@Muse - This circuit combines the precision of an op-amp (#U1, LM324), the high power-handling and zero DC drive current of a power MOSFET (#M1, IRLZ44N), and active short-circuit protection via an NPN BJT (#Q1, 2N3904 / BC547).
### Circuit Architecture
1. **Voltage Regulation Loop**:
* Potentiometer #RV1 provides a smooth adjustable control voltage #adj from $0\text{ V}$ to the supply rail.
* Resistors #R1 and #R2 form a negative feedback divider from the output #vout back to the inverting input #fb:
$$V_{\text{fb}} = V_{\text{out}} \times \frac{R_2}{R_1 + R_2}$$
* Op-amp #U1 drives the gate (#gate) of NMOS pass transistor #M1 so that $V_{\text{fb}} = V_{\text{adj}}$, providing stable, load-independent voltage regulation.
2. **Active Current Limiting (#Q1 and #Rs)**:
* Sense resistor #Rs ($0.68\,\Omega$) monitors the total current delivered by the source of #M1.
* Transistor #Q1 monitors the voltage drop across #Rs with its base tied to #isense and its emitter tied to #vout.
* **Under normal loads:** The voltage drop $I_{\text{load}} \times R_{\text{s}} < 0.6\text{ V}$, keeping #Q1 completely off.
* **Under overload or dead short:** Once current reaches:
$$I_{\text{limit}} \approx \frac{V_{\text{BE(on)}}}{R_{\text{s}}} \approx \frac{0.65\text{ V}}{0.68\,\Omega} \approx 0.95\text{ A}$$
#Q1 turns on and diverts gate drive current directly away from #gate down to #vout, immediately throttling #M1 and saving it from catastrophic thermal failure.
### Practical Bench Build Tips
* **Pass Transistor Heatsinking**: In linear regulators, power dissipation in #M1 is $P_{\text{D}} = (V_{\text{in}} - V_{\text{out}}) \times I_{\text{load}}$. At low output voltages and high currents, mount #M1 to an extruded aluminum heatsink.
* **Sense Resistor Rating**: Under a full $1\text{ A}$ current limit, #Rs dissipates $P = I^2 R \approx (1)^2 \times 0.68 \approx 0.68\text{ W}$. Use a $1\text{ W}$ or $2\text{ W}$ wirewound/metal-oxide resistor.
* **Expanding Output Current**: To achieve higher currents (e.g., $3\text{ A}$ to $5\text{ A}$), reduce #Rs to $0.15\,\Omega$ ($5\text{ W}$) and parallel additional MOSFETs or use a power Darlington BJT (e.g., TIP142) on a large heatsink.
