@ Anachrocomputer:
That doesn't mean that you can get away with not fitting a heatsink! There will still be 20W or so of heat produced at the transistor, which will need to be removed. Also, remember to use large enough PCB tracks and wiring to handle your motor current. And do fit a large enough back-EMF diode to protect the transistor from voltage spikes.
Thanks for the info. I was considering using the metal body (of the electric scooter) as a heatsink because there's no airflow for a standard heatsink. There'll be plenty of metal to dissipate the heat generated by the transistor.
And thanks for the reminder about the diode. I'd forgotten that one...
@Lefty:
Well first you need to define the motor application a little further. Do you want the motor to be able to reverse turning direction or will always be rotating in just one direction.
Just one direction, being a scooter with only 2 collinear wheels...backwards could be a real challenge! I'm glad that simplifies things...
The best choise is a logic level N channel power MOSFET transistor. Most MOSFETs require a 10vdc gate turn-on voltage to fully saturate the transistor on which is the key to keeping it within it's max heat dissapation ratings. A logic level MOSFET will saturate on with as little as 4-5vdc and therefore can be directly driven by a Arduino output pin.
Are you referring to two different MOSFETs in this sentence? Or are you simply saying that I can saturate with as little as 4-5VDC, but it'd be better with more voltage, say at least 10VDC? Either way I guess the solution is, saturate with more voltage to get better heat dissipation! Please correct me if I'm wrong...
The scooter is running from a 24VDC source. I will have a 12V rail using a voltage regulator IC (I've definitely learned something in the past couple of months!) that I will use to power the Arduino and I can use that 12V rail to switch the gate of the MOSFET (providing 12V is within the limits of the gate turn-on voltage). Sounds like the plan. Or, I could use the 24-28V straight from the battery. Which would be better, or am I splitting hairs?
You should probably also have a pull-down resistor, say 5K, to make sure the device stays off if the Ardunio is powered off but the 24vdc motor voltage is still on.
I'll do this too, even though one can't run without the other, it seems like a good practice to get into. I assume you mean a pull-down resistor on the gate pin of the MOSFET to ensure the gate is pulled to ground when there is no PWM pulse?
Thanks for the help Lefty. I'll keep digging and double check with you when I find something else...