Which Hexfet?

I have 6.7v/1.2A stepper motors, I havent ordered but i want to get the stepgenie IC's to build drivers. I dont know how to select the proper Hexfet for my motors. I would be driving with 5v, i know thats lower than my rated voltage but for the particular application in mind it shouldnt be a big deal. I know nothing about electroinics and components and such.... So any insight is a blessing.

Thanks!
Shadow

[www.mcmanis.com]

then take into account Rds(on) at Vgs=4.5v, 175C which is usually listed in the datasheet if the Rds(on) is usably low at that voltage.

would something like this work?
[www.fairchildsemi.com]
I really dont understand these diagrams. im sorry :/

yeah looks good. Since they don't supply Rds(on,Vgs=4.5v,Tj=175C), I'd estimate it at Rds(on,Vgs=10,Tj=175)/Rds(on,Vgs=10)*Rds(on,Vgs=4.5) or 24/15*18.5 = 29.6mR.

With Tj-amb=100C per Watt, Tj(max)=175C and ambient of 50C, you get Tdelta=(175 - 50)=125C

125C/(100C per watt)= 1.066W. P=I^2.R, so your max current is sqrt(1.066W / 29.6mR) = about 6 amps.

This means, if you put 6 amps through your fet and its case is at 50 celsius, the silicon inside will be at 175 celsius, on the knife edge before burning out. Better call it 5 amps for some safety margin

Also note that this fet is rated at 30A! This figure comes from assuming that the case is held at 25 celsius by some sort of active cooler like a peltier or a block of ice or something, and that all other conditions are perfect too - so don't take fet current ratings at face value, but a higher rated fet will always run cooler and/or survive higher currents than a lower rated one, all else being equal.

So, putting 5v 25ma through it wouldn't cause me any problems at all. What do you think the mosfet will output amp wise? The step genie says output current is limited to 25ma on the output pins, which is why we have to use mosfet in the first place. But as said earlier, I have 6.7/1.2a motors.

Completley unrelated, but if a stepper motor is driven at its rated voltage, is there a risk of providing too much amps to it? Won't they only draw what they need? If using a 5v power supply would I still have to use current limiting resistors in the two center tap wires? Thanks for all the help, I'm going to find some basic electronic books and start learning as much as I can. Any good reads?

only 25mA output current? hm, your mosfets will get very hot at high speeds. Mosfet gates "look like" capacitors, so while it doesn't take any current to keep them on, it does take current to switch them on and off quickly. Ask google about calculating switching times given Qc (total gate charge, should be in datasheet) and gate drive current.

power dissipated is voltage * current (P=VI). When a mosfet is off, there's no current. When a mosfet is on, there's almost no voltage across it. BUT, when they're halfway between on and off they experience both current and voltage in significant amounts, and can get extremely hot with relatively small amounts of both. SO, you want them to go from on to off quickly, spending almost no time in the intervening "linear" zone. This requires high gate drive current only during the change period. This is why dedicated mosfet gate driver chips are available, although a couple of BJTs or signal fets will do it as well.

steppers can overheat if you put too much current through them, not sure how related that is to rated voltage. I've seen steppers "rated" for 1.6v, but recommended to drive them from up to 50v as long as current is tightly controlled so they maintain torque at high speeds. AFAIK the reprap stepper controllers implement some sort of current control, because when the stepper is stalled or between steps, the current is defined solely by the winding resistance, but when they spin, the winding inductance comes into play and reduces the current markedly, in turn reducing torque.

best beginner reads:
amasci.com/ele-edu.html
allaboutcircuits.com