PA Protect include:
1 Anticlick 5sec delay
2 DC cut-out 
3 Thermal cut-out on 90deg
4 Fan high speed on 60deg
5 Over current protect (power-on reset)

Conectors:
1 2x18V 500mA AC
2 +/-15V DC stab for XLR circuit
3 PRO overcurrent detect
4 Protect LED
5 Thermal sensor BD241C as diode on heatsink
6 24V DC Fan cooler (12V for low speed)
7 Speaker IN/OUT

http://www.diyaudio.com/forums/solid-state/164208-500w-pa-amplifier-limiter-12.html

http://www.diyaudio.com/forums/solid-state/162043-mosfet-amplifier-irfp240-irfp9240-20.html

PA Protect can be used with Hi-Fi amps also. For silent operation 47R/5W do not stafed, and fan do not work on low speed, when heatsink temperature is over 60deg it will turn on to high speed, and even without fan when heatsink is over 90deg protect will cut-out speakers anyway.

Short circuit protect cut-out speakers, and not turn on again until amplifier was turn off, and turn on again (power on reset). DC protect turn on speakers when DC gone, and thermal protect turn speakers again when amp get cold.

http://www.diyaudio.com/forums/solid-state/164093-100w-ultimate-fidelity-amplifier-10.html
PRO pin must be connect to over-curent detector on amplifier circuit. Protect will be activeted with positive DC voltage (mosfet amp hv23), or ac voltage (PA amp B500). Unfortunaly this Hi-Fi amp have not over-curent detector circuit, and it must be add if you want use this pa protect with short-circuit protect. PA protect can work witout short-circuit protect, just not use PRO pin. Protect which I suggest early in this thread have short-circuit protect without over-curent detector on amplifier circuit. If you have no expirience to add over-curent detector circuit on this amp, is better to use this circuit.
Regards
Attached Thumbnails
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http://www.diyaudio.com/forums/attachments/solid-state/168541d1272150296-100w-ultimate-fidelity-amplifier-apex-20b500-20sch.jpg