Air Purifier with Resettable PolySwitch Device

This reference design is a simple circuit that can be used as an air filter motor. The circuit is composed of a motor, a motor driver, and a power supply. The source of the power supply comes from a socket that has a voltage of 220VAC. The 220VAC is stepped down by the transformer to a voltage of 12VAC and rectified by D1 and D2 to make it into a DC voltage. The pulsating DC and the noise from the output of the rectifier is filtered by the capacitors C1 and C2 to make the DC voltage smooth. In order to remove the ripples from the output of the capacitor filter and regulate the DC voltage to 12VDC, a voltage regulator is added to the power supply circuit. The capacitor C3 is an additional filter to remove unwanted noise from the output of the voltage regulator.

The motor driver circuit is a transistor switched ON and OFF by a timer configured as a PWM generator. The rotating speed of the motor is controlled by the frequency and the duty cycle of the signal at the base terminal of the transistor. The frequency and the duty cycle of the PWM signal generated by the timer is dependent on the values of resistors R1, R2, R3 and capacitor C4. The duty cycle of the PWM signal is adjusted through the variable resistor R3. The motor and its driver is protected by the TE Connectivity RUSBF110-2 polyswitch PTC resettable fuse. This device protects the circuit by going from a low-resistance to a high resistance state whenever an overcurrent and overtemperature condition occurs due to the malfunctioning of the motor.