General > General Technical Chat
How do you prove an LED is actually lit?
Fraser:
A Google search for “LED failure detector” revealed that On-Semi produce a component specifically designed to detect LED failures and it can even activate a back-up LED. Likely not what the OP wants due to his use of a 12V LED indicator package but it demonstrates that such detection devices exist as a COTS component.
https://www.onsemi.com/pub/Collateral/DN05066-D.PDF
https://www.onsemi.com/pdf/datasheet/hbl5006-d.pdf
Fraser
jmh:
--- Quote from: Siwastaja on January 25, 2023, 07:02:47 pm ---Do you think it's a common failure mode to stop producing light while continuing to pull similar amount of current? I'd say it's far-fetched.
--- End quote ---
My issue is I have no idea how these things may fail. A single LED yes, but all the gubbins in these bulbs, no. I would not expect it to pull the same current of course but it may pull some and I don't know the difference between good and bad. I may be worrying too much! This is why I asked rather the my usual cobble bits together and experiment.
--- Quote from: mariush on January 25, 2023, 07:41:00 pm ---If there's a led driver and the circuit fails, at most you're gonna have pulses of current draw, but average the current consumption would be very low, I'd say maybe a couple mA at most... much less compared to at least 20-50mA or whatever such bulbs would use.
--- End quote ---
Thanks, in which case then current sensing seems to best way to go.
--- Quote from: Siwastaja on January 25, 2023, 07:02:47 pm ---TLDR; what are you actually doing?
--- End quote ---
It's to modify railway signal lamps to use LEDs instead of filament bulbs. The bulbs, each has 2 filaments / 3 contacts, take over an amp at 12V and are very expensive things and don't last that long. The trouble here is, with a filament bulb we simply use a relay to determine if it is dead, switch to the second filament and raise an alarm. We need to replicate this for the LED - not to switch to a backup LED but to drop a relay. What is supposed to happen then is all the further signals go to danger, and that bit is easy. These signal lamps are somewhat ancient and will not take the 'real railway' drop-in replacement LED units and anyway we could never afford them.
(I am of course asking in other relevant places but for me I want to understand how it works / make it work myself rather than just being given a widget)
IanB:
I have some LED bulbs that take anything from 12 V to 24 V, AC or DC.
When I tested one of them, I found that it had an internal regulator behaving something like an SMPS, so that it would draw constant power at any supply voltage and maintain more or less constant light output. So if I doubled the voltage from 12 V to 24 V, the current drawn was reduced by half.
I would guess, if the bulb failed, that the current would change from the nominal value. Most likely it would decrease. If it increased then some internal component would probably overheat and fail, and then the current would decrease after that. Or the electronics might fail short circuit and blow a supply fuse. There are all sorts of possibilities here.
The reliable solution I suppose, in a safety critical application, is to continue using the traditional incandescent bulb, or to completely replace the signal lamp fixture with a modern replacement designed to meet regulatory standards for performance and reliability?
I believe there is some similar issue with traffic lights on the highway. You might research how they handle the same question?
Tomorokoshi:
I'm lit, therefore i amps?
Sorry!
SiliconWizard:
Note that the current sensing method may not work if the failure mode of the LED is a short-circuit (never seen it myself, but I guess that could happpen) and the LED is driven with a constant current source. Obvious, but hey, what do ya know.
If it's just powered with a voltage source and a limiting resistor, it should work fine though.
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