I received some K2 emitters, and Rebels today. I decided to do a direct comparison since the bins of each that I got are very close, so they should have very similar performance.
I received PW12-S00 K2 emitters, bin SYAH, manufacture date 07/03 (week 3, 2007) (SYAH = 51-67 lumens @ 350mA, 3.03-3.27 Vf)
The rebels I received are PWC1-0050, bin HWOD, manufacture date 07/09 (week 9, 2007) (HW0D = 50-60 lumens @ 350mA, 3.03-3.27 Vf)
Here are the contenders:
The rebel is extremely tiny, and difficult to work with.
Here's a closer image:
The entire top surface of the rebel is covered in phosphor. If you shine a blue LED at a rebel, the entire thing glows yellow. With a K2 (and all other Luxeons), only the die glows yellow, as that's the only place where phosphor is applied.
The heat sink and electrical contacts are both on the bottom of the rebel. This almost requires reflow soldering to achive thermal and electrical connections.
I decided to take a different approach. I made a rebel "star", by cutting out an 0.8" circle of 1/16" aluminium, then cutting a notch in the circle for the electrical contacts. I then used arctic alumina thermal epoxy to attach the rebel to the heat sink:
Here's the backside, showing the electrical connections:
I did this mostly for testing purposes, but maybe it'll give people ideas about alternate ways to mount the rebel to a heat sink without requiring reflow soldering.
Both the rebel and K2 were mounted to a 2"x2" heat sink with a fan blowing on it during all testing.
I mapped out Vf and brightness across a wide range of currents, and used my lumen integration rig to estimate luminous output. I measure the brightness at regular intervals, from on-axis, to 90 degrees to create a beam profile, then use some geometry and the definition of brightness in lux to convert bare-LED measurements to an approximate lumen output.
Here are the results:
First the Vf curves:
The rebel maintains a lower Vf at high currents. I wouldn't be surprised to the the K2's Vf drop after running for some time.
And brightness:
The K2 maintains brightness at high currents a bit better. This could be because of a slightly better thermal path - for this testing, the K2 was mounted directly to the heat sink, whereas the rebel was mounted to a "star" with thermal epoxy, which was then mounted to the heat sink. So the extra thermal interfaces could be accounting for the performance difference at high current.
The K2 and rebel give very similar levels of performance. Given that both were manufactured about the same time, it wouldn't surprise me if they were using the same dies, just in different packages.
The K2 has the advantage that it's in a package that's easy to handle, one that we're used to using.
The rebel definitely is made for more specialized applications, where space is a premium, and cost is a factor. They are more suited for a manufacturer with part placement and reflow soldering capability.
Future Electronics has the rebels for $2 each when you buy 2 or more, whereas the -12 S-bin K2s are $4 each.
If you can deal with the package and mounting, the rebel is a pretty good deal. But, for a small amount more, the S-bin K2s perform equally as well, and are much easier to use.
I received PW12-S00 K2 emitters, bin SYAH, manufacture date 07/03 (week 3, 2007) (SYAH = 51-67 lumens @ 350mA, 3.03-3.27 Vf)
The rebels I received are PWC1-0050, bin HWOD, manufacture date 07/09 (week 9, 2007) (HW0D = 50-60 lumens @ 350mA, 3.03-3.27 Vf)
Here are the contenders:
The rebel is extremely tiny, and difficult to work with.
Here's a closer image:
The entire top surface of the rebel is covered in phosphor. If you shine a blue LED at a rebel, the entire thing glows yellow. With a K2 (and all other Luxeons), only the die glows yellow, as that's the only place where phosphor is applied.
The heat sink and electrical contacts are both on the bottom of the rebel. This almost requires reflow soldering to achive thermal and electrical connections.
I decided to take a different approach. I made a rebel "star", by cutting out an 0.8" circle of 1/16" aluminium, then cutting a notch in the circle for the electrical contacts. I then used arctic alumina thermal epoxy to attach the rebel to the heat sink:
Here's the backside, showing the electrical connections:
I did this mostly for testing purposes, but maybe it'll give people ideas about alternate ways to mount the rebel to a heat sink without requiring reflow soldering.
Both the rebel and K2 were mounted to a 2"x2" heat sink with a fan blowing on it during all testing.
I mapped out Vf and brightness across a wide range of currents, and used my lumen integration rig to estimate luminous output. I measure the brightness at regular intervals, from on-axis, to 90 degrees to create a beam profile, then use some geometry and the definition of brightness in lux to convert bare-LED measurements to an approximate lumen output.
Here are the results:
First the Vf curves:
The rebel maintains a lower Vf at high currents. I wouldn't be surprised to the the K2's Vf drop after running for some time.
And brightness:
The K2 maintains brightness at high currents a bit better. This could be because of a slightly better thermal path - for this testing, the K2 was mounted directly to the heat sink, whereas the rebel was mounted to a "star" with thermal epoxy, which was then mounted to the heat sink. So the extra thermal interfaces could be accounting for the performance difference at high current.
The K2 and rebel give very similar levels of performance. Given that both were manufactured about the same time, it wouldn't surprise me if they were using the same dies, just in different packages.
The K2 has the advantage that it's in a package that's easy to handle, one that we're used to using.
The rebel definitely is made for more specialized applications, where space is a premium, and cost is a factor. They are more suited for a manufacturer with part placement and reflow soldering capability.
Future Electronics has the rebels for $2 each when you buy 2 or more, whereas the -12 S-bin K2s are $4 each.
If you can deal with the package and mounting, the rebel is a pretty good deal. But, for a small amount more, the S-bin K2s perform equally as well, and are much easier to use.
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