High Voltage Cree XT-E for use with generator hub

itsmee

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Hi folks,

http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XTE-HVW

Would there any application for the 12V version in a generator hub setup?

4.5W at 0.375A (12V)

I don't know enough to know how best to drive it, since a generators 500mA would kill it.

Simply limit voltage to 12V?

Split off 150mA to a rear light?

It seems well suited to my untrained eye, but I've no idea really and am likely missing something obvious that would explain why people aren't talking about them.

-Glenn
 

Steve K

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most generators don't produce much power with a 12V load until the bike has reached at least modest speeds (it varies from generator to generator), so this would be a compromise for typical operation.

You could apply a current regulator in series with it to limit the current, and other methods could work too.

One reason to not use it for a bike light is that the web site says "Optimized for non-directional and small LED replacement lamps such as B10 candelabra....", which seems to be the opposite of most bike lights which are quite directional.
 

itsmee

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Yeah, I was wondering about what the required speed might be to achieve full brightness. I guessed if it was comparable to that required for the popular triple XP-G configuration then the XTE might be a decent alternative to it.

All else being equal, it seems more practical to me to use a single LED. Not that all else is ever equal.

The XTE actually has identical footprint and equivalent efficiency and viewing angles as an XP-G though, so presumably the same optics could be used.
 

Steve K

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Yeah, I was wondering about what the required speed might be to achieve full brightness. I guessed if it was comparable to that required for the popular triple XP-G configuration then the XTE might be a decent alternative to it.

All else being equal, it seems more practical to me to use a single LED. Not that all else is ever equal.

The XTE actually has identical footprint and equivalent efficiency and viewing angles as an XP-G though, so presumably the same optics could be used.

part of the learning curve of bike light design is figuring out what beam angle is suitable. The triple optics tend to have beams that are wider than the +/- 4 degrees that I find to work well for road use.

A significant difference between the XTE and the XP-G that doesn't get much mention, if any, in the datasheet is the size of the spot that is emitting the light. i.e. the size of the LED die in the XP-E compared to the bunch of little dice in the XTE and the phosphors that cover them. The large size of the light source in the XTE will produce a wider beam than the XP-G, and I think that's why it is described as optimized for non-directional lights.
 

itsmee

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I'd noticed the wide spread from the triangular arrangement. I think I'd also prefer three separate LED housings.

I had a pretty good look at the datasheets. The XTE beam ought to be only marginally wider, judging by the "typical spatial distribution" charts.

The form factors are also damn close, with both having a 3.45 x 3.45 footprint, the only difference being the XTE die is slightly more raised at 2.60 vs the XPGs 2.36.

Comparing the "electrical characteristics" charts, with a triple X-G setup at 500mA you ought to need 8.85V (3 x 2.95V) for approximately 4.5W.

For an equivalent 4.5W from a single XTE it would be around 365mA at 12.5V.

Alternatively, for the same 500mA generator output I'm guessing two XTEs could be used in parallel to yield two lots of 250mA at 12.1V for a total of 6W.

From what I understand the generator hubs never exceed 700mA, so presumably running two XTEs in parallel would also negate the need for any current limiting?
 

Steve K

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......

Alternatively, for the same 500mA generator output I'm guessing two XTEs could be used in parallel to yield two lots of 250mA at 12.1V for a total of 6W.

From what I understand the generator hubs never exceed 700mA, so presumably running two XTEs in parallel would also negate the need for any current limiting?

that is correct.

it does assume that the two XTE's are very similar, which is usually a safe bet when using individual LEDs, especially when they are sold on the same tape. For XTE's which use a number of smaller LED dice, I would assume that the dice are all from the same batch, but don't have any personal experience.

When running a 12V load with a bike dynamo, I doubt the dynamo will produce anything over 500mA at typical speeds.
 

Steve K

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I'd noticed the wide spread from the triangular arrangement. I think I'd also prefer three separate LED housings.

I had a pretty good look at the datasheets. The XTE beam ought to be only marginally wider, judging by the "typical spatial distribution" charts.

bear in mind that the spatial distribution charts describe the LED's radiation pattern without any optics. It doesn't indicate how the LED will interact with a particular optic.

I'd suggest reviewing what optics are designed for use with the XTE and see what the beam characteristics are.

edit: a check of Ledil's website shows that they do have data on optics use with the XT-E. I've used the Ledil Heidi before, and for the Heidi-D, p/n CA11264, the beam angle with the XT-E is 10 degrees. With the XP-E, it is 9 degrees.
For this optic, at least, the penalty for using the XT-E is very minimal.
<end of edit>
 
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itsmee

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Thanks for that Steve.

In that case it seems a simple way to achieve up to 6W. I think it would be four XPGs required for same.

I'd like to try them out. Sourcing the 12V XTE version could be a little problematic though, a quick search turned up no obvious suppliers.

It might have to wait, since I'll have my hands on a Shutter Precision SV-8 hub next week and want lights up and running ASAP.

(Had to share that, very excited ... my first ever generator hub).
 

Steve K

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the development of quality hub dynamos made a huge difference for bike lights! I started bike commuting around 1997, and all you could get were bottle dynamos or lights powered by heavy nicads. The first generation Schmidt hub dynamos came on the market around 2000, and it was sooo nice to not worry about battery run time or hauling a battery that weighed a couple of pounds. The development of the first high power white LEDs was another thing to celebrate!

I will offer a suggestion, though... bring a small battery powered light, just in case there is a failure in your home built light.
 

itsmee

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Much like yourself I started commuting in 1998. I used to put up with lights that dimmed over the course of a ride so that by the time I arrive home it felt like I was squinting and leaning over the handlebars to be able to see the road. More recently I've been using a 1W LED with single battery that throws little light, but is at least consistently dim for over an hour ... much like myself.

I appreciate the need for a backup light source. I'm all for redundancy, which is the main underlying reason for the parallel battery pack configuration that you've been kindly helping with in my other thread:

http://www.candlepowerforums.com/vb...ith-dyno-powered-triple-led-with-battery-pack
 

Steve K

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I may need to clarify that when I suggest keeping an extra light around, I mean a completely separate light with nothing in common with the dynamo light. Stuff breaks, and if you use the stuff long enough, it will certainly break... especially if it is designed and built by someone who doesn't have a lot of experience.

I keep a little Petzl headlamp in the bag just for fixing a flat or something, and a Cateye headlight powered by 4 AAs (and converted to LED) as a backup to the dynamo. I also use the Cateye as a headlight on some of my other bikes, just in case I end up riding in poor conditions or at dusk.
 

itsmee

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Thanks for the clarification. I did understand and will follow your advice on this.

Nonetheless, I'd still like to have redundancy in the dynamo light setup, by way of two power sources (dynamo and battery), at least two separate light fixtures, and the ability to switch any combination in and out such that a complete failure could only result from simultaneous failure of either both lights or both power sources.
 

Steve K

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There are complications that may need to be considered. Most dynamo lights use the dynamo's internal impedance to control the current going to the light.

Most battery powered lights use a current regulator to control the current going to the LED. It assumes that the voltage coming in from the battery will be within a fairly narrow range. Dynamo voltages can change a lot, depending on the speed and load.

In general, I think there is a chance that bad things will happen if you don't understand the details of all of the parts you are working with.
 

itsmee

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Yep, point taken. I've come to realize that dynamos are unfathomable beasts that only really belong in the hands of electrical engineers. It's a bit late to turn back now though - I'm picking up the new wheel tomorrow.
 
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