Resistor R2 is shorted through pins 1 and 3 of S1, connecting VR1 and VR2 at pin 1 of IC1. Output depends on the positions of VR1 and VR2. VR1 is configured as a coarse voltage-adjusting knob and VR2 is configured as fine voltage-adjusting knob. By varying the potmeters (VR1 and VR2), voltage output is varied in the range of 1.2V to 27.8V (at TP2).
With S1 in position 2, its pins 1 and 3 are opened and pins 4 and 6 are shorted. This results in connecting 40V AC to the bridge rectifier. Thus 40V AC is rectified, producing around 56.4V DC, which is connected as input to LM317. Shorting of R2 is now opened. Due to a series connection of R2 with VR1 and VR2, lower limit of the output voltage is increased from 1.2V to 27V. At position 2 of S1, differential voltage of LM317 is 56V-27V=29V. By varying potmeters VR1 and VR2, in position 2 of S1, voltage is varied in the range of 27V to 50V at TP2.
At output stage, three diodes (D5 through D7) are connected in series to obtain 0V output, assuming each diode voltage drop of 0.5V (that is, 3 x 0.5 = 1.5V) nullifies output residue voltage of 1.2V from LM317. Zero-volt selector switch S2 works in 0V or 1.2V mode. When it is in 0V mode, effect of diodes in series is present and so 0V output is obtained. When the switch is in 1.2V mode, the three diodes are shorted and actual output of 1.2V is passed on to the output.
Resistor R6 helps series diodes D5 through D7 give better regulation at voltages lower than 4V. If you are operating above 4V, it is better to use 1.2V mode, as presence of series diodes affects the regulation of the power supply. R3 is used to switch on T1 when current exceeds 600mA through LM317.
Construction and testing
A single-side PCB pattern for the 0-50V, 1A variable power supply is shown in Fig. 3 and its component layout in Fig. 4. After assembling all components on the designed PCB, enclose it in a suitable box in such a way that 230V AC mains can be connected easily.
Fix the output terminals on front side of the cabinet along with switches, potmeters and LED1 as shown in Fig. 5. Connect jumper J1 externally on the PCB. Fix switches, LED1 and potmeters VR1 and VR2 with proper labels on the front panel. Verify the test points table before using the circuit.
IC1 and current-boost transistor TIP127/2N6107 should be placed on suitable heat-sinks. S1 is marked as the range-selector switch, while S2 is marked as zero-selection switch. Use 2-pin terminal connectors for output connection. VR1 is labelled for coarse adjustment and VR2 for fine adjustment.
LED1 is used as a power-on indicator. D8, D9 and C2 are used for protection. At 50V, power supply can supply a maximum current of 1A. Choose a good-quality transformer (X1) to support IC1.
1. Switch S2 to 1.2V when output requirement is above 4V.
2. Switch off AC mains power while switching S1 to high range. Wait for a few seconds until power LED1 goes off.
3. Do not short output terminals.
4. Use proper heat-sinks for IC1 and T1.
5. During testing we used a 20V-0-20V, 2A transformer and output was in the range of 29V, 1.5A to 50V, 220mA when S1 was at position 2 and S2 at 1.2V. Similarly, output was 12V, 800mA to 21V, 100mA when S1 was at position 1 and S2 at 1.2V.
Download PCB and component layout PDFs: CLICK HERE
K. Murali Krishna is a former faculty of Aditya Engineering College, Surampalem, Andhra Pradesh. Presently, he is working as telecom technical assistant, BSNL Rajahmundry, Andhra Pradesh