Schematic diagram of an electronic motor starting circuit

The figure shows a schematic diagram of an electronic motor starter circuit. This motor starter protects single-phase motors from voltage fluctuations and overloads. Its outstanding feature is the simple-to-operate soft on/off electronic switch.

The transformer steps down the AC voltage from 230V to 15V. Diodes D1 and D2 rectify the AC voltage to DC. The unregulated power is fed to the protection circuit. Inside the protection circuit, transistor T1 is used to protect the motor from overvoltage.

The overvoltage setting is performed with the preset VR1 so that T1 turns on when the voltage exceeds the upper limit (e.g., 260V). When T1 turns on, it turns off T2. Transistor T2 acts as an undervoltage protector. The undervoltage setting is performed with the support of the preset VR2 so that when the voltage falls below the lower limit (e.g., 180V), T2 stops conducting. Zener diodes ZD1 and ZD2 provide base bias to transistors T1 and T2, respectively. Transistors T3 and T4 are connected back-to-back to form an SCR configuration, which behaves like an "on" / "off" control. Switch S1 is used to turn on the pump, while switch S2 is used to turn off the pump.

When an overvoltage/undervoltage setting occurs, C2 is temporarily disconnected. Capacitor C2 prevents relay chattering due to rapid voltage fluctuations.

The voltage regulator IC7809, after filtering by capacitor C4, together with the relay, provides a 9V regulated power supply for the soft switch. An appropriate miniature circuit breaker is used for automatic over-current protection. The green LED (LED1) indicates that the motor is "on" and the red LED (LED2) indicates that the power is "on". The motor is connected to the normally open contacts of the relay. When the relay is energized, the motor starts.