How hard can I thrash a 5 watter?

I'm sitting here writing this message on an overclocked, water cooled PC. I'm considering building a Luxeon bike light so I wondered how much I could overdrive a 5 Watt emitter with reasonable heatsinking - I don't want to water cool my bike! I've read of people putting 1.5A throught the 5Watter, but how does the light output scale at these levels?

Also, who is selling well binned 5Watters?

-Mark.

It would make more sense trying to overdrive a 3-watter than a 5-watter. The 5-watt LS is obsolete with the introduction of the 3-watter, anyway. A 3-watter is rated for 20,000 hours running at 1A, a 5-watter is only rated for 500 hours at its rated current of 750 mA. Doubling the current will probably drop the lifetme to a few hours. In fact, using a 5-watter on a bike, even at rated current, makes no sense as you can exceed the rated lifetime in a year or less if you ride a lot at night. On the other hand, you can probably overdrive a 3-watter to maybe 1.3A (about the point of diminishing returns as far as light output goes), and still get a few thousand hours of life. I think a good idea is to have a few power settings to conserve battery life when you don't need full light output regardless of which LED you use. An LED array is another option that seems to have fallen out of favor with the availability of the Luxeons.

A Luxeon V can produce 120 lumens at 700mA. A Luxeon 3 can only produce 65 lumens at 700mA.

That's true about the lumens. My point was more about lifetime-you can overdrive the L3, get fairly close to the output of the L5, and still have a fairly decent lifetime. An L3 at 1A puts out 80 lumens. At 1.3A it would put out maybe 25% more, or 100 lumens. The L5 has too short of a lifetime even at 700mA, much less heavily overdriven, to consider using on a bike. For a flashlight, it's fine, but you can rack up hundreds of hours a year on a bike light if you ride a lot. Lifetime is an inversely proportional exponent of LED current-doubling the current might result in 10% of the life or less (the exact function depends upon the type of LED). While worrying about getting 20,000 or 50,000 hours out of a bike light is ludicrous, aiming for a 1000 or 2000 is not. An L5 overdriven at 1.5A would probably only last a few nights, which kind of defeats one of the reasons for using an LED bike light in the first place, the other reasons being greater efficiency, especially when dimmed, and a truer white color (which results in greater visibility per photopic lumen).

It goes without saying here that regardless of which Luxeon is used you need some sort of collimating optics as Luxeons without one are better suited as area lights rather than headlights.

.

It's true, it reduces the lifespan of them, but you can still push them very hard if shorter lifespan isn't an issue. Mr Bulk routinely drives them with no resistance from 3 123 cells! I have one of those lights. It's super bright and gets very warm in a hurry. For a limited use flashlight that is all OK, but for a headlight, you're better off with an array of L3's I think.

Yeah, MrBulk pushes them very hard. I measured the current in mine at 1.98 amps with fresh 123s. Of course, we have to keep in mind that he does a superior heatsinking job that probably mitigates any harm caused by the severe overdrive.

Paul

Thanks for all the info. I wasn't aware that the 5W had such a short life.

This is for a bike light which will only really get used in the winter, when it's nice and cold so I'll have an edge when it come to cooling - It doesn't get dark much in the summer /ubbthreads/images/graemlins/ooo.gif

I'm definately doing to go for the L3 (or, rather, 3 L3s), driven at 1000mA.

Mark.

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Sprout said:
Thanks for all the info. I wasn't aware that the 5W had such a short life.Mark.

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It's designed for 500 hrs, but it won't die after 500 hrs. They just can't say what the output will be. My guess is that the output will still exceed the L3 even after 500 hrs.
120 lumens at 700mA is certainly better than 80 lumens at 1000 mA. But it is easier to create a hot spot with the L3 than with the LV.

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120 lumens at 700mA is certainly better than 80 lumens at 1000 mA.

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Just remember that the LuxV takes higher voltage too, since there are 4 dies in paired series-parallel configuration. So in actual fact, for the same amount of power, you should get close to 160 lumens for 2 LuxIII. Downside is you need 2 lenses to focus and you get some parallex spread.

Even a 5 watt LS though isn't a whole lot of light for a bike. It's one thing to need light for close work or walking, quite another at speed on a bike.

I built one with a 5 watt LS and a Micropuck running from a 12 volt water bottle battery. It's at least twice as bright as my SF L4 with fresh batteries so it's probably running at, or above spec. Despite that it's only suitable for reasonable riding conditions on the road.

On the road with some street lighting to help, the LS works fine. It's plenty bright enough up to about 17-18 MPH, after that it gets dicey. If I'm out for an easy training ride it works fine. If I'm really pushing (23 average - 27-28 peak mph) or I'm going off-road with my mountain bike, I use an HID. The LS just isn't adequate for any but the slowest trail and unsafe beyond 20 mph on any road. There just isn't enough throw and it doesn't put enough light on the ground to see problems soon enough. Above 20 mph, by the time you see something, it's too late.

Another problem with the LS is that while it's OK for riding at moderate speed on the road, is it's poor beam control with a 30mm optic, it blinds oncoming car drivers. I've been cursed at quite a few times, once told by a local cop that the miserable thing was worse than most high beams.

If you want the best, save your money and get an HID if you can find a way to afford it. I get 4+ hours on mine and it puts out probably 6-10 times the light of the LS. That sucker puts out REAL light.

Al

I've ruled out the L5 now, so I'm thinking more along the lines of 3 L3s with a trilens. Having never seen a luxeon though I'm a bit in the dark /ubbthreads/images/graemlins/smile.gif I have considered HID, and actually a Catey Stadium 3 wouldn't cost me much more than the LED system. The Cateye is a 21Watt HID, and should be very bright, but it seems a little fragile compared to the LEDs. It would only cost me about 200Euros for the lamp+ballast though.

-Mark

Sprout,

as long as you're not going off-road why not use a halogen bulb, they make bike lights that use a standard Pr base and you could use a halogen 6 volt bulb with Lithium-Ion cells; plenty bright! And inexpensive.

Btw, I'm overclocked: 1533@2016 mhz Amd.
And water-cooled too; Dangerden+Black Ice Extreme.
82 farenheit as i type this.

If you are going to consider thrashing your LED, please don't forget about thermal transfer...here is something to consider.


Take a look at the light output verses temperature on the Luxeon datasheet.

The light output is heavily dependent on the die temperature.

The cooler you keep the Luxeon, the brighter it is. You can loose an easy 40% or more of your light just because you don't keep the emitter cool.

Folks who try to keep and LED cool on a FR-4 board with
one layer of copper running to the flashlight body are
just loosing out hardcore. It is due to the thermal
resistance of that super thin layer of copper, doing a
very poor job of tranferring the heat, due to it's thermal
resistance.

PCB Weight Copper
(oz/ft2)...(mils)
1/2........ 0.7
1 ......... 1.4
2 ......... 2.8
3 ......... 4.2

Most consumer PCBs are 1/2 or 1 oz copper, which means the copper thickness is just like foil, 0.7 to 1.4 mils thick, or 0.007" to 0.0014" thick. This makes for a nice thermal resistor. Luckily in most flashlights, the distance isn't so short that we don't incur major thermal resistance in the super thin copper layer and cause big issues.

If you want to get more light, mounting the emitter to a slug that is 0.090" of aluminum, or 0.030" of copper, will do a very nice job of tranferring the heat from the Luxeon to the flashlight housing. Thermal epoxy is highly suggested for mounting the emitter to the metal slug, and either thermal grease or thermal epoxy between the slug and the flashlight housing will also help.