Make a Motor Controller 4 Starter-Motor ?

Thread Starter


Joined Apr 15, 2007
Hi, I have some general questions please if you can help.
can anyone help me design a starter-motor controller and using a 12 volt car battery.(for a small go-kart) . Or give me some tips as to whats involved as Im trying to learn from doing these projects.. My new/limited experience is yesterday soldering up a kit to handle a motor up to 1 amp.
I have Been running a motor from a windscreen wiper off this little experiment using a 12 volt 0.39 amp battery. It gets extremely hot , the heatsink bolted to the transistor. Using some kind of pulse modulation to control speed. Is the transistor likly to be turnining on and off furiously to controll speed ?
So I am guessing that a speed controller is more complicated than simply controlling the amount of resistance. Which I previously thought was all that had to be done. Also the battery specs of the little battery from Jaycar, it says initial current 0.39 amps. Does this mean that 0.39 Amps is all I can pull from that battery? Does having a pretty heafty motor such as the windscreen wiper pull any more amps ?


Joined Apr 20, 2004
Your starter motor controller is usually a relay with contacts large enough to handle the starter motor current.

Your PWM controller is getting hot because the current draw of the motor is probably pretty large. The time it takes to switch the FET on and off has a lot to do with the heating, as well. When off, the FET passes no current. When on, the FET has a resistance that is very low. But while going from on to off, the FET looks like a resistance, and heats up. The formula is P - I^2 * R (current squared times resistance).

I'm not quite sure how to interpret the battery notation, but I don't think it will run the motor for very long. I'm sure it can supply more than 390 milliamps, but it may not be happy doing so.