2way Circuit Breaker instead of melting fuse inside amp - a good idea?

[Lee Knatta]

Hi all

I'm building a powerful Sym-A-Sym like amp at the moment and break my head about profer fusing of the amp.

I used LT spice to simulate what would happen if one of the rail fuses tripped and was shocked to see that the absence of the positive rail makes the output go to the negative rail. This is not a nice behavior. After an overload condition I could have serious DC at the output. Now I'd have to rely on a DC protection circuit - given installed and it works.

I think about using 2way circuit breakers as fuses to solve this problem. If one channel trips the other one is forced off as well.

They are designed for AC mainly but can switch some DC, too.

According to the datasheets I'd have to use two in series for each rail because of 90V rail voltage, thus needing a 4 way circuit breaker for each channel.

I've never seen those circuit breakers inside amplifiers. Almost everyone has them in the house electricity instead.

Do you think it is a good idea to put circuit breakers inside an amplifier?
Maybe it's crazy, stupid or even innovative?

Thanks,
Lee

[llwhtt]

IIRC the old Mark Levinson ML-2 used a three-way CB. One section handled the mains, and a section for each rail. The multi-input protection circuit would crow bar the two unregulated rails and trip the CB. You can find schematics for the ML-2 but I don't think the circuit breakers are shown on what's available.

Craig

[atx]

Originally Posted by Lee Knatta

Hi all

I'm building a powerful Sym-A-Sym like amp at the moment and break my head about profer fusing of the amp.

I used LT spice to simulate what would happen if one of the rail fuses tripped and was shocked to see that the absence of the positive rail makes the output go to the negative rail. This is not a nice behavior. After an overload condition I could have serious DC at the output.

They are designed for AC mainly but can switch some DC, too.

Thanks,
Lee

I believe it is a good idea. The use of magnetic-thermal breaker is common use in industrial DC panels. Most of those breakers would work just well with 90 V DC. Just take care :
1) Use a real bi-polar one, i.e. having the magnetic and thermal protection built in on both sides. Get one with In= rated fuses.
2) take one with a high interruption current. This parameter has nothing to do with the nominal current (normally written as In, and that means the thermal overload threshold) , is normally marked in KA and that means the maximum short-cicuit current that the contacts can safely interrupt.Note that on case of short circuit the magnetic protection (that is faster) will take place, but at a value that is several times In.
A 6KA (6000 A)swich should be robust enough to safely habdle the bigger arcs generated by DC. A 10KA, better.
3)choose a "B" intervention curve that should comonly available in Germany and has a faster intervention than C and D.

ATX

[ontoaba]

Hi, how about make it also for overvoltage protection. I am using them for my amplifiers.

[jaycee]

TBH you could do something easier. Connect only the drivers and output transistors to the fuse. Connect the VAS and LTP stages via 100 ohm resistors. If you want safety, make them 100 ohm fusible resistors. If you have a failure then it is likely to be drivers or outputs. If you now lose a rail, you will simply get a distorted signal instead of DC.

This should even be readily possible on the Symasym design - from memory it connects those small signal stages via resistors anyway.

[Lee Knatta]

Thanks to all who replied. This helped me a lot.

I thought about all this for some days now, gathered more information and came to some conclusions:

The circuit breakers might be a good idea because they have a much higher current interruption capability than the 5x20mm fuses used in most amps. For most amps the 5x20mm fuses should be sufficient.

In my case a 8A 5x20 with M characteristic could interrupt 300A, a standard CB can do 10kA (didn't find the 6kA anywhere).

Rating and de-rating of the circuit breakers is a bit difficult but could be very rewarding.
Downside is that they are expensive and I ran out of money a bit.

Thanks Jaycee for pointing me on what I've totally overseen. I already split rails for frontend and output stage but didn't in simulation. With split rails the catastrophic hangup shouldn't occur.

I'll let you know whether the CBs made it into the final amp.

So far,
Thanks,
Lee