This stabilized power supply circuit can be connected directly to 230V AC mains to get output voltages of 3V to 12V DC to connect to the prototyping board. Stabilized power supply circuit Step-down transformer X1 takes 230V AC from the mains and turns it into 15V AC. Its secondary winding can handle a current of 2 amperes. The AC is changed into pulsating DC with a peak voltage of 21V (151.4142). This is done by a bridge rectifier. When LED1 lights up, it means that the rectifier is ready to send out power. Resistor

R1 (2.2 k) keeps the current through LED1 below 10 mA, which is safe. The 470F capacitor C1 smooths out the output of the bridge rectifier. High frequency ripple is blocked by capacitor C2. At the end of the rectifier section, a 3-terminal, positive-voltage regulator from the LM317T series is used to control the voltage. It can give out more than 1.5A over a range of output voltages from 1.2V to 37V. But in this case, it is being used to provide discrete voltages in steps of 3V, 5V, 6V, 9V, and 12V. This is done with the help of a 5- way rotary switch S2, which connects different resistor values between the regulator's Adj pin and ground. R2, on the other hand, is a fixed 220 ohm resistor between the Adj pin and the output pin. The output voltage (Vo) is given by: where "Rx" is the resistance between the regulator's "Adj" pin and "ground." In the 12V position (the "off" position of the switch), the value of Rx is R3+R4=1900 ohms. In other positions, it is the series equivalent of 1900 ohms in shunt with another resistance chosen by the rotary switch. In different places on the rotary switch, the table shows the equivalent series resistance. Note In the circuit diagram, X1 rating is written wrong. That is, you should read 15V-0-15V as 0-15V. Switching with a discrete resistor (with 1% tolerance) is better than using a variable resistor because the wiper contact gets messed up after some use, and a variable resistor's tolerance (change with temperature) is also much higher.

To get the most heat away from the LM317T regulator, a heat sink should be put between it and the PCB. Note that the regulator will give off more heat if the load current is high or if the voltage across the load is low. Assuming you set the output to 3V and the load draws 1.5A, the voltage drop across IC1 is about 10V. The amount of power lost at IC1 is 101.5, or 15 watts. To get rid of this heat, you need a heat sink that is about 4x10cm. If you screw a 3mm aluminum plate of that size to the regulator, it will work well. For proper regulation, the difference between the input and output voltages must be at least 3 to 4V.

The best way to mount switch S1, transformer X1, LED1, fuse F1, and rotary switch S2 is in a metal box. After putting heat-sink paste on the metal part of LM317T, the heat-sink (a flat piece of aluminum) is put between the regulator and the PCB and held in place with a nut and bolt. Use a rotary switch that is mounted on the box and extend the wires from the PCB to the position of the rotary switch. The common wire should go to the pole of the rotary switch. Since LM317T has built-in protection against short circuits, there is no need to put a fuse at its output. A PCB should be used to connect the wires in the circuit.