Power & reliability issues for UV leds

[distar97]

I recently got an inexpensive HK keychain light with a UV led. It
worked great at first. After a very short amount of time, the
brightness dimmed to a useless level.

Thinking it had cheap 2016 (2) cells to begin with, I replaced them. No
effect. Then I noticed there is no current resistor or even space for
one. The led leads are simply placed above and below the 2 cell stack,
finger pressure makes the contact. So the 2 cells (6Volts) go directly
to the led leads. Does this sound right? Not to me.

I remember reading about reliability problems with UV leds. Is that
still the case?

 

[Allodoxaphobia]

I recently got an inexpensive HK keychain light with a UV led. It
worked great at first. After a very short amount of time, the
brightness dimmed to a useless level.

Thinking it had cheap 2016 (2) cells to begin with, I replaced them. No
effect. Then I noticed there is no current resistor or even space for
one. The led leads are simply placed above and below the 2 cell stack,
finger pressure makes the contact. So the 2 cells (6Volts) go directly
to the led leads. Does this sound right? Not to me.

If LEDs are not current limited, then what you have observed is execatly
the End Result. It's all Made In China crap now-a-day -- so I would
question the 'design' behind it. Maybe, just maybe, it was designed to
'ruin' the LED at about the same time the 'non-replaceable' battery
died. Their junk - our landfills.

I remember reading about reliability problems with UV leds. Is that
still the case?

Separate topic, I guess? Duuno 'bout UV LEDs -- even if they exist.
(It's only been recently that blue LEDs have reached The Masses.)

Jonesy

 

[Dave D]

I recently got an inexpensive HK keychain light with a UV led. It
worked great at first. After a very short amount of time, the
brightness dimmed to a useless level.

Thinking it had cheap 2016 (2) cells to begin with, I replaced them. No
effect. Then I noticed there is no current resistor or even space for
one. The led leads are simply placed above and below the 2 cell stack,
finger pressure makes the contact. So the 2 cells (6Volts) go directly
to the led leads. Does this sound right? Not to me.

Most of the cheap keyfob LED lights are like that. They are designed with
the idea that the internal resistance of the batteries is sufficient to
limit the current, and that although the LED is overdriven very hard, it
should last the couple of hours the batteries last. Also, they generally
have a momentary switch which may extend their life a bit as the LED is
unlikely to be powered for more than a few seconds at a time..

I remember reading about reliability problems with UV leds. Is that
still the case?

Don't know, but my blue keyfob drew 60mA, and taking the resistance of my
AVO into account, it was probably much higher than that. No standard LED is
going to survive that sort of abuse for long.

Dave

 

[[email protected]]

Exactly right on the use of the internal battery resistance to limit
the current. A local inventer in Bend had us build a bunch of LED light
units for him. They go in skate board lights! The units had two coin
batteres and three LEDs, as I recall. These were white LEDs and I think
they would run for a couple of hours and then the batteries had to be
replaced.

We were way too expensive for him, so he is having the units mfg.
elsewhere.

I wonder what the UV LEDs are being used for and how did the poster
know if they worked or not?

Paul,
Jodeco, Inc.
Redmond, OR

 

[Dave D]

Exactly right on the use of the internal battery resistance to limit
the current. A local inventer in Bend had us build a bunch of LED light
units for him. They go in skate board lights! The units had two coin
batteres and three LEDs, as I recall. These were white LEDs and I think
they would run for a couple of hours and then the batteries had to be
replaced.

We were way too expensive for him, so he is having the units mfg.
elsewhere.

I wonder what the UV LEDs are being used for and how did the poster
know if they worked or not?

AIUI some UV LEDs emit a lot of visible violet light, though I've never
played with them myself.

Dave

 

[distar97]

Thanks for all the observations...Some additional comments:

I've worked with LED's since their introduction way back when, but this
little keychain model is my first UV led. It is indeed a true UV,
probably 395nm or so. I didn't get much use out of it, the mom switch
allowed for quick on-off use on some fluorescent items I had laying
around. I don't think it lasted more than 60 seconds in total time. It
was quite bright in the UV with a bit more visible light than a
traditional "black light" tube UV-B source. I wanted it as a small
convenient UV source for quick document checks but just as a little toy
really. (something I can admit to this group)

The seller said he will send a replacement. If he does, I'll check and
see what current typical UV leds run at and figure what resistance I
need with 2 2016 lithiums, maybe I can run it on a single 2016 for a
lower, more reasonable voltage, and leave some space for a resistor.

I questioned the reliability for a few reasons, mostly web chatter, but
in fact Inova, a reliable company, dropped their UV offering. I wonder
if reliability issues lead to that decision, or simply lack of market
interest.

Dennis Harper
[email protected]

 

[James Sweet]

Thanks for all the observations...Some additional comments:

I've worked with LED's since their introduction way back when, but this
little keychain model is my first UV led. It is indeed a true UV,
probably 395nm or so. I didn't get much use out of it, the mom switch
allowed for quick on-off use on some fluorescent items I had laying
around. I don't think it lasted more than 60 seconds in total time. It
was quite bright in the UV with a bit more visible light than a
traditional "black light" tube UV-B source. I wanted it as a small
convenient UV source for quick document checks but just as a little toy
really. (something I can admit to this group)

You might have better luck with a blacklight CCFL, that technology is
pretty robust and the blacklight performance is comparable to a standard
blacklight fluorescent tube.

 

[distar97]

Thanks to James Sweet for the CCFL tip. I totally forgot All
Electronics had these things. Its currently out of stock but they had a
25mm CCFL for 8 bucks or so, they also have the necessary inverter to
provide the proper juice. It's not as small as my little keychain led
but it would make a very nice "robust" portable UV source. I can
probably find the Velleman CCFL elsewhere. Curiously, All doesn't carry
UV LED's. BTW, you can see the keychain version I got on ebay. Search
under UV LED, you'll see just one HK seller offering the small keychain
coin-cell type. A guy in the states has a different pointer style UV
LED light.

Dennis H.

 

[Ken Weitzel]

Thanks to James Sweet for the CCFL tip. I totally forgot All
Electronics had these things. Its currently out of stock but they had a
25mm CCFL for 8 bucks or so, they also have the necessary inverter to
provide the proper juice. It's not as small as my little keychain led
but it would make a very nice "robust" portable UV source. I can
probably find the Velleman CCFL elsewhere. Curiously, All doesn't carry
UV LED's. BTW, you can see the keychain version I got on ebay. Search
under UV LED, you'll see just one HK seller offering the small keychain
coin-cell type. A guy in the states has a different pointer style UV
LED light.

Dennis H.

Hi...

And for those in Canada, if you have a chequing account with the
Royal Bank of Canada, next time you need more blank cheques you can
ask for the cheque "kit", rather than just more blank cheques.

You'll get one in that kit.

Take care.

Ken