majenko:
Any of those will do nicely.The three parameters you are interested in:
Continuous Drain Current Id:
This is the amount of current the MOSFET can handle, and hence the maximum size of your load.
I disagree, you should choose a device based on Rds(on) and the power dissipation you can afford - a MOSFET rated at 10A will probably melt if you pass 10A through it (since its the thermal limit with infinite heatsinking)
Drain Source Voltage Vds:
This is basically the voltage rating of the MOSFET. When the MOSFET is turned off, the whole supply voltage will be measurable across it, so you must not exceed this value.
Ensure this rating is twice your supply voltage if you want reliable operation. Rapid switching can cause spikes of twice the supply voltage via stray inductance very easily.
Threshold Voltage Vgs Typ:
This is the gate voltage at which the MOSFET will switch on. It ideally needs to be somewhere in the "dead" zone of the logic levels in use. 1.7 to 1.8v is perfect.
NO NO NO - the threshold is the threshold of conduction, this is the voltage at which it switches OFF (a few microamps only). Anyway ignore this parameter, you use the Vgs / Rds(on) specs for choosing a switching MOSFET. Logic-level MOSFETs usually have Vthr of 0.5 to 1.0V, non-logic-level MOSFETs usually 2.0 to 4.0V. The threshold is always greater than 0V for a MOSFET since they are enhancment-mode FETs. The only way to know if a MOSFET is logic level is seeing "Rds(on) at Vgs=4.5V" or similar in the spec.
You can directly drive a MOSFET with an Arduino IO port. You don't need a current limiting resistor as the MOSFET has a very very high impedence. What you do need, though, is a pull-down resistor (say 10K) between Gate and Source. This stops the gate from floating when the IO pin isn't set as an output.
No, a power MOSFET gate has a VERY LOW IMPEDANCE of about 1 ohm in series with 5--20nF or so. At logic speeds this is effectively 1 ohm. You must use a resistor of about 180 ohms to protect the Arduino's output transistors from over-current. You are confusing DC resistance with impedance. MOSFETs have exceedingly high DC resistance, but when switching the gate current can be large (in fact you want it to be large to achieve fast switching). When driving directly from a logic output you have to limit the current to what the logic output can provide (abs max of 40mA for Arduino).
This is the basic layout:
but with the additional resistor between "To Arduino Output Pin" and "To Arduino 0V".
