Self-switching power supply

One of the main features of the proposed regulated power supply circuit is that although a fixed voltage regulator LM7805 is used in the circuit, its output voltage is variable. This is done by connecting a common terminal to the potential between the regulator IC and ground. For every 100 ohm increment in the value of the circuit the resistance value of potentiometer VR1 increases by 1 volt in the output voltage. Therefore, the output changes from 3.7V to 8.7V (taking into account the 1.3 volt drop across diodes D1 and D2).

Another important feature of the supply is that it automatically shuts down when no load is connected across its outputs. This is achieved with the help of transistors T1 and T2, diodes D1, D2 and capacitor C2. When a load is connected at the output, the potential voltage drop across diodes D1 and D2 (approximately 1.3V) is sufficient for transistors T2 and T1 to conduct. The result is that the relay gets energized and remains in that state as long as the load remains connected. At the same time, through transistor T2, capacitor C2 is charged to a potential of about 7-8 volts. However, when the load is disconnected, transistor T2 is switched off. However, capacitor C2 is still charged and it begins to discharge through base transistor T1. After a period of time (which is basically the determined C2 value), relay RL1 is de-energized, turning off the power input to main transformer X1. To restore power, press switch S1 for a moment. The higher the value of capacitor C2, the more delay there will be in turning off the power supply and disconnecting the load, and vice versa.

Although in the prototype the transformer's secondary voltage of 12V-0V, 250 mA was used, however, it can vary per user's requirement (up to 30V maximum rated current of 1 amp). To draw currents over 300mA, the regulator IC must be equipped with a small heat sink above the mica insulation. When the transformer secondary voltage increases beyond 12 volts (rms), potentiometer VR1 must be redimensioned. In addition, the rated voltage of the relay should be re-determined.