Building a light source for a Stanton Instruments, Ltd. B15 balance.
A quick project for another bigger project.
I am in the middle of restoring a Stanton Instruments, Ltd. B15 aperiodic auto-loading analytical balance. The original light source for the display optics consisted of a high intensity 6 volt AC light bulb powered by a large 240 volt to 6 volt 50 Hertz AC transformer.
I wanted a smaller light source with less heat produced, so I designed a small circuit board to hold a Cree MX6 XLamp surface mount LED chip.
The circuit is very simple, consisting of the footprint for the Cree MX6, two pads for connecting the wires, and a ceramic capacitor. I tried to maximize the surface areas connected to the LED pads for heat dissipation.
I had some Carclo 20mm LED lenses #10138 salvaged from a large floodlamp that had failed, that would focus the wide angle light dispersion of the XLamp chip into a more focused beam needed for the balance. The inner set of four holes are for mounting the #10737 lens holder.
The Cree MX6 chip has an isolated thermal pad which transfers heat to the back side of the board thru vias included in the foot print and additional vias I added on both sides of the chip. I also removed all solder mask from the back side of the board for maximum thermal coupling to the thermal pad and heatsink.
I did get a little (a lot) carried away with the heat sink. I had a spare CPU cooler with a nice copper button and aluminum fins that matched up with board very well, but is much larger than needed for this 1 watt LED. At least it will stay nice and cool.
The typical current for this LED is 300 mA at 3.16 volts DC, with a maximum of 1,000 mA current with proper cooling. I will most likely be operating this at 300 mA or less from an external wall mounted 3.3 volt supply.
Below 300 mA the board by itself with no heatsink stays below 38 degrees C at room temperature with only convection cooling.
Eagle CAD V7.7 Schematic and Board files for Cree XLED V1.1 ZIP file
OSH Park project page for board fabrication.