Author Topic: Just noticed an odd quirk with Osram LEDs  (Read 2344 times)

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Offline SirNick

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Just noticed an odd quirk with Osram LEDs
« on: July 08, 2014, 11:02:50 pm »
I built an SMD LED demo board (to easily audition my stock parts) and of course managed to swap the power leads on its first power-up.  Now, there are some bi-color LEDs on there, but none of them are in an anti-parallel configuration.  They just use 4 pads, with all diodes forward biased.

So, with reversed leads (and a safe, low voltage input), what should have happened is -- nothing.  At first glance, I wondered why only three LEDs were lit.  Then, I noticed the three that were lit, were lit red.  Which is weird, since they're white LEDs.  (Osram LW / LCW)  Of course, once I sorted the polarity, the whole board lit up in a painfully bright display of every possible color all at once.  (Each LED is individually controllable via a breakout header, but they were paralleled for the initial test.)

How do you explain that?  Dummy light?  It seems unlikely they would go to the expense of providing a reverse-polarity indicator for giggles.  Maybe they're intended to be used in LED flashlight applications, and the red "night light" is an undisclosed feature?  (Or maybe I just haven't scoured the datasheet adequately.)
 

Offline thm_w

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Re: Just noticed an odd quirk with Osram LEDs
« Reply #1 on: July 08, 2014, 11:09:24 pm »
Some high power LEDs have ESD diodes or TVS', perhaps those were heating up or lighting up.

« Last Edit: July 08, 2014, 11:11:57 pm by thm_w »
 

Offline ConnorGames

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Re: Just noticed an odd quirk with Osram LEDs
« Reply #2 on: July 08, 2014, 11:36:36 pm »
I had a similar thing happen with a laser diode. How strange!
 

Offline PedroDaGr8

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Re: Just noticed an odd quirk with Osram LEDs
« Reply #3 on: July 08, 2014, 11:44:52 pm »
I built an SMD LED demo board (to easily audition my stock parts) and of course managed to swap the power leads on its first power-up.  Now, there are some bi-color LEDs on there, but none of them are in an anti-parallel configuration.  They just use 4 pads, with all diodes forward biased.

So, with reversed leads (and a safe, low voltage input), what should have happened is -- nothing.  At first glance, I wondered why only three LEDs were lit.  Then, I noticed the three that were lit, were lit red.  Which is weird, since they're white LEDs.  (Osram LW / LCW)  Of course, once I sorted the polarity, the whole board lit up in a painfully bright display of every possible color all at once.  (Each LED is individually controllable via a breakout header, but they were paralleled for the initial test.)

How do you explain that?  Dummy light?  It seems unlikely they would go to the expense of providing a reverse-polarity indicator for giggles.  Maybe they're intended to be used in LED flashlight applications, and the red "night light" is an undisclosed feature?  (Or maybe I just haven't scoured the datasheet adequately.)

It's an undisclosed feature, supposedly, instead of using a standard diode for the reverse voltage protection they used a small red led. I remember this coming up on some automotive forums years ago now. I can't find any info on it now but there was some available at that time. Actually not undisclosed, just not well described. Many of their datasheets say reverse voltage will result in red or no emission.
« Last Edit: July 08, 2014, 11:47:34 pm by PedroDaGr8 »
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Offline SirNick

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Re: Just noticed an odd quirk with Osram LEDs
« Reply #4 on: July 09, 2014, 12:09:16 am »
Huh.  I never knew LEDs came with reverse polarity protection diodes.  All the ones I've used up until now just don't conduct in reverse.  Unless the avalanche rating is exceeded.

The latter was a relatively recent discovery as well.  I blew out an LED on a breadboard with -5v.  Had no idea that could happen.  It was only still connected because I changed the circuit around it, and didn't think it would care whatsoever.  I was quite wrong.  Even more the surprise since it had a rather hefty rating of 50mA.

I guess it goes to show, even simple components aren't always as simple as you think.  So anyway, thanks for that little tidbit.  :)
 


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