Variable Power Supply For Up To 3A Current

The author’s prototype is shown in Fig. 1. The block diagram of the variable power supply is shown in Fig. 2 and its circuit diagram in Fig. 3.

Circuit and working

The variable power supply is built around step-down transformer X1, four 1N5408 rectifier diodes D1 through D4, an LED, TIP127 transistor T1, LM317 adjustable voltage regulator IC1, 9V voltage regulator IC2, and a DC digital panel meter (DPM). A 22-ohm, 2-watt resistor (R1) is connected between the base and emitter of T1 to control the current. Pot VR1 is used to control the output voltage. The output voltage can be seen on the DPM.

The output voltage can be set to 9V and the current drawn is around 3A. The voltage across resistor R1 makes transistor T1 to conduct and most of the current is drawn from emitter to collector of T1. As lesser current passes through LM317 and most of the current flows through T1, heatsink of IC1 is less warm as compared to the heat sink of transistor T1 heat sink. So the heat sink for T1 needs to be bigger than the heat sink for IC2 and IC3. Use of exhaust fan helps in proper dissipation of the heat. With more current needed by the load, transistor T1 conducts more and the current bypasses the LM317.

LED1 glows when 230V AC supply is on. VR1 is used to vary the output voltages according to your requirement. The voltage is shown on DPM.

Construction and testing

An actual-size, single-side PCB for the 3A variable power supply is shown in Fig. 4 and its component layout in Fig. 5. After assembling the circuit on PCB, enclose it in a suitable cabinet. Do not forget to use suitable heat-sinks for transistors TIP127, IC LM317 and IC LM7809.

Download PCB and Component Layout PDFs: click here

Connect +12V power supply to exhaust fan from CON2 provided in the PCB. Also connect VR1 and LED1 via 2-pin wires from PCB to front panel.

Fix a small exhaust fan just behind the heat sink of TIP127 to get proper output. Also provide a window on the rear side of the cabinet for the exhaust fan’s air outlet. Fix LED1, pot VR1, and DPM on the front side, and CON1 and CON2 at rear side of the cabinet. A placement of the parts in the cabinet is shown in Fig. 6.

S.C. Dwivedi is an electronics enthusiast, circuit designer and technical article writer