I should be able to make a 200W LED lamp using the Cree XPG for just about $130 in total. I think that is a very good price, for a high quality lamp, right?
Feel free to ask any questions you might have, and I will try to do my best to help
Thanks
The best way to run LEDs is off a constant current source. I don't know your level of electronics knowledge, so I will explain it very basically.
When you connect a lamp to a wall outlet you connect it to a constant voltage source (constant in the sense that it's RMS value stays the same over time). This means that the voltage doesn't change, and that the device you connect decides how much current will be needed. In a constant current source the circuit will adjust the voltage until there flows a specific current, for example 700mA.
With a pure ohmic resistance you you have linear relationship between the voltage and the current. With LEDs this relationship is highly nonlinear. It itself is not a problem, but as the LED gets warmer the voltage across it drops slightly - this results in an increased current, which again increases the temperature.. and there's your thermal runaway, which in the end will destroy your LEDs.
So when the voltage slightly drop as the LED gets warmer, and the current increases, a constant current driver will lower the voltage until the specified current again is achieved - thereby protecting your LEDs.
This, however, does not protect the LEDs against overheating. A simple bi-metal switch can do that though.