Quick High power led question.

I have a 10 watt led that draws about 800mA at 10 volts. It's really bright so I want to control it via either 555 timer, or attiny with its PWM properties. So the question is, should I use an IRF830 or a TIP120 to control the led. I know that they can both work, but I want to use the best one. Which would you choose.

The MOSFET will probably be more efficient. The 555 has the advantage that it can run directly on 10V (the ATtiny is limited to 5.5V maximum). A circuit like this would probably work:

http://schematiccircuit.com/pwm-dc-motor-driver-with-ic-ne555/

http://www.candlepowerforums.com/vb/showthread.php?206249-Using-LM317-in-LED-driving-applications

The IRF830 is completely the wrong sort of MOSFET - firstly its not logic level so you
cannot drive it directly, it needs 10V gate drive.

Secondly its an ultra-high voltage part with a whopping on-resistance of 1.5ohms.

A sensible choice of logic-level MOSFET for low voltage use would have 100 times
lower on-resistance, 15 milliohms or so and not require any heatsinking for this load.

A cheap 2N5551 can handle 600mA continuous, so it would be fine up to a 75% duty cycle. With a small heatsink or with the collector soldered to a ground plane it should even handle the full 800mA.