Especially when the amplifier design involves costly mosfets, the design becomes specifically vulnerable to short circuits at the outputs.A short circuit at the output may be caused due to mishandling or ignorance from the part of the user.
Whatever might be the reason, the end results in the destruction of the precious mosfets inside the amplifier box.
The above mishap can be prevented by adding a small circuit for detecting a short circuit conditions at the outputs of an amplifier.
The given amplifier short/overload protection circuit diagram, shows an inexpensive design using just a single transistor for implementing the intended feature.
Normally a low value resistor is usually employed at the output of mosfet amplifiers, the current developed across this resistor can be well exploited for tripping a relay in case it exceeds the safe maximum current value.
The current threshold across the above resistor is sensed by an LED inside an optocoupler, which lights up the moment a short or overload conditions is sensed.
This instantly triggers the opto transistor which in turn switches ON the transistor driver and the associated relay mechanism.
Since the relay coils support the amplifier connection with the speaker output, disconnects the amplifier from the output connection, preventing the amplifier devices from a possible damage.
The capacitor at the base of the transistor keeps the transistor switched for a few seconds so that the relay does not oscillate randomly.