This is a very useful project for anyone working in electronics.
It is a versatile power supply that will solve most of the supply problems arising in the everyday work of any electronics work shop.
It covers a wide range of voltages being continuously variable from 30 V down to 3 V. The output current is 2.5 A maximum, more than enough for most applications. The circuit is completely stabilised even at the extremes of its output range and is fully protected against short-circuits and overloading.
How it Works
The power supply is using a well known and quite popular VOLTAGE STABILIZER IC the LM 723. The IC can be adjusted for out put voltages that vary continuously between 2 and 37 VDC and has a current rating of 150 mA which is of course too low for any serious use.
In order to increase the current handling capacity of the circuit the output of the IC is used to drive a darlington pair formed by two power transistors the BD 135 and the 2N3055.
The use of the transistors to increase the maximum current output limits the range of output voltages somewhat and this is why the circuit has been designed to operate from 3 to 30 VDC.
The resistor R5 that you see connected in series with the output of the supply is used for the protection of the circuit from overloading. If an excessively large current flows through R5, the voltage across it increases and any voltage greater than 0.3 V across it has as a result to cut the supply off, thus effectively protecting it from overloads.
This protection feature is built in the LM 723 and the voltage drop across R5 is sensed by the IC itself between pins 2 and 3. At the same time the IC is continuously comparing the output voltage to its internal reference and if the difference exceeds the designerâ€™s standards it corrects it automatically.
This ensures great stability under different loads. The potentiometer P1 is used to adjust the out put voltage at the desired level. If the full range from 3 to 30 V is desired then you should use a mains transformer with a secondary winding having a rating of at least 24 V/3 A.
If the maxi mum voltage output is not desired you can of course use a transformer with a lower secondary voltage output. (However, once rectified the voltage across the capacitor C2 should exceed by 4-5 volts the maximum output expected from the circuit.
Technical Specifications – Characteristics
Input voltage: 24V DC
Output current: 2.5 A
Output volytage: 3-30V DC
R1 = 560R 1/4W C1 = 100nF
R2 = 1,2 K 1/4W C2 = 2200uF 35-40V
R3 = 3,9 K 1/4W C3 = 100 pF
R4 = 15K 1/4W C4 = 100uF/ 35V
R5 = 0,15R 5W
D = B40 C3300/2200, 3A rectifier bridge
P1 = 10K potesiometer TR1 = BD 135
IC = LM723 TR2 = 2N3055