Looking for the right LEDs....

Biker Bear

Enlightened
Joined
Sep 9, 2006
Messages
279
Location
The Greater Los Angeles Metropolitan Sprawl
Greetings, all - I'm new here, hope I'm putting this in the right place.

I made some trail-marker lights for a group camping trip I went on over Labor Day weekend from some white LEDs I bought from BestHongKong.com a while back, CR2032 coin cells and tape. Granted my trail marker lights didn't run at maximum brightness for the LEDs I have - but that was actually a good thing, as the marker lights lasted longer than the 4-day trip without anyone having to fuss with them.

I recently realized that finding a batch of white LEDs that work pretty well (down to 2.75v!) on lithium coin cells might be a rather rare thing. While I did retrieve the LEDs I used for re-use on another trip, eventually I'm going to need more and I'm not sure where to go looking.

I don't have a manufacturer or any other significant details on the batch of LEDs I have, other than they were listed as 10,000 mCd brightness. It's been quite a while since I bought these and they're long gone from BHK's website.

So - any pointers on finding 5mm (or 3mm, I suppose) white LEDs that will run decently off a single lithium coin cell?
 

Taxythingy

Newly Enlightened
Joined
Sep 10, 2006
Messages
7
Location
Dublin, Ireland
Hi BB!

I'm fairly new here myself, but I'll see what I can do.

Try the following links for a similar LED:

http://www.ultraleds.co.uk/product_info.php?cPath=25&products_id=94
http://www.cascadesurplus.com/catalog/product_info.php/products_id/29
http://www.theledlight.com/5mmwhleds.html <- product spec sheets

As to whether it will run for 4 days, I offer the following verbiage which may or may not be of any use... :grin2: (others may step in and correct my limited electronics - hey, so I'm a chemist, not an electrician)

Your LiIon cell puts out 3V, and these LEDs are rated at around 3.5V. The cell is rated at 210mAh and the LED draws 20mA around 3.5V. This would give you a runtime of around 10hrs. If you lower the voltage available to the LED junction, it will draw significantly less current. Since the cell can only supply 3V, I'd guess that the current drop-off is quite significant and the cell should continue to drive for several days. As a comparison, 3.72V will drive a Luxeon K2 white at 1000mA while a drop to 3.42V will drive it at only 350mA.

So, I'd expect the current flow to be closer to 5-10mA on a mostly full cell, dropping to under 5mA as the cell runs down. This should give a run-time of something like what you are looking for.

As to whether the LEDs are going to run at 2.75V, if I can read the spec sheet correctly, the K2s (std Vf of 3.4ish) will operate down to the 2.7V area, so I'd expect a standard 5mm to handle something similar. A browse of a product spec sheet (see theledlight.com) suggested a minimum operating voltage of 2.9, but that is probably being conservative.

Hope all this helps - if it doesn't, try copying, pasting into notepad, then firing it into the recycling bin with as much vengance as can be mustered! :xyxgun:

Regards,
Chris
 

Biker Bear

Enlightened
Joined
Sep 9, 2006
Messages
279
Location
The Greater Los Angeles Metropolitan Sprawl
Maybe I wasn't clear, but I know for a fact from having done it - that the LEDs I have now - because of the low current draw at the working voltage - do indeed run for nearly 5 days at a useful level (for trail markers, anyway ;) on a single CR2032. I just want to be able to repeat the feat!

Thanks very much for the pointers. I noticed on a couple of the spec sheets from TheLEDLight.com that luminous efficiency ramps up dramatically past a certain voltage, but they'd still light up at a much lower voltage.

You've answered my main question - the LEDs I got are not some freak of nature; I might have to sample around a bit to find another batch that does well at low-ish voltages, but it's not going to be impossible to find them.
 

Steve K

Flashlight Enthusiast
Joined
Jun 10, 2002
Messages
2,786
Location
Peoria, IL
just a thought.... is there a reason to use white leds?

the eye is most sensitive to green light, so it may be easier to produce a perceived level of brightness with green leds instead of white.

Also, green leds usually have a lower forward voltage than white leds, so at a given current, the power draw is less. And a battery may be able to power it longer, since the difference between the battery voltage and the led voltage is greater.

Amber leds may also be an option.
Red leds have the lowest forward voltage, but the eye is much less sensitive to red.


Steve K.
 
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