Soft Starter + Soft Stopper

Some AC / DC motors and Universal motors has to connect to mains power supply smoothly.  For high current flow motors, the mechanical switches or relays create sparks, while connecting and disconnecting to the mains.  So, the circuit explained here, connects to power supply by slowly increasing to mains supply and disconnects from the power supply also slowly decreasing.


The project is divided into two parts for easy understanding and making of the circuit.

1) Power Board Circuit using BTA41A-600B Triac. DB3 Diac, LDR etc.

2) Control Board Circuit using DB107 Bridge Rectifier, 12V zener diode, LEDs, Capacitor etc.


BTA41-600B TRIAC is used to control the power supply to the motor through DB3 DIAC.  An LDR (Light Depending Resistor) is connected in series with 180K resistance and two numbers of 0.1uF capacitors.  By varying the light falling on the LDR, the power of the motor (or Load) can be controlled. 

A single 0.22uF capacitor may be used instead of two numbers of 0.1uF capacitors shown in the circuit.

Proper wire (by cross section and insulation) should be used for load, as shown in thick line in the circuit diagram.

Refer Power Board circuit diagram: 

Soft-Starter-POWER CIRCUIT.png


The AC mains supply is converted to DC using Bridge Rectifier (DB107) and Zener Diode regulates to 12VDC, through the series resistance 220K. When the SPDT switch is connected to ON position, the 470uF capacitor starts charging and both the LEDs (Red and White) starts glowing slowly.  Similarly, when the SPDT switch is pushed to OFF position, the same charge in 470uF capacitor starts discharging and both the LEDs goes to OFF mode slowly.

The ON time delay may be changed by adjusting VR1 (20k) and OFF time delay may be changed by adjusting VR2 (20K).

The delay for Start time may be further increased by increasing the series resistance 220K (up to 470K).

Refer Control Board circuit diagram: 

Soft-Starter-CONTRL CIRCUIT.png


Both the circuit boards are to be connected to single circuit for proper working of Soft Starting and Soft Stopping of the motor.  The White LED of the control board and LDR of power board are to be concealed in a plastic tube with little gap.  As the brightness of white LED increases (which may be visualized by outside Red LED), the resistance of LDR decreases and more power is transmitted through the BTA41 triac. Adding a semi-transparent sheet in between them, like a curtain, is added advantage, which will allow controlled lighting from LED to LDR for more precision control.

Refer Full circuit diagram: 

Soft-Starter-BTA41600B CIRCUIT.png


1) Basic Electrical wiring knowledge is mandatory for making and testing this project.

2) Circuit should not be touched, while mains power is ON.