dc42:
For a 3A motor, you would be much better off using a logic level N-channel mosfet instead of a transistor. It will run much cooler and you won't have a problem providing enough base current. In that circuit, replace T1 by a logic level mosfet, and replace the 3k3 resistor by one in range 100 to 220 ohms.My rule of thumb:
- to switch up to 500mA, use a transistor
- to switch more than 500mA, use a mosfet
- never use a darlington
PS - that motor has a current drain of 21A @ 7.2v when stalled. So unless you implement some form of current limiting, the mosfet needs to be rated at 21A or more. The diode in parallel with the motor similarly needs a peak (not necessarily continuous) rating of 21A.
Thanks for the quick response. Can I just ask how you came to 100 - 220 ohms?
I'm struggling to find diodes rated to 21A - the current limiting plan might be a better option. Also, if I want to limit this to 10A then I'll need a 0.72 ohm resistor. Finding it hard to find a 0.72 ohm resistor too, so would having a 1 ohm resistor limit the current to 7.2A matter? The motors normal current is ~3A so it would only limit the spin up current, meaning it would just be slightly slower when accelerating? If I were to use a 1 ohm resistor its not going to drop a significant voltage across it as I assume the resistance in the motor is much higher?