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| Luxeon III on Star MCPCB | Cree XR-E on Star MCPCB | SSC P4 on Star MCPCB |
| Updated 03/22/08. Superbright LED technology is always advancing. So the LED you choose today will probably be surpassed in output/efficiency within six months to a year from now. For the year 2005 and most of 2006, the best choice was to choose one of Lumiled's Luxeon emitters. In October of 2006 Cree released the XLamp 7090, XR-E series LED. About 4 months later Seoul Semiconductor (SSC) released the Z-Power P4 LED, which is acutally based on Cree's chip, but doped and packaged differently. Both of these offerings totally surpassed what Luxeon was offering, more than doubling the available luminous output for the same amount of power (ie. lumens/watt). But for historical purposes, I still list the Luxeon offerings below. |
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| 2005 - 2006 Luxeon I - one watt, 45 lumens @ 350 mA (mA = milliamps) Luxeon III - three watt, 80 lumens @ 1000 mA Luxeon V - five watt, 120 lumens @ 700 mA Luxeon K2 - up to 140 lumens @ 1500 mA |
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| Late 2006 - 2007 Cree XR-E (Q5 bin) - three watt, ~240 lumens @ 1000mA (last quarter of 2007) Seoul Semiconductor Z-Power P4 (U bin) - three watt, ~240 lumens @ 1000mA (2007) |
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| 2007 has been the year of the Cree XR-E and the Seoul Semiconductor P4 for bike lighting applications. Back in November of 2006 I built my first triple Cree XR-E bike light using the P3 bin of the XR-E that was available at that time. It was more than twice as bright as the triple Luxeon III that I'd built a year earlier. Then in January of 2007, I ordered a few Seoul Semiconductor (SSC) P4 emitters. | |
| Shown
below is a Seoul Semiconductor P4 (U-bin) emitter. Notice that Arctic
Alumina Adhesive is used to mount the emitter and to electrically isolate the emitter's slug (back surface) and solder leads from contact with the aluminum surface onto which it is mounted. I totally smeared the AAA all over the surface of this light... probably not the best way to do it... since the stuff is so darned expensive.  |
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| Stars and Emitters -
As shown above, LEDs can be purchased as just a bare emitter
or
with the emitter soldered to a slightly larger star-shaped or circular
circuit
board that allows for easier mounting and soldering. (Emitters mounted
on stars are shown at the top of this page). The stars also
have a special aluminum core and backing that lets the heat transfer
through
from the emitter to dissopate heat to whatever surface that you mount
it to. The stars can be mounted to your case with screws (provided you
don't short them out) or fastened
with an adhesive that transfers heat as well, such as Arctic
Alumina
Adhesive. For most bike light construction situations, I
recommend
using the star. There are some situations where it is better to use the
bare emitter, but we won't get into that (using the bare emitter can
result in better heat dissipation, but it is more difficult to work
with... it's up to you). Note: most star MCPCBs provide electrical isolation from the metal heatsink surface that the star is usually mounted to, but there have been exceptions with some Luxeon Stars. This is not usually true when using just the bare emitter. For example, the Seoul emitters' heatsink slug, on the back of the emitter, is electrically connected to the positive lead of the package. Stars generally isolate the slug electrically, which makes life easier when building a light. |
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| Lambertian versus Side-Emitting - Luxeon LEDs are available in two different light output patterns. Lambertian and side-emitting. The Lambertian pattern concentrates most of the light out the front of the LED in sort of a parabolic dispersion pattern, which is ideal for use with most reflectors and optics. The side emitters are probably not well-suited for bike lighting as I have not seen any reflectors or optics available for them that would provide the kind of forward-facing beam that we need. Also I have not seen the Cree XR-E or SSC P4 available in anything but a Lambertian emitting pattern. | |
LED Binning and the Lottery - LEDs (Luxeon, Cree and SSC) are graded, like diamonds, into certain bin groupings. The binning represents several qualities of the LED which include: lumens output, color, and required foward voltage to light the diode. Generally, you'll try to select a bin that gives you the most lumens of white light with the minimal foward voltage. As you might expect, the better grade LEDs demand a higher price tag and are sometimes more difficult to find. This table from Cree shows the different luminous output ratings for the different bins of the LED @350mA.  Cree and SSC use different binning notation. The table below shows SSC's luminocity binning. Currently the SSC U-bin is roughly equivalent to Cree's Q-bin, except that Cree further subdivides the Q-bin into 4 tighter subranges (Q2, Q3, Q4, Q5). If you're lucky, you might be able to find a Cree R2 bin, which is just a step above the Q5 in brightness.  You can see that your odds are better at getting closer to your desired output with the tighter Cree binning system. With a U-binned Seoul Semiconductor P4 LED there is a greater variance in the binning, thus a little more of a gamble at getting a bright LED. In practice, however, I have had quite good luck with the SSC U-bins and find most of them to be toward the higher end of the bin... though I have no real way of measuring the lumen output other than just observation and comparison. |
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The graph below, from Cree, shows how driving the LED at higher current levels will result in greater intensity. For instance a P3-binned Cree XR-E will average about 77 lumens at 350mA. When driven at 1000mA, the intensity will be about 220% more than at 350mA. Thus, a P3-binned XR-E will be jamming at over 160 lumens when driven at 1A, providing you can keep it cool enough.  |
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| LED Drivers LEDs are rated to output a specific number of lumens of light at a given current (typically, 350mA, 700mA, 1000 mA, etc) and a specified forward voltage (typically around 3.75 volts for the ones we discuss here). We could drive an LED directly from a battery if the battery voltage was close to the forward voltage of the LED, but we would probably need to add a resistor to limit the amount of current flowing to the LED, otherwise it might get too much juice and burn out.. Many inexpensive LED flashlights and bikelights work this way. So if we had 2 x 1.5v batteries, in series, and an appropriate resistor, we could probably get enough to light our LEDs sufficiently. The preferred method to light an LED is to use a constant current regulator/driver. It will provide a constant current to the LED even as the battery's voltage changes (as it drains)... down to a certain point. Some drivers offer support for dimming the LEDs to a lower output than the full rated drive level. The nFlex and bFlex (taskled.com) offer several fixed maximum drive current settings as well as 5 levels of dimming within each of those. The 3021 Buckpuck controller overs continuous dimming via a 5k potentiometer across its control and reference pins. The 3023 Buckpuck controller offers the convenience of being pre-wired, but typically does not have wires connected for the control and reference pins, so you just get full power... no dimming. (Note: just to add to the confusion, I have seen some wired 3023 Buckpucks that do offer dimming... just be careful if you are looking for dimming capability). |
