2021
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I'm trying to figure out the failure rate of an LED using the mil-hdbk217F .
The problem that I have is there is no category for an LED in the diode section. I was thinking of using the general diode category, but the calculation requires the max voltage / applied voltage.
With an LED it's about how much current not voltage so I'm a bit stuck
 

Bluejets

Oct 5, 2014
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Not much point as any driver will fail long before the LED. (Practical experience)
 

Harald Kapp

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Nov 17, 2011
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Try this.

but the calculation requires the max voltage / applied voltage.
With an LED it's about how much current not voltage so I'm a bit stuck
Section 6.1 of the MIL Hdbk states that voltage is diode reverse voltage:
This is not applicable to LEDs as these are operated in forward mode only (at least that's how it should be).
Since "voltage applied" in reverse is 0, the voltage stress ration VS = 0, thus πS = 0.054:

upload_2021-11-8_10-15-24.png
The relevant factor is temperature stress which can be calculated from the other tables in the Hdbk , the power dissipation of the LED, the thermal resistance of the LED and ambient temperature.

Not much point as any driver will fail long before the LED.
Good point but doesn't help if one is required to deliver a calculated MTBF value for a product.
 

dave9

Mar 5, 2017
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Not much point as any driver will fail long before the LED. (Practical experience)
Tell that to my landscape lights. The LED bulbs are rated for automotive use (so should be good up to 14-ish volts) but much about 12V their lifespan is only a few hundred hours due to having only 2 x series resistors to limit current. I ended up putting a buck regulator board in series.
 

dave9

Mar 5, 2017
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^ If not for being generic-Chinese, the bulbs would cost about what an entire new landscape light set cost.

These generics, it was a wacky promotional deal, buy a 20 pack of something else, get these thrown in free, but instead of one ten pack free, I was sent 10 x 10 packs free (seller didn't want them back, return shipping must exceed their loss), plus a pair of fog lights were free with that 10 pack product, so I ended up spending $10 for 120 T10 bulbs and 20W fog bulbs. The buck driver board to fix the issue was only $4 from a US shipper.
 
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