I bought a cool-looking vintage photography spotlight at a garage sale that I thought would make a great looking desktop lamp or reading light. The bulb was burned out and the base was a rare twist lock that is no longer available. For the conversion I bought a chip-on-board (COB) LED. LEDs require constant current and precise thermal management. I'll use a driver with a constant current of 300ma and a forward voltage of 21. A quick calculation using Ohms law has my LED working at 6.3 watts. A finned heat sink rated up to 10 watts will handle the heat.
Step 1: Pick You LED First
I chose a high power COB LED with a color temperature of 3,200 Kelvin, or about the warmth of an incandescent bulb. COB chips pack small LED together on a single chip. The maximum drive current for this LED is 500ma, but I chose a driver at 300ma to stay within the wattage rating of the heat sink.
Step 2: Heat Sink for Thermal Management
High-powered LEDs require you to stay below a certain temperature or they will fail. My LED forward voltage is 21 volts with a driver delivering 300ma. Volts times Amps = Watts (21v x .3a) gives me 6.3 Watts. I chose a heatsink rated at 10 Watts to be safe. I'll use thermal paste before attaching the LED to ensure good heat transfer.
Step 3: Mount the Heat Sink and LED
After tearing the guts out of the spotlight, I fashioned a 90-degree bracket that put my LED at the center of the reflector. I coated the heatsink with thermal paste and screwed down the LED. This LED is mounted to a star that allows two soldered connection on the front of the chip. It has two positive and negative connection so you can daisy chain chips. Just remember that if you chain the chips, your driver's voltage must be greater than the combined forward voltage of the chips and be of sufficient wattage.
Step 4: LED Driver and Mounted Chip
I pulled the case off the driver to enable it to fit inside the spotlight. I'll cover it in plastic to keep it from shorting out on metal case. It is a 120 volt AC to 21 volt DC constant current driver I bought on Ebay.
I added a reflector to keep light spill to a minimum. You can add optics to focus the light, but this spotlight has a glass Fresnel lens to handle that task.
Step 5: Testing Before Assembly
A quick test shows the LED is working properly. I realize the old reflector is not doing anything, but it's not in the way. High-powered single LEDs throw a fairly narrow beam, while COB chips are quite wide.
At 6.3 Watts, the bulb is equivalent to a 65 Watt incandescent, or about 850 lumens at 3,200 kelvin. I intend the light to be on a desktop, so the light is not overpowering or too blue.
Step 6: The Assembled Case
The LED is dead-center on the fresnel lens. The spotlight has a twist knob to adjust the distance to the fresnel and the beam angle. Yes, I could have converted the light using a small 30 Watt halogen, but I decided to go small and efficient. The case is much too small for a standard bulb. You may see LEDs rated with a CRI. The rating is a measure of the quality of light. Colors look better with higher CRI ratings. This LED's CRI comes in a 85. That's a B+ for light quality. Anything above 90 will render colors that are the most pleasing to the eye. The rated life of this chip is 40,000 hours, or about 40 times longer than a standard incandescent.