Simple Variable Power Supply

050 SIMPLE VARIABLE POWER SUPPLY This low-cost power supply has an output voltage range of 1.5-15 V at a maximum current of 500 mA. Regulation is better than 2% for output currents not exceeding 350 mA. Voltage adjustment is effected by a potentiometer and an acoustic over- load indication is provided. Transistor T4 compares the voltage at the wiper of P1 with the output voltage. When this is 0.65 V higher than the set voltage, T2 is switched on, which removes the base current from darlington power stage T3-T5. In this manner, the output volt- age of the supply is 0.65 V higher than the refer- ence potential at the base of T4, which is derived from a 15 V zener diode, D5. The voltage at the 18 V, 1 A winding of the external mains transformer is rectified by bridge B1 and smoothed by C1. A simple acoustic over- load alarm (Bzl) is actuated when the output cur- rent exceeds around 500 mA. Note that the exact level of actuation depends on the electrical specifi- cation of the buzzer,...