A photographer friend gave me a few of these:
I am having a hard time figuring out what the forward current would be so I can't figure out what size resistor to use.
10W at 12V is 830mA. They make 'standard' drivers for 850mA so I guess that would be the current (the 12V will be approximate, LEDs are driven by current, NOT voltage).
My experience is that eBay power ratings are optimistic. You might be able to run them at the stated rating in a laboratory with some sort of super heat sink but it's best not to try it at home. Those things get HOT. Burn-your-house-down type hot.
I'd try running it at about 60% (eg. 500mA) and see how it goes. Even so, make sure you get a big heatsink.
Also...do you think it would be okay to control these through an arduino and a TIP120? so i'd have the LED cathode grounding controlled through the TIP 120, the anode would have a resistor on it, going into 12v as well as the tip120 would have a 1k resistor going to arduino pin
What's the reason for doing it? I assume you're going to try PWM control. If so, 1k for the gate resistor is much too high. 150 Ohms is the right size.
It might be OK to do it that way at 60% power but don't try going anywhere near 850mA with just a resistor to control the current. Even at 850mA you'll need a really high wattage resistor, eg. 10W, because the resistor is going to generate a lot of heat of its own.
To figure out the right value for the resistor you can put a potentiometer on the gate of your MOSFET and use it to adjust the MOSFET's resistance. Start at zero gate voltage then turn it up gradually until you get 500mA flowing through the circuit. Measure the voltage drop across the MOSFET. Use that voltage drop+current reading to calculate the value of the resistor you need. Be quick because your MOSFET is getting hotter all the time, even with the heatsink. Two multimeters is very useful here (one for the current, one for the voltage).
and also...anyone know where to get good heatsinks for them?