dc42:
The DC resistance of the motor is 7.2v/21A = 0.34 ohms. At 3A, a 1 ohm resistor would drop 3v, which is quite significant given that the power supply is only 7.2v. You could use a 0.4 ohm resistor (which you can make up from three 1.2 ohm resistors in parallel), which when added to the 0.34 ohm motor resistance will limit the current to 10A.
Right. The current limiting seems the best route as it will also make sure I can preserve the battery for that little bit longer. I'm actually using 4 motors so I'll probably have to buy smaller ones that use less current or else I will need one beefy battery.
Now the current is limited to 10A I can get 10A rated parts. I just want to make sure I've got the right thing.
I've made this circuit diagram which I think is correct.
Diode: http://uk.farnell.com/multicomp/mbr1060/diode-schottky-10a-60v/dp/1625119
MOSFET: http://uk.farnell.com/international-rectifier/irlu014npbf/mosfet-n-55v-10a-i-pak/dp/8660220
Farnell is so much better than RS for searching.
I'm slightly worried about the MOSFET as it was the cheapest one on the list, though I can't see much difference between it and others, other than a slightly lower voltage rating which is still much higher than I'm using.
Is the temperature a risk on relatively low current circuits like this? I see that most transistors/MOSFETs/diodes have a hole for a heat sink, but I won't need that right? They are not going to be in an enclosed position or anything, and will be quite close to a propeller which is what the motors are driving.
So yeah, are these OK?
Thanks for all the help.