This is definitely an effective 4-output stage stabilized DC power supply unit for testing electronic circuits. It delivers very well regulated and stabilised output, that is important for most electronic circuits to provide good results. The circuit gives you an audio-visual indication if there is a short circuit in the PCB under test, so the power supply to the circuit “under test” could be cut-off instantly to protect the important components from damage.
The circuit gives you four different regulated DC outputs (12V, 9V, 6V and 5V) and an unregulated 18V DC output, that are selectable by way of rotary switch S2. The selected output is showed on the analogue voltmeter connected to the outputs rails.
The circuit works by using a typical 18V-0-18V, 500mA step-down transformer to deliver 18V AC. A rectifier diode comprising diodes D1 and D2 supplies 18V DC, that is smoothed by capacitor C1 and given towards the combination of regulator ICs (IC1 through IC4). The regulator ICs deliver fixed, regulated outputs of 12V, 9V, 6V and 5V, respectively, that are joined to the rotary switch contacts. This power supply is useful and effective for loads requiring up to 200mA current.
Complementary transistors T1 and T2 conduct in the event the power to the circuit is switched on. Full chosen supply voltage is obtainable at the collector of transistor T2, that is applied to power the load. LED3 signifies the presence of output voltage. The negative terminal of piezobuzzer PZ1 is joined to the output rail via LED2, so the piezobuzzer stays silent as its negative terminal is also at full supply voltage (selected). If there is a short circuit at the output, LED2 glows to activate the piezobuzzer.
A fuse-failure indicator distinguishes short circuit at the output and input failure. It includes a bicolour LED (LED1) and resistors R1 and R2. When power is available and also the fuse is intact, red and green halves of LED1 are effectively in parallel to output a yellowish light. When fuse fails, green LED goes off and red LED lights up to tell us fuse breakdown.
The circuit could be simply assembled on a general-purpose PCB. Use small heatsinks properly for all ICs to dissipate the IC temperature. The output voltage could be check out on a voltmeter. Assmble and mount the circuit inside a metal box / case with provisions for voltmeter, LEDs, rotary switch, and so on.