Stabilized Power Supply With Short-Circuit Indication
Here is an efficient 4-stage stabilized power supply unit for testing electronics circuits. It provides well regulated and stabilized output, which is essential for most electronics circuits to give proper results. The circuit provides an audio-visual indication if there is a short circuit in the PCB under test, so the power supply to the circuit ‘under test’ can be cut-off immediately to save the valuable components from damage.
The circuit provides four different regulated outputs (12V, 9V, 6V and 5V) and an unregulated 18V output, which are selected through rotary switch SW2. The selected output is indicated to the output rails.
The circuit uses a standard 18V–0–18V, 500mA step-down transformer to generate 18V AC. A rectifier diode comprising diodes D1 and D2 provides 18V DC, which is smoothed by capacitor C1 and given to the combination of regulated ICs (IC1 through IC4). The regulated ICs produce fixed, regulated outputs of 12V, 9V, 6V and 5V, respectively, which and connected to the rotary switch contacts. The power supply for loads requiring up to 200mA current.
Completely transistors T1 and T2 conduct when the power to the circuit is switched on. Full selected supply voltage is available at the collector of transistor T2, which is used to power the load. LED3 indicates the presence of output voltage. The indicate the presence of output voltage. The negative terminal of piezobuzzer remains silent at 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 consists at the output and input failure. It consists of a bicolour LED (LED1) and resistor R1 and R2. When power is available and 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 light up to indicate fuse breakdown.
The circuit can be easily constructed on a general-purpose PCB. Use small heat-sink for all ICs to dissipate heat. The output voltage can be read on a voltmeter. Enclose the circuit in a metal box with provisions for voltmeter, LEDs, rotary switch, etc.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 820 Ω
R2 = 1 KΩ
R3 = 1.5 KΩ
R4 = 10 KΩ
R5 = 2.2 KΩ
R6 = 100 Ω
R7 = 1.2 KΩ