Hi, my name is Gerrard Shaw, I am doing my A-levels at the moment, including one in Electronics (Systems and Control Technology). My final design paper concerns cordless power tools and variable electronic speed controls. After looking at various systems available to do this job (including the ones here) I have what I think is quite a good circuit. Check it out at: -
http://www.orient.fsworld.co.uk/Motors/Motor1.GIF
(you probably need to take a look to understand any of the stuff below)
The voltage regulator would be a 7805 to give 5V regulated from a 12/24V battery whilst still giving the motor the high voltage. As seen on this site, the DPDT switch gives motor direction control.
It uses two MOSFETS driven by a PIC microcontroller to give out the PWM pulses. This would allow input from a linear potentiometer on the trigger and an algorithm to produce the right pulse train. Another input switch could set high or low speed setting as well, which would be easy to do in the PIC program.
The circuit turns on one MOSFET in antiphase to the other from the output of the PIC (i.e. when 1 on the other is off).
The flywheel MOSFET works like a reverse bias diode to prevent voltage spikes under normal conditions but when the output of the controller decreases (i.e. motor slowing down / stopping) it also allows reverse current to flow back into the battery, thus recharging it and slowing down the motor in one go.
I could also interface a temperature sensor that checks the temperature of the motor / drive MOSFET to ensure that the system doesn't overheat.
What I wanted to check is that MOSFETS do conduct in either direction when switched on and that this circuit would work in practice. What do you guys think? A second opinion would be very useful!
Any help much appreciated.
Regards,
Gerrard Shaw |
