Slightly OT LED questions

[APKhaos]

This board is impressive! Hope its OK to ask a non-flashlight LED question......

I'm working on a project to develop a LED replacement for the gauge lighting on older Porsche 911s. The stock incandescent lighting sucks, and there are lots of 911 DIY guys who are looking for something better. This is more a labor of love than a commercial project - it looks like the fabrication process will be daunting! For more details on the project, look here - http://forums.pelicanparts.com/showthread.php?s=&threadid=89372

The project needs a lot of light. Space is restricted to 9mm overall diameter for the LED assembly. The internal gauge lighting design relies on reflected light from the incandescent bulbs [1 or 2 per gauge].

I'm currently testing with 4 x 3mm LED arrays from superbrightleds.com, but I really need to fabricate the arrays myself to solve the dispersion problem. I can get four 3mm packages lying on their sides for better dispersal around the gauge, or maybe up to 6 x 3mm LEDs mounted verically in the available space.

Have also looked at the Luxeon emitters, but not sure the 1 watt version derated to 100mA [no heat sink] would be brighter than 4 or 6 x 3mm packages.

So...some dumb questions:

- What are the brightest 3mm LEDS to use? Beam dispersal is important if there are more than 4.
Need white, plus perhaps red as an optional color.
The Nichias seem to be highly rated - is there anything better?

- Which will deliver more light - an array of 4 or 6 x 3mm packages or a Luxeon 1W emitter derated to 100mA?

- This sounds agricultural, but it may be possible to mould form thermal epoxy around the emitter slug to bulb base to sink some heat via the bulb base to the gauge case. Is thermal epoxy mechanically strong enough to form a 9mm x 4mm slug which would bond the emitter to the 7mm base?

- Anybody have a reference design for a constant current supply w/ 12V to 14V input range, output current for 32 or 48 3mm LEDS, and dimmable?

Really appreciate any info.
Thanx,
Tony K

 

[Entropy]

At their full rating, most 5mm Nichias are rated at 20 mA current. So a Luxeon at 100 mA would be equivalent to 5-6 of them. (More likely 6-7 - LEDs usually become more efficient when underdriven.)

That said - Are you sure you couldn't get some sort of thermal path to allow for a higher current? Could you provide pictures of the area where the LED would be mounted?

 

[highlandsun]

I'm running Amber Luxeons at 100mA on my car. They're plenty bright. See this thread for info: http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=15;t=000090

I think for instrument panel lighting 100mA would be blinding; you need much less. Even 30mA in a white Luxeon is piercing.

For dimming, I guess you could use a potentiometer instead of the fixed 5ohm resistor that I used.
High-Dome Luxeons have a very broad dispersion, should be excellent for this purpose.

 

[APKhaos]

Well, if the Luxeon is equivalent to 5+ Nichias, I'd better look hard at using the emitter. There is a 10mm overall diameter limit, so what might be possible is to terminate the emitter onto the bulb base, then use some 10mm ID polycarbonate tubing as a mouldform and flow thermal epoxy to bond the emitter to the bulb base. Hoping the thermal epoxy is strong enough to support the emitter to base joint without failing.

Andy - this will provide a thermal path from the emitter to the bulb base/holder/gauge case, which should be enough to drive the emitter at more than 100mA.

This rough diagram shows a cross section of the idea. There will be a series resistor in each base.

So, Now its time to buy some Luxeon emitters to test - will try both batwing and side emitter types for this application.

- Is Future Electronics the best place to buy??
- Whats the best thermal epoxy to use??

Many thanx,

 

[Harrkev]

It seems to me that using several LEDs would allow you to spread them out more. This would eliminate "hot spots." I don't know if this may be a problem.

LEDs also tend to "thow" their beam. If you attack them with some 120-grit sandpaper, then they should glow more like a regular light bulb.

For the ultimate in customizability, get some RGB diodes. Then you could customize the color by twiddling a couple of knobs. If you wanted to rig up a few op-amp circuits and a couple of oscillators, you could have a color-shifing dash similar to the Sauce products.

 

[APKhaos]

Eliminating 'hot spots' and overall intensity have been the gnarly issues with this project. Not helped by the gauge design, which basically relies on light from the one or two incand bulbs being reflected off the gauge case onto the gauge face. Tried squaring off the tops of the cases, which helped....but not quite enough. This was with 4x3mm. Plan on trying 6x3mm and 4x3mmm lying on their sides to see if that helps.

Ummm....will probably stick with plain white or red - the multicolor trick is a little rice-ish

Got some new Nichia 3mms today, including the NSPW315BS - the milky case version. Will try these while sorting out the Luxeon approach.

Is there a better source for Luxeon than Future Electronics??

BTW, the Luxeon specs say that dimming must be via switching - not current limiting. Is that right?

 

[highlandsun]

"Must" is subjective. The output characteristics of the LED (brightness and color) are only guaranteed at the particular current rating. At lower current, the color changes as well as the brightness. My amber luxeons are a pale yellow at 100mA, they become more saturated/orange at full power.

 

[highlandsun]

According to this approximation : lumens = cd * .00024 * angle^2 the brightest 3mm Nichia is the NSPW310BS at 1.296lm. Looking at the Luxeon datasheet, I figure a white LS at 100mA will be at about 35% of normal output, or about 6.3lm. That's assuming a low dome or high dome emitter, the side-emitter config is dimmer. Also larger, probably not suitable for your purpose...

I'm referencing the Nichia data from http://www.nichia.co.jp/product/lamp-white.html

and the Luxeon data from their DS23.pdf ...