alexxander_foster:
Far-seeker:
MOSFETs are primarily voltage based devices, so it's the voltage applied to the gate that determines how open the channel is between the source and drain. While MOSFETs do have a minimum current through the gate necessary to function, basically it's a very small threshold value. For most MOSFETs it should be well below the limits an Arduino output can supply. Now if you were using BJTs then the current of the controling signal would be a much more important consideration...Gotcha! So its a voltage based device. Well, thats definitely helpful, but it brings me back to my original question: how do you calculate the voltage required to open the drain completely in a MOSFET based on Rds(On)? Is it just Ohms law?
No, not Ohms law, but rather analysis of the device's datasheet. For example: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdf
has two key things to look at. First there is the Ron spec will show the minimum resistance at some stated gate source voltage in this case at two different values of gate voltage. Second they almost always publish a graph showing current flow at several gate voltage values, as in Figure 1 in that data sheet which shows drain current Vs gate voltage at eight different gate voltage values.What I'm finding is that, in the power range I'm looking for (25 amps), not all MOSFETs will have a completely open drain at the voltage and amperage levels the Arduino's pins can produce. Nearly all of them seem to have a threshold around logic levels, but I want full voltage potential at logic level.
That is because you are looking at non-logic level mosfets that normally rate their Ron value at +10vdc gate voltage and are indeed not suitable for direct control from a arduino +5vdc logic level. Gate threshold is not the correct specification to look at but rather the gate voltage Vs current flow graph at the gate voltage of interest to you.
Far-seeker:
The diode needs to be in parallel with the motor. That doesn't necessarily mean directly soldered between the fan motor's connection tabs, but if you put it else were you'll have to run the wiring appropriate to keep electically in parallel with the motor.Actually the effectiveness of the freewheeling diode is best/better if mounted right at the motor terminals as the dissipated currents won't be effected by the resistance of the motor lead wires.
LeftyFantastic! That answers my question, thank you!