Will this DC protection work?

Status
Not open for further replies.
Jump to Latest
#7
A quite common solution is to use two NPN transistor for this detection circuit. One is connected in common emitter like the left one an the other one is connected in common base with the base to ground and the input diode to it's emitter.

It doesn't cost a lot to build the detection circuit for both cannels seperately because there is a DC failure mode (which might of course occur very seldom) that this circuit can't detect: If one output goes fully positive and the other one fully negative the circuit wouldn't recognize it !!!

Regards

Charles
 
#8
phase_accurate said:
A quite common solution is to use two NPN transistor for this detection circuit. One is connected in common emitter like the left one an the other one is connected in common base with the base to ground and the input diode to it's emitter.
]
Click to expand...


I agree. I use diode bridge, then NPN. Work ok.

It doesn't cost a lot to build the detection circuit for both cannels seperately because there is a DC failure mode (which might of course occur very seldom) that this circuit can't detect: If one output goes fully positive and the other one fully negative the circuit wouldn't recognize it !!!
Click to expand...

🙂 Than sense resistors can't be equal. (22k and 10k will be OK)

Regards
 
#11
Note also that if you have a powerful amp the protection must be fast because the speaker will smell in a few seconds!

My DC-protection used 4th order Tchebuchev (definitely wrong spelling) at 7 Hz and a detection level at 1 V. The circuit was designed to not to trigger at 200 W/8 ohms and 20 Hz.

The filter can be seen at the bottom of the picture, the LM324 IC's.
http://home5.swipnet.se/~w-50674/hifi_pics/hifi_100pr/qro_pcb_top.jpg

My other amp was a little but unreliable and put out 70 volts a few times and guess what happened to my small 10 W speakers? Yes, hardmelt, speaker nomore!
 
#12
A point regarding relay contacts : If the amp goes DC to rail, there is a large DC current through the relay contacts as they open. The resultant arc can weld the contacts together, thereby not disconnecting the speaker. One solution is to add a large capacitor (1uF or more) across the relay contacts to quench the arc. another idea I have seen is to use changeover relay contacts to switch the speakers to ground - then if the contacts arc they will short out the amp (blowing it's fuses) and saving the speaker.

Cheers
 
Status
Not open for further replies.
Top Bottom