Short Circuit Protection Circuit

http://www.daycounter.com/Circuits/Short-Circuit-Protection/Short-Circuit-Protection.phtml

Short circuit protection is often a desirable feature to add to power amplifiers or power supplies, for both safety concerns and protection of circuitry.  

The circuit in the schematic below can be used to protect the output power transistor Q1. Under normal circumstances, where there is a typical load, the voltage across R1 is negligible, an Q2 is turned off and the circuit acts normally.   When Vout is shorted a voltage develops across R1, resulting in Q2 Turning off.  Since the base of Q1 is pulled towards ground by Q2, it is turned off, and current through it is limited or completely shut off, thus protecting it from overheating.