a logic level mosfet can be used for both linear and switching mode, just like a normal mosfet butrequiring less gate voltage for any given operation mode.
No, logic level mosfet is just a mosfet with a lower gate threshold and gate saturation voltage values. It works the sameas a normal mosfet, just with lower gate voltage values, making it easier to interface directly to 5vdc logiclevel voltage values. Both normal and logic level mosfets do not have internal series resistors. I saidmany recommend using such a series gate resistor to limit gate current, it doesn't matter which kind of mosfet.
Yes, however a logic level mosfet can be used for both linear and switching mode, just like a normal mosfet but requiring less gate voltage for any given operation mode.
On your soft-start drawing. Yes if you disconnect the top of the 47k ohm resistor and wire it instead to an Arduino output pin, along with a common ground wire from the arduino to common ground from the 6 volt source, it should work in principle. Keep in mind this circuit may be designed specifically with the current requirements for the specific motor used, that is the 0R5 resistor and the 100ufd cap and the 47k may have to be adjusted to work with a motor with a different winding resistance. Also keep in mind this circuit will also act like a soft-stop circuit in the turn off will be as slow as turn-on until the cap discharges below gate threshold value, keep that in mind for any possible safety concerns.
Rather than re-writing it badly, I suggest you read International Rectifier's AN-1084, Power MOSFET Basics. http://www.irf.com/technical-info/appnotes/an-1084.pdf. It will give you a good grounding in what's going on.
Most MOSFETs are designed these days to be used in "switching" applications rather than in their linear region, which means that they want to be either all the way on, or all the way off.
If they are used "in between", the power dissipation will get very high, and you won't get anywhere close to the rated current. The "danger" of a normal MOSFET in a logic application is that the logic control voltage is not enough to turn the MOSFET all the way on, which will leave it in the linear "partially on" region, leading to high power dissipation and ... smoke.
(Say you have a 24V 10A motor switched by a MOSFET, so you go and pick an IRF6613 rated at 18+A with nothing but PCB as heatsink and 3.4mOhm on resistance (at Vgs = 10V); so at 10A you have a power dissipation of 0.34W (IIR), which is well within specification. But instead you drive it with a logic output that only gets up to 3.6V for some reason, increasing Rdson to 50mOhm (which would normally still be pretty good!) Now your power dissipation is 5W, which is pretty substantial, well above the spec, and likely to result in smoke...)
Can someone explain:VGS(th) Gate Threshold Voltage 2.0 --- 4.0 V where VDS = VGS, ID = 250?AAccording to above, I need to input 10v at gate? If this is so, how would I do that using the digital pin of arduino?
Vgs(th) is the gate voltage where the device just starts to conduct, perhaps just a few millamps. If you are using the mosfet in a switching mode, on or off only, then you need a gate voltage that fully saturates the device on, 10vdc for your device. That of course can not be provided by directly from an Arduino output pin. You can however wire up a normal npn transistor as a switching device and switch your +12vdc battery voltage to the gate of the mosfet, but that is adding a lot of extra components and you might just be better off using just a NPN power transistor alone to switch the motor on and off via the arduino.
The series resistor between the output pin and a mosfet gate is not required by the mosfet, it will work without one, however it can be a protection device for the arduino output pin to prevent drawing too much current from the pin while trying to charge up or discharge the gate capacitance. It really depends on the specific mosfet being used as to how much gate capacitance it has and the maximum output pin current is available. All the logic level mosfets I worked with worked fine without a series gate resistor, however those recommending them have a valid point in using them. There is no reason to 'compute' this resistor, anything from 200-1k ohm should protect the output pin and still allow fast enough switching speed for your motor.
Last point, once a mosfet is fully turned on (saturated) the switched load determines how much current will flow using standard ohms law formula.