Circuit breaker for amplifier

I am thinking about using a Live-Neutral current balance circuit breaker, also known as, 'salva-vita'. This is the closest mechanical circuit breaker to what I need. I am calling it a different name for the reason I will not be using earth leakage currents to trip it, but a Live-Neutral current imbalance.

The connections are planned to be as follows:

Input terminals to breaker: L0, N0
Output terminals from breaker: L1, N1

A fault sensing current path consisting of a resistor and a very low current triac will be connected between N0 and L1. A low resistor will have the load current flowing through it: this will be connected to L1. The triac's gate will be connected to sense current through this resistor. Hopefully, when the current reaches the trip current the triac will conduct leading to a Live-Neutral current imbalance.

I am attaching a schematic of the contraption.
 

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This circuit will be sensitive to trip even when a current of a few milliamps flows. The problem with it is, it is intolerant of inrush currents which are normal for many power supplies employing rectification and smoothing.

I do NOT want to damage the amplifier while testing this circuit breaker with repetitive tripping during power-on. So, I am asking here.

In the case of the amplifier design, the forum's help was indispensable. In particular, it helped me design a more robust circuit with thermal compensation for the output stage at two levels. One level is the Vbe multiplifier being affixed to the main heatsink to respond to temperature rise, and get a ~ -2mV/K multiplied by a fixed factor determined by the Vbe multiplier's multiplication factor. The other is the use of 0.1 Ohm resistor to sense the output current and negatively feed back the voltage so generated to the output drivers. As a result of these two suggestions, the amplifier doesn't get hot even during the hottest season.

Other useful suggestions which vastly improved the amplifier's persformance are the suggestion to make the input ground a star connection and the suggestion to use additional filtering for the power rails supplying the input stage and VAS.


Alone I would have never arrived at using these mitigations to get better performance. Therefore, I am indebted to these fora, and to anyone who made very helpful suggestions.
 
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Since, this circuit breaker operates before the amplifier mains transformer, when it trips, the amplifier's large electrolytic reservoir capacitors will still contain stored charge. At 50V DC and 6600uF, the stored energy will be 8.25 Joules. Besides that, there is also the magnetic energy in the toroidal transformer, which is transferred to the output winding, whenever the primary current is suddenly turned off. This means, this kind of circuit may not be quick enough to protect the amplifier when it trips because of the residual energy.

Suggestions are most welcome.
 
Circuit breakers don't prevent damage to semiconductors. They prevent your house from catching on fire if the appliance has a fault. Most circuit breakers tolerate 11 x the rated current for a second, to allow for motor starting. Fast trip circuit breakers can be bought; in quantities of 1000 due to the special run at the factory required. Even a breaker that was designed to trip at rated current is waay too slow to protect semiconductors. As are fuses series the speaker too slow, proved by the SWTC Tiger products.
If you want to protect the semiconductors, you need to design an Michael Bean nfet disconnect between rail caps and output transistors. These typically sense DC on the speaker for more than a half second, then set a flip flop to remember that there was a fault, then the flip flop removes the drive current to the nfets that allowed the rail current to flow. I'm using a FDP52N20 fet to allow 55 amp current to my output transistors, and an APV1122 fet driver to allow the gate to get 5 v above the positive rail to drive the plus fet gate. I'm using a 74HC74 flip flop the Q of which has enough drive current to drive a pn2222 transistor to drive the fet driver @ 10 ma in series with a green "okay" led. The Qbar of the flip flop can drive a pn2222 to drive a red "fault" led. Mains power stays on the amp during a fault, which allows the other channel to play on. Fault circuit DC power 5 v is from a different transformer than the DC mains.
 
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Using a rail trip electronic circuit would be extremely invasive to the way the amplifier is built. As it is, this requires too many changes to the existing circuit. The latter, is not easily achievable as the remaining space in the amplifier box is limited. Consequently, I have to opt to using external protection circuitry, which at the moment, I think, would consist of a fast to respond mains circuit breaker and 2A - 4A speaker quick blow fuses.

My aim is to protect the amplifier when a non-fault electronically tolerable high current flows. This should be possible to implement using a circuit breaker that trips whenever current reaches a threshold or a condition. For instance, an earth leakage circuit breaker is fast to trip and is easily available for the common user. Such a device trips when the currents flowing through the Live and Neutral connections become unequal. This property can be used to implement a quick trip current circuit breaker.

However, nobody is replying to this thread apart from two users. With the circuit I posted, an earth leakage circuit breaker can still sense and trip whenever current flows from one terminal to earth. There may, however, be problems caused by power line high voltage spikes. These may trip the circuit on their own.

My strategy is to set the circuit breaker to a minimum of current that lets me use the amplifier with adequate output power (a few tens of Watts).
 
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If you were to proceed with your thoughts then your proposed circuit seems at odds with itself. The type of earth leakage breaker you appear to want to use relies on a flux imbalance in a common mode choke.

As such your triac shorting line post breaker to neutral pre breaker just blows up the triac.It would seem that what you want to do is short live post breaker to live pre breaker to cause a proper imbalance in the live/neutral windings.

Seems feasible but it might be wise to short post neutral to pre neutral.

I see no obvious way to avoid inrush currents other than incorporating some form of sense delay and if you accept as much then it is likely that your single triac is going to need some extra help from a bit of signal processing electronics.

That might be possible using some form of capacitative divider rectifier thing. They do exist and are used as long as you obey the rules and can work out how to reference things to your circuit.

Probably best to place things down at neutral, as above, and short the neutral winding. Depending on where you are in the World neutral is or should be close to ground.

Conceptual piccy attached.

Assuming the rails come up quickly enough, 2 line cycles, now you have to get your head around low power analog things that might do something to trigger your triac.


...
 

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Mechanical breakers trip in halves of seconds. Fuses blow in tenths of seconds. Semiconductors burn their 35 ga bond wires in thousandths of a second. As you said, the rail caps and transformer have joules of energy stored up to blast current through any stressed semiconductors. You're whistling in the dark, trying experiment that were proved total failures in the 1970's.
The best thing then, if you can't disconnect the rail caps from the semiconductors, is to disconnect the speaker from the amp on dc detect with nfets, and let the semiconductors burn. Another name is solid state relay. there are dozens of threads that cover how to do that. This can be done outside the amp box. Such a device will save the speaker.
You can detect fault current flowing from the speaker ground to the transformer center tap, and do something electronic with it. Disconnecting rail cap - output transistor with rail fets is the obvious thing. Tiefbassisue recommends doubling the rail cap voltage, and lifting the connection speaker ground to the transformer center tap entirely. Leads to problems like the amp running off center from the two rails, but that can be handled with center control circuitry.
What I do is connect speaker to negative rail instead of transformer center tap, and insert a 3300 to 4700 uf capacitor between output transistors & speaker. Thus no DC on speaker. Criticized as incredibly prehistoric by people who are afraid of phase shift in speaker currents, but the walls/furniture in my room do phase shifts all the time to the actual acoustic wave.
 
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indianajo said:
Mechanical breakers trip in halves of seconds. Fuses blow in tenths of seconds. Semiconductors burn their 35 ga bond wires in thousandths of a second. As you said, the rail caps and transformer have joules of energy stored up to blast current through any stressed semiconductors.
Which means, my hypothesised setup is inferior compared to a fuse, and is far more expensive to implement.

You're whistling in the dark, trying experiment that were proved total failures in the 1970's
So, from a protection perspective, suddenly turning off mains power to an amplifier in the event of a fault, was shown experimentally to provide little or no protection at all.

Thanks for both these tips, greatly appreciated. Money saving tips on DIY fora are the equivalent of a gold mine.

This means, my only option to protect the amplifier and speaker is to separate the rails smoothed DC supplies from the amplifier circuitry using MOSFETs.
 
Most manufacturers use anti surge fuses in the mains supply and fast blow for HT supplies after the main smoothing. This protects the output transistors/power output stage and then NAD used 2Amp thermal breakers in series with the loudspeaker connection, this helped to protect the loudspeaker from being cooked whilst having a time delay when overloaded.
That was quite successful but a relay and detecting circuit can protect against unwanted DC potentials.
 
JonSnell Electronic said:
Most manufacturers use anti surge fuses in the mains supply and fast blow for HT supplies after the main smoothing. This protects the output transistors/power output stage
I searched for quick blow SMD fuses with high current ratings like 10A and found they exist. The reason for choosing SMD, is because space is limited and using connecting wires to fuse carriers, would impact negatively the amplifier's performance. A narrow cut in the rails tracks should be enough.

For the loudspeaker there is already a fuse, but this is rated at 20A slow blow. Since this is huge, I would like to use a quick blow fuse with a rating of around 2A - 3A in series with the speaker. At 8Ω, output power should peak to 8*2^2 = 32W and 8*3^2 = 72W. For me, the latter seems enough.

I found the 10A quick blow SMD fuses at Farnell. Suggestions of other suppliers, and alternative fuse ratings but in SMD format, are most welcome.

AC mains input to the amplifier can be interrupted with an existing spring loaded 'fuse'. The only obvious condition when this fuse should activate, is to protect the large toroidal transformer against excessive current which is 10A per rail, if my estimates are correct. [This is basing on the fact the loadspeaker is already 'protected' with a 20A slow blow fuse. So, I am concluding, this implies 10A per rail.]
 
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I would like to know whether ready made 0.5A to 5A presettable circuit breakers exist. The circuit breaker is required to trip immediately when a preset current limit is reached.

Since, this circuit breaker operates before the amplifier mains transformer, when it trips, the amplifier's large electrolytic reservoir capacitors will still contain stored charge. At 50V DC and 6600uF, the stored energy will be 8.25 Joules
 
This amplifier in question is vastly modified, with only the power supply, the driver transistors, and the power transistors remaining connected as in the original circuit. The output stage itself has been modified by adding a 0.1Ω resistance to add some negative feedback and provide for thermal stability.

Fuse protection is currently provided as in the original circuit with a huge 20A slow blow fuse between the amplifier and 0V node. To tell the truth, I am NOT comfortable with this setup and would like to change it. The current fuse can be removed and the track shorted with a thick wire, so that, the 0V track always remains intact. Fuse protection can be implemented by breaking the +/- rails just after the smoothing stage and inserting two 10A fast blow fuses as is usually done. Had it not been, for the utter bizarre design of this amplifier, I would have preferred to repair the original circuit, instead of going the treacherous route of a complete circuit overhaul.

Datasheets for quick blow fuses is indicating they blow after a delay of around 10ms. This may be a long time for semiconductors, but it should offer protection against getting the entire power stage destroyed whenever a fault develops. The latter is an opinion, and I may be wrong. Amplifier service technicians know far better than me in this regard, and their advice is vastly appreciated.
 
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Silicon can fail much faster than 10ms, especially in a cascade where a power device shorts, puts rail-voltage on the driver output, which melts shorted, putting rail voltage on the pre-driver, which then pops, etc etc. Of course you can be lucky, but its probably a matter of luck whether the fuses protect the amp, or the amp protects the fuse! Its not an argument against having the fuse (it can save the pcb tracks for one thing).
 
I captured the driver outputs, always below 1 v, with 3 v 3 W zener diodes to analog ground. Then I replaced a jumper going from driver to the 5 output transistor bases with a 5 amp automotive fuse. Hopefully the zeners will save the drivers predrivers op amps, 50 v rated capacitors, etc etc the next time the output transistors go. I put 124 parts in that amp. Learned a lot crawling through the circuit, though.
I drilled a #45 hole in the blades of the automotive fuses to solder in a component lead. Then they soldered right in the holes the jumper used to be in.
 
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Fuses protect wiring, to protect the sand you need something very much faster.

I quite like Bob Cordells approach of over sized output stage and using diodes from the output to either side of the bias splitter so that a large over current event will develop sufficient voltage across the emitter resistors to cause the diodes to steal base current from the drivers.

SOA is an issue so you want something else to disconnect the load quick like, but a couple of small signal mosfets and a couple of those Vishay opto gate drivers would get it done. Going for the 'short the output to the ends of the VBE multiplier feels workable and does not need any high current doings.
VI limiting is the alternative but that tends to sound really gnarly if it ever activates.
 
Fuses protect wiring, to protect the sand you need something very much faster.
In my scheme if an output transistor lets current out the base line, the zener clamp conducts in nanoseconds, keeping high voltage off driver emitters. Max zener current is 487 ma; hopefully the die shorts across and blows the 5 a fuse before the bond wire goes. Cost about $.40/channel and required no circuit board mods.
TVS diodes take more current but the lowest voltage one I've found in stock is about 6 vac. EB junctions blow about 7 v backwards.
 

PRR

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... ... ... Datasheets for quick blow fuses ... 10ms. This may be a long time for semiconductors, but it should offer protection against getting the entire power stage destroyed...

All your transistors, and even power diodes, CAN be toasted in 10mS.

Power transformers and large capacitors take *minutes* to be "crispy".

Since the large passives may be more than half the cost of a build, protecting them is wise, but also easy.

A complete set of transistors is what, 25% of the cost? Declining in recent years (you don't wanna know what I paid for my first TIP122). Try to add $2 of "protection" circuit, Volt-Amp. This only protects against expected faults, not The UnExpected, but is cheap. Then also put your first-repair power devices in your First build, double-up, so it has way more beef than "should" be needed. Even the for-profit companies now go with rows and rows of power devices.
 
If you do the VI limiter thing, make the stage really butch so it can be configured to never even come close to activation with program material (VI limiters sound NASTY when they activate), this is not the expense it once was when even at Mouser a 200W, 15A, 150V part with reasonable SOA and not too painful second breakdown can be had for a couple of bucks.

Do fuse the transformer secondary before the rectifiers, the objective here being the protection of the transformer from a failed rectifier bridge, fuses way bigger then the design RMS current are a good thing here, and remember that the current here has high RMS/Average ratio and is subject to the cap charging inrush.

Personally I would never trust a fuse to protect sand (It generally goes the other way), use electronics to protect the transistors, fuses to protect the wiring and large passives.
I might make an exception for the kind of UFF fuse sometimes used to protect butch thyristor packs, but that thing will cost you more then your transistors.