This is a basic, Universal Power Supply voltage regulator circuit using an LM317, 3-terminal regulator in a TO-220package. The Universal Power Supply output voltage can be set to anywhere in the range one.5V to 30V by selecting resistances. By using a potentiometer, R2, as of the resistors you can dial up the output voltage wanted. Either AC or DC input can be supplied to the PCB by a socket or terminal block. Connection can be either way around. This is because they have provided a bridge rectifier on board. The input DC voltage to the regulator must be at least two.5V above the necessary output voltage. An off/on switch is provided. For lots of applications (say 12V at 60mA) a heat sink won't be required. The LM317 will provide slightly higher output voltages than 30 volts. However, for most hobbyists over 30V won't be needed. So to make a small PCB they have used some electrolytic capacitors rated to 35 volts. To be safe for continuous operation the maximum input DC voltage...