12V Dual Power Supply (±12V) With Short Circuit Protection

This project is a ±12V dual regulated power supply with short-circuit protection.

It uses 7812 and 7912 regulators with power transistors for high current, and a current shunt + opto-isolator + NE555 circuit for fast and safe short-circuit protection.

A separate 12V SMPS powers the sensing, display, and protection circuit.

1. AC input is rectified and filtered
2. 7812 provides +12V and 7912 provides −12V
3. Power transistors increase output current
4. Load current flows through a shunt resistor
5. Excess current triggers opto-isolator
6. NE555 latches OFF the output during short circuit
7. Reset button restores operation

1. 7812 & 7912 voltage regulators
2. Power transistors (TIP3055 / TIP2955 or similar)
3. Bridge rectifier & filter capacitors
4. Current shunt resistor (0.1Ω–0.22Ω)
5. Opto-isolator (PC817 / 4N35)
6. NE555 timer IC
7. 12V SMPS module
8. Resistors, capacitors, diodes, reset switch
9. Heatsinks

1. Center-tapped transformer with rectifier
2. Large capacitors reduce ripple
3. 7812 regulates +12V
4. 7912 regulates −12V
5. Power transistors handle high load current

1. Shunt resistor senses output current
2. Voltage across shunt drives opto-isolator
3. Opto output triggers NE555
4. NE555 disables output and latches OFF
5. Manual reset require

1. SMPS powers protection and display circuits
2. Assemble power and control circuits separately
3. Test no-load, normal-load, and short-circuit conditions
4. Output shuts down instantly on short circuit

This compact design provides a stable ±12V output with reliable short-circuit protection.

It is suitable for bench power supplies, audio circuits, and analog electronics, and can be expanded with current adjustment or digital monitoring.