In this project, we are going to build a variable power supply for medium applications using 555 Timer IC. This variable power supply circuit can provide up to 300mA current and is variable between 6 volts to 13 volts. This project is very cheaper and easy to design. The 555 Timer from a monostable multivibrator with a time period of 0.2ms and Zener diode forms reference voltage and potentiometer control the output.
Must Read Top 555 Timer IC Projects
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Working Principle of Variable Power Supply
The working principle of variable power supply is very simple. Only five transistors and a 555 Timer IC is used as main components.
The T1 or BC148 forms an emitter follower, where the emitter voltage is equal to the voltage at the varying point of 10KΩ Pot or VR1. T2 or second BC148 is used to sense the output voltage. By this time T3 or BC558 triggers the 555 Timer in accordance with the output level.
When the whole circuit is switched on, the voltage at T1 is equal to the voltage at the centre point of VR1. Therefore, the emitter of T2 is at 5 volts and the base is at zero. As it is in the off state, no voltage is passed across R4 and T3. So this also remains off state.
Pin 2 of the 555 Timer IC is grounded through a 100KΩ resistor. The output of the IC is triggered to VCC. This switches the T5 or third BC148 so that it goes into a saturation state. At this time C1 or 1000uF/50V capacitor gets charged by the voltage across it reaches 5.6V.
Now T2 turns on and develops a voltage across 100KΩ. This turns on T3 and voltage at pin 2 becomes more than VCC/2. So, the output of the IC goes low after the time period of 0.2ms. The capacitor C3 discharges through the load.
When the output voltage decreases from 5.6V, T2 goes off. This process produces a negative trigger to pin 2 of the 555 Timer. The output of it turns on T4 or SK100 transistor. The whole process is repeated continuously. We use T4 in a saturation state so the power loss becomes almost zero.
There is a chance that the capacitor gets charged due to leakage current, increasing the output voltage on the load. We use a 1KΩ resistor to avoid this. You can increase the load current capacity by adding one more SK100 transistor in parallel.