10VDC Motor Controller

Hi. In this project I’m making a 10VDC Motor Controller without any programming. I try to create a PWM signal with a Schmitt Trigger. This PWM signal then controls a MOSFET so the motor can be regulated in speed.

The circuit

You can see the used circuit in the picture. It’s a good thing to add a flyback diode next to the motor. Otherwise the other component could damage. This is not drawn in the circuit.

The starter circuit is for generating a pulse to start the motor/fan. If you start with a PWM duty cycle of 20%, the motor won’t start. This is because of the mass inertia from the motor/fan!? The starter circuit has a PNP transistor which is ‘normaly open’ and that means we have ((R1/(R1+R2)*V=)9,1V on the Gate of the MOSFET which is plenty to ‘open’ the MOSFET more than the specification Gate-Source Threshold Voltage of the IRF540. It’s better to have this between the 2-4VDC. This ‘opens’ the MOSFET and the motor starts spinning with a 100% duty cycle.

The capacitor voltage in the starter circuit is slowly rising due to the current limiting resistor (10kΩ). At some point the Base current is enough and the Transitor closes (because is’t a PNP). This means the PWM signal generator is now generating the pulses to control the motor.

Conclusion

The PWM signal generated has a range from 20% to 70% which is reasonable in this application. The PWM frequency fluctuates between 30-34Hz.

Also the circuit could be improved by putting diodes in the right places so the components don’t get damaged over time.