Something different in speaker protection.
http://www.diyaudio.com/forums/chip...ng-speaker-output-using-microprocessor-5.html
http://www.diyaudio.com/forums/chip...ng-speaker-output-using-microprocessor-5.html
DC detection relay
This DIY design is original made for +- 45Vdc. It runs with nothing more than 100-200mA, and you can add a 220R/5w resistor or more in series with the plus and minus rails for higher voltages.:
https://picasaweb.google.com/103176...nAndDelayStereoLevelMeter#5570679135858129058
For stereo layout there have been used two Erni 24V relays, but you can change the layout for any multi-connection 48V relay ( or two 24V in series):
https://picasaweb.google.com/103176...nAndDelayStereoLevelMeter#5570678685275924978
Besides the DC detection on the amplifier output, the circuit also "feels" on the AC rectifier bridge. If the powertransformer "tilt" to either the plus or minus voltage side the output relays disconnects rapidly.
The circuit has saved all speakers here, and has been used since a KEF B139 burned off more than 25 years ago.
This DIY design is original made for +- 45Vdc. It runs with nothing more than 100-200mA, and you can add a 220R/5w resistor or more in series with the plus and minus rails for higher voltages.:
https://picasaweb.google.com/103176...nAndDelayStereoLevelMeter#5570679135858129058
For stereo layout there have been used two Erni 24V relays, but you can change the layout for any multi-connection 48V relay ( or two 24V in series):
https://picasaweb.google.com/103176...nAndDelayStereoLevelMeter#5570678685275924978
Besides the DC detection on the amplifier output, the circuit also "feels" on the AC rectifier bridge. If the powertransformer "tilt" to either the plus or minus voltage side the output relays disconnects rapidly.
The circuit has saved all speakers here, and has been used since a KEF B139 burned off more than 25 years ago.
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The detection threshold is +- 8V DC, and the delay time is approx. 3 sec. . The delay time can be increased by use of a higher value of the C20 capasitor.
Check the circuit with a 100K resistor from either + or - rails to the plus speaker terminal on the output relay. The protection circuit should disconnect rapidly.
Rgds.
Thanks. Isn't a DC detection threshold of + - 8V DC a little high. Voltages below that threshold would destroy delicate FR speakers and small woofers even before the protection is triggered.
I saw some Elektor design achieving + - 600 mV DC threshold but am not able to get hold of the schematic now.
I saw some Elektor design achieving + - 600 mV DC threshold but am not able to get hold of the schematic now.
Yes, that was the basic idea. If something goes wrong and the chip melts, which in many cases is the PNP , the full rail is applied to the output as mentioned by Richie.
Even, + - 8Vdc applied in a longer period of time, say minutes, will be in need of a lot of amperes before being able to melt the voicecoil of a modern woofer...I think. The crossover seriel capasitor blocks for DC voltage to midrange and tweeter.
If something goes wrong inside a poweramplifier there are normally a higher voltage value applied to the output than the mentioned +-8V dc.
Rgds
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Change of threshold voltage
It might be possible lowering the sensor resistor values from 220K, in order to get lower threshold DC voltage values...say from the 600mV you suggested to 1V DC. This have not been on the testbench, and have to be checked.
rgds.
It might be possible lowering the sensor resistor values from 220K, in order to get lower threshold DC voltage values...say from the 600mV you suggested to 1V DC. This have not been on the testbench, and have to be checked.
rgds.
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