Tuesday, December 27, 2011

Upgrading the wedge base bulb in a Sturmey Archer tail lamp

I'm quite partial to the bullet-shaped tail lamps that were part of the Sturmey Archer Dynohub lighting set (and I'm not the only one). While the head lamp takes a standard E10/MES screw base bulb, the taillight takes a hard-to-find wedge base bulb.

Original capless wedge base bulb. Get 'em while you can at sjscycles.

When I converted my first set of Sturmey Archer lamps to LEDs, I resorted to soldering a jumbo leaded LED onto a piece of PCB that I could wedge into the the bulb holder.  Now, in order to take advantage of the extremely bright red Cree XP-E, I used a slightly modified version of my E10/MES LED heat sink to make a wedge base LED bulb.

Cree XP-E red. 131 lumens @ 0.7A!

Copper heat sink for red Cree XP-E
LED board and wedge base board.
The copper heat sink has a #4-40 tapped hole to mount a rectangular piece of PCB perpendicular to the LED platform with a machine screw.  Wires come through the heat sink platform and are soldered to the wedge contacts:

Completed wedge base bulb with copper heat sink and red Cree XP-E

To get a good fit in the lamp's spring contacts I added a blob of solder at the base of each contact, on both sides.   A tight fit is essential as the heat sink assembly is much heavier than the original glass bulb.

Original bulb and Cree XP-E replacement bulb mounted in lamp
The beautiful bullet!
The Cree XP-E has a maximum current rating of 0.7A.  In a passive dynamo system, it'll get up to 0.6A, so I'll need to ensure that the heat sinking is sufficient.  Unlike my screw base LED bulb, there isn't really any thermal connection between the heat sink and the bulb base, preventing much heat transfer to the lamp body.  I'm hoping that it will run up to 0.7A in flashing mode, the off time lowering the steady state heat sink temperature.

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