Hi, little off the subject of THD impact of mosfet protection. Also, litte late...
The question is: in a case of output device short, the amplifier put all of its current capabilities on the speaker, specially the woofer coil or what ever coil there are in the crossover that can pass current.
When shutting off, Faraday's law implies a high back emf.
With normal mechanical relay usually one shorts the speaker to ground in fault occasion. It is for protecting the relay contacts from the arc.
Is it also a measure that should be taken to protect the speakers? Maybe it may harm the speakers to short this back emf?
I am asking as in a normal SS speaker protection this connection to GND measure can't be taken.
Is it safe for the speakers to leave this back emf un shorted?
For the mosfet themselves, what protect them agains this huge back emf? Internal diodes?
Last, I saw in many places that people write that mosfets are fast vs EMR but who care if most of the time is spent to sense the DC by the LPF and then activate?
Maybe the main issue is that the relay might not opened at all (only after fuses are burnt) with high voltages.
Thanks
The question is: in a case of output device short, the amplifier put all of its current capabilities on the speaker, specially the woofer coil or what ever coil there are in the crossover that can pass current.
When shutting off, Faraday's law implies a high back emf.
With normal mechanical relay usually one shorts the speaker to ground in fault occasion. It is for protecting the relay contacts from the arc.
Is it also a measure that should be taken to protect the speakers? Maybe it may harm the speakers to short this back emf?
I am asking as in a normal SS speaker protection this connection to GND measure can't be taken.
Is it safe for the speakers to leave this back emf un shorted?
For the mosfet themselves, what protect them agains this huge back emf? Internal diodes?
Last, I saw in many places that people write that mosfets are fast vs EMR but who care if most of the time is spent to sense the DC by the LPF and then activate?
Maybe the main issue is that the relay might not opened at all (only after fuses are burnt) with high voltages.
Thanks
We usually set up TVS protection diodes rated at the upper end of the MOSFET’s max voltage.
I think the TVS is a useful addition to a mosfet output relay. You could alternatively run reverse biased diodes from the output side of the SSR to the amp supply rails so any inductive energy from the speaker load is dumped back into the supply rails. This is a little more inconvenient than a TVS for a stand alone SSR, but if you are integrating the SSR on your amp modules, it just means placing the protection diodes that you would in any event place across the output bipolar devices after the SSR. mosfets come with drain-source body diodes that offer back emf protection but not if you are worried about speaker back emf and you are using a SSR. In that case, if you opted for diode protectio, you would still have to add the. After the SSR to the supply rails.
The distortion introduced by SSR’s is negligible - Mark’s measurements attest to that as are the other measurements done by DIY audio members. It’s easy enough to see why. If your SSR is say 5 milli Ohms and it’s it’s 1% non linear, that’s 1% of 5 milli Ohms and 5 milli Ohms is .0625% of 8 Ohms, so the actual worst case effect is .000625 ie 6.25 ppm. But, when turned on properly, mosfet Rdson variation with load current is much less than 1%, so the distortion contribution is in effect much lower than 6.25 ppm.
The distortion introduced by SSR’s is negligible - Mark’s measurements attest to that as are the other measurements done by DIY audio members. It’s easy enough to see why. If your SSR is say 5 milli Ohms and it’s it’s 1% non linear, that’s 1% of 5 milli Ohms and 5 milli Ohms is .0625% of 8 Ohms, so the actual worst case effect is .000625 ie 6.25 ppm. But, when turned on properly, mosfet Rdson variation with load current is much less than 1%, so the distortion contribution is in effect much lower than 6.25 ppm.
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No added distortion- measurements essentially show no difference. If anything, there is less distortion with SSR. I think there is a reason for this, as it was also observed by tomchr in his SSR protection.
As function of power:
As function of frequency with and without:
Thanks for your answer.
Of course I have diodes but to protect the bipolar outputs so before protection.
If I understand you correctly, the mosfets are safe by its internal diodes.
So putting the TVS for the speaker?
The question: The speaker could be damaged by back emf? In EMR a practice (to protect relay) is to short to GND in fault scenario.
I wonder if it is not more harmful for the speaker to give it a way to pass this large current.
I don't know.
Of course I have diodes but to protect the bipolar outputs so before protection.
If I understand you correctly, the mosfets are safe by its internal diodes.
So putting the TVS for the speaker?
The question: The speaker could be damaged by back emf? In EMR a practice (to protect relay) is to short to GND in fault scenario.
I wonder if it is not more harmful for the speaker to give it a way to pass this large current.
I don't know.
Thanks. Also Nuerochrom has nice graphs.
I have already build SSR protection and indeed no added distortion.
I did not put TVS and recently I was thinking maybe it is a mistake.
I started to think about that after I understood that the speed of the mosfet is not obtained becuasd it is the DC sense delay that dominant. That the reason the inductance charge fully and then one obtain a huge emf because the shut down is now fast by mosfet 😊 .
Then I started to think of I need go protect the mosfet .
I think a promotion to mosfet protection is not the speed but other thing.
Maybe to say that EMR sometimes does not open at all!
Yes, completely logical.
I wonder if back emf is harmful for speaker? Maybe it makes the situation even worse with clamping from the speaker point of view (not the mosfet).
Many articles can be found on the internet giving answers to a lot of basic questions regarding MOSFET relay for speaker protection.
https://sound-au.com/articles/mosfet-relay.htm
https://sound-au.com/project198.htm
Just two quick examples.
Being able to measure the actual distortion of the relay itself, without being masked by that of the amplifier, is the topic of this thread.
Cheers,
Patrick
https://sound-au.com/articles/mosfet-relay.htm
https://sound-au.com/project198.htm
Just two quick examples.
Being able to measure the actual distortion of the relay itself, without being masked by that of the amplifier, is the topic of this thread.
Cheers,
Patrick
Several years ago I measured the THD (10W@4 Ohm) of the power amplifier with and without MOSFET relay protection made by my colleague (@VEC7OR). I could not detect any difference in distortion with and without the relay made using modern (=low Rds) MOSFET switches.
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