SSR for speaker protection?

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#301
How can 0.4VDC produce 20A through a speaker (DCR 3 ohm) ?

captJackSparrow
Yes, the amp ground and the aux ground should be connected somewhere.
The implemented overcurrent protection is mainly about accidental short circuit.
I will try to add some calculations to the documentation.
 
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#302
How can 0.4VDC produce 20A through a speaker (DCR 3 ohm) ?
Click to expand...
Have to be a short in the cabling - not something DC protect circuits care about, since this cannot fry a speaker - the DC voltage offset detect is perfectly servicable for protecting speakers, and to limit current you'd use current limiting circuitry on the amp output stage, and of course fuses to prevent wiring catching fire if the semiconductors fail shorted.

Fuses - stop wires and transformers catching fire - only purpose of a fuse really.

output current limiting - stops an output stage burning up on a short-circuit, may also protects output devices from secondary breakdown if suitably designed.

speaker-protect circuit - protects speaker voicecoils from fusing if a large DC offset happens for some reason (clipping is one cause, large RF oscillations also perhaps) - nothing to do with handling short circuits or protecting the amp from drivers that are too low an impedance.
 
#304
1. Class A can deliver high current at low voltage so although 20A might normally not be possible, up to 10A definitely is.

2. It might be possible to extend DC protection to FET protection but I haven't seen it done. Output (IV) limiting seems to be most popular way but purists would argue that these circuits can affect output quality. I would be surprised if anyone could hear the difference.

It all comes down to whether you want to spend money on circuits to protect FETs or if protecting your expensive speakers is good enough. Lateral MOSFETs are extremely rugged and so if you can shut off power quickly, it is unlikely that they will blow. Other types of FETs might not be so rugged.
 
#308
The protection schematic from post 295 will not protect the speakers in all cases. Specifically, in the most dangerous cases for speakers, the protection with MOS-FET's will also break and will then burn the speakers.
Why? Simple! A short-circuited transistor will cause a very high current through the speakers. This current will charge the coil from the woofer filter with energy. When the relay opens, the energy in the coil will cause a very large overvoltage at the terminals of the MOS-FETs and the transistors will short-circuit. The transient voltage suppressor diode present at the terminals of the transistors will limit the voltage but if the energy is high it will also enter a short circuit because this is how it is designed to work.
 
#310
Exactly!

Do the following test: monitor the voltage at the terminals of the MOS-FET transistors with an oscilloscope when it switches OFF a current of minimum 5A through a coil of at least 2mH.

We'll talk after you will post the results from the scope.
 
#311
I tried it years ago.
PSU45VDC - 7R5 - 10mH - SSR(clamped by transil P6KE75CA)
So, the current was 6A.

And I don't want to talk to you.
 

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#313
0.18J is 0.18W for 1sec or 180W for 1msec

Did you notice the D6, D7 clamping diodes as well ?
My recommendation for max safety
- power the SSR3 by the poweramp psu
- apply diodes D6,D7,D8
- use current or VI limiter (SOA protection)
- immediately switch-off the SSR when short circuit happens
 
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