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Old 19th June 2006, 12:45 AM   #11
clem_o is offline clem_o  Philippines
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Ok, how about this one - LEDs have a finite life. Not sure if their Vf changes as much, as the light output drops...

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Old 19th June 2006, 07:07 AM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
at the junction, the self emitted light must surely overwhelm any daylight leaking in.
What is the issue?

LEDs are one of our more reliable components with an extended lifetime compared to many components used within their ratings.
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Old 19th June 2006, 07:46 AM   #13
clem_o is offline clem_o  Philippines
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True, though they are rated for several "thousand" hours. However, a phono preamp we made had almost all of its parts still ok after 10+ years - save for the LEDs used to bias the current sources...

Cheers!

Ok - maybe the electrolytics were a bit dry. Not sure...
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Old 19th June 2006, 08:46 AM   #14
forr is offline forr  France
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---They modulate with light...---
I think Christer gave some values for this modulation in a thread. I can't localize it. I think it was not significant.
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Old 19th June 2006, 11:45 AM   #15
djk is offline djk
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It depends on the circuit. For audio you're probably OK.

IIRC, Bob Pease of National Semiconductor wrote a heads up note in Electronic Design a few years back.

http://www.national.com/rap/

Dear Bob:
Roger Sheker's letter about the photosensitivity of input-protection diodes, which appeared in your recent Mailbox column (Electronic Design, Aug. 19, p. 66), reminded me of a trick that I started using many years ago when working on low-level circuits. In the days before SMT components, I always found it very frustrating that signal diodes, including low-leakage diodes, were inevitably in clear glass packages. (Not all are. Fairchild made many low-leakage diodes in black-painted packages. But when the paint gets scratched, it's almost as bad. /rap)

As such, the photocurrents often far exceeded the actual leakage of the diode, particularly when the board was out on the bench for calibration, etc. Putting black heatshrink tubing over the diode packages improved the problem, but was a pain in production. Then I realized that I could use an inexpensive small-signal transistor (2N3904, etc.), in a completely opaque TO-92 package, as a diode. The C-B junction is fairly low in leakage, has a high breakdown voltage, and the transistor may actually cost less than special low-leakage diodes of lesser performance.

But if you can tolerate the modest reverse-breakdown voltage (such as in back-to-back diode applications), the B-E junction makes an exceptionally low-leakage diode. Although transistor data sheets caution against breaking down the B-E junction, I have found that in protection or clamp applications this isn't particularly harmful if the current is limited to reasonable values. Hence, the B-E junction can make a useful low-leakage bidirectional input clamp, providing a forward drop in one direction, and a clamp of about 6 to 9 V in the otherówithout any problems due to photosensitivity.

By the way, amazingly, some small metal-can transistors DO show photosensitivity because the ceramic/glass header and epoxy seal at the bottom are translucent. Of course, today with SMT technology, you can finally buy signal diodes in opaque packages! (Yeah, but many of those junctions are still LOUSY in terms of conduction and leakage. /rap)
Eric Kinast
via e-mail

~snip~

The main subject I wanted to comment on was your mentioning of typical signal diodes' sensitivity to light. I had an amplifier in which I couldn't tame the 60-Hz noise. It had vent holes in the top lid, and an incandescent light was shining down on it in its usual place of use. (Incandescent lamps tend to be bright at dc, with only a small 120-Hz component of noise. That's because the large thermal mass of the filament doesn't let the light intensity fluctuate much. It's the fluorescent lamps that make a lot of 120-Hz hum! And at 360 Hz. Not much at 60 Hz. /rap)

When I moved it to the test bench, removed the cover, and focused my shop high-intensity light on it, it measured very high noise. (I haven't studied halogen lamps' emissions, nor noises, very much. /rap) I was lucky as the light generated enough heat to cause other problems. When I moved the light away from the unit, the noise stopped. This was when I remembered reading of this phenomenon in your article. Taking an ultra-black magic marker to the input limiting diodes solved the problem. Thanks again and please publish more practical engineering stuff. I love it!
Roger Sheker
via e-mail

Isn't it funny how removing a noisy source correlates well with the noise going down a lot?óRAP

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Old 19th June 2006, 03:41 PM   #16
AndrewT is offline AndrewT  Scotland
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Hi,
I am sure glad my LEDs are not painted black, I would lie awake at night worrying that I might have scratched them.

I think we are really referring to other diodes being sugject to light modulation, not LEDs.

I have seen figures for LEDs of 50khrs and 100khrs. before performance drops off significantly.
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Old 19th June 2006, 06:14 PM   #17
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Does this explain why my hifi always sounds better at night with the lights turned down?
BAM
(a little Merlot seems to help too)
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