# 0-50V Variable Power Supply Using LM317

K. Murali Krishna

48381

Presented here is a 0-50V, 1A variable power supply that has been designed using positive-adjustable voltage regulator LM317 IC. This IC normally generates voltages in the range of 1.2V to 25V, but here it produces 0 to 50V with a current of 1A. The circuit used increases the upper voltage from 25V to 50V and decreases the lower voltage from 1.2V to 0V.

## Variable power supply circuit

Fig. 1 shows the block diagram of the variable power supply. Mains input is connected to transformer X1. Output of the transformer is given to LM317 via rectifier diodes. Final output is provided by LM317 and transistor T1.

Heart of the circuit is LM317. Data sheet of LM317 IC states that it can produce voltages between 1.2V and 37V with a maximum current of 1.5A. LM317 has an internal reference voltage of 1.25V. This voltage is reflected in the output and is set as the lower-limit voltage.

Maximum differential voltage between input and output should not exceed 40V to operate within safe limits. For this, voltage drop of 3V is maintained between input and output. To obtain maximum voltage with LM317, input should be 40V. So, if LM317 is fed with a maximum of 40V, output will be 40V-3V=37V

Circuit diagram of the 0-50V, 1A variable power supply is shown in Fig. 2. It is built around step-down transformer X1, bridge rectifier comprising D1 through D4 (1N5408), adjustable voltage regulator LM317 (IC1), power transistor TIP127 or 2N6107 (T1) and diodes 1N5408 (D5 through D9).

The circuit is centred around LM317, whose output depends on resistors R1, R2, VR1 and VR2. A centre-tapped transformer (X1) with 230V AC primary to 20V-0-20V, 3A secondary is chosen, whose centre-tap point is at 0V and voltage between both ends of secondary windings is 40V AC. So, voltage between the centre tap and either end of the secondary winding is 20V AC. One side (half) of the secondary (20V AC) windings is connected to anode of D1 and cathode of D4.

Secondary of X1 is connected to the bridge rectifier through range selector switch S1. When S1 is in position 1, its pins 3 and 1 as well as pins 4 and 2 are shorted. This results in connecting 20V AC to the bridge rectifier. The 20V AC is rectified, producing around 29V DC, which is connected as input to LM317 IC.