How fast does offset protection have to be

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Wondered if anyone has any info on how fast speaker protection has to be in the event of a fault. If you were to apply say 40 volts.dc across a typical speaker how long ( we are talking miiliseconds ) could it withstand this without damage.
I ask this because I am trying to develop a different type of offset protection that eliminates any RC integrating network time constant.
Karl
 
the general rule of thumb is that the DC detector has a cutoff frequency of 5hz. the use of RC time constants and some threshold (such as 500mV) does not necessarily mean that it will take 200mS for a relay to shut off if 40V DC appears at the output. it might take 200mS for 1V DC to shut the relay off, but with 40V DC, the rate of change through the time constant is 40 times greater, so the relay will shut off in 5mS.
 
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Hi Kostya-M,

Kostya-M said:
Protection circuit can’t confuse full amplitude signal with lowest frequency. If we accept 20Hz, time must be greater then 25ms. One speaker passed this test, but another can get mechanical damage.
This is one of the classic problems with conventional offset protection, selecting a suitable time constant. I have been trying to develop a circuit that gets round this by comparing input/output to the power amp. There is no lower frequency limit and the trip time is very fast, 10's of microseconds for the detector, the relay is another matter.
I guess what I am really asking is if you could apply 40v D.C. across a typical speaker for say 1ms then 2ms and so on at what point does damage occur.
We all hope we never need to find out how good are protection is, and there are all these designs out there (commercial and D.I.Y.) but are any actually tested. Take a screwdriver, poke the output transistor, does the speaker survive. As far as I am aware you never see total trip times for these designs.
Thanks for your interest,
Karl
 
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Unclejed, I had'nt calculated the rate of the exponential voltage rise on the cap. My maths is useless I'm afraid, should have paid more attention at college. When you say 5ms to reach 1 volt we still have the relay drop out time to add to this. I have seen some figures quoted for this, about 2ms I think. I am trying to eliminate the 5ms part of the problem, and then to see if I can speed up the relay drop out time by applying a "short" via a MOSFET, across the coil. The instant the coil voltage drops out the MOSFET pulls in. I have not tested this part of the design yet, been trying to get the detector to work reliably.
There will probably be more circuitry in the protection than in the amp :D
Regards Karl
p.s. It was much easier with a coupling cap :)
 
Hi,
using 40Vdc as your worst case seems to indicate you're looking at 50W to 100W speakers.
The drivers inside these are likely to have less than 50mm voice coils, probably a mix between 16mm and 35mm diameter.

These will not last long for a prolonged DC offset. But I suspect no damage from tens of mS and maybe even from a very few hundred mS.

How about testing using Fuses as your timed pulse interrupter?

A 1A fuse from a charged 40Vdc supply to a driver VC. See what happens! Or start with a F200mA and work your way up.
 
Hi, Mooly!
There are several aspects of your question.
1. If you want to select only amplifier damage. Then your differential method is better of all.
2. If you want to prevent speaker damage. Then you must know speaker power limit as function of frequency.
3. Speaker damage can be produced by input signal offset if amplifier has no low frequency limit.
 
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I might try Andrews idea of using fuses. Thats a possible !
Prolonged offset, that could overheat the voice coils but the differential method detects this although the trip level is higher than I would like at the moment (about -/+ 3 volts). You would have to see it drawn out really as it has to integrate with the power amp design. You could always add conventional offset detection as well with a long time constant to pick up any long term offset problem. I said it was getting more complex than the amp. When I refer to no lower frequency limit, I am talking of the detector, it trips just as fast with any offset over the trip level, whether or not there is an AC signal superimposed on it or not.
The fact that the power amp probably has an AC coupled feedback arm means that at full output, as the frequency is reduced, the output and input of the amp begin to differ in phase and amplitude and again the detector will then trip.
Nordic mentions pull in and drop out times, these are the most important factors now, most of the relay specs don't always quote this. The one I am experimenting with is 8ms pull in and 2ms drop out. Its the drop out part that's important.
 
speakers with 100W or more ratings have enough of a thermal time constant that even a few hundred mS of offset will not usually cause a problem. the usual DC detection schemes are usually quite effective. same goes for protecting output devices from shorted speaker wires. the thermal time constants of output transistors are usually long enough that a protection relay has more than enough time to react to a short on the output to protect the devices. as a matter of fact i have seen SOA detection circuits that have these time constants accounted for. if you look at SOA graphs for transistors, you will see that there is a time variable in the SOA, and that a transistor can operate for tens and hundreds of mS far outside of it's DC SOA.
 
I've spent a lot of time with the UPC1237 chip. Most of the time was spent determining the R/C time constants. The relay dropout time is dependent on the clamp on the relay coil. A diode holds in the relay longer. An R/C network drops out faster but allows for more over voltage. Conventional ice cube type relays take anywhere from 10-20ms.

Something I worry about is not damaging the voice coil due to heating. I once dropped a tonearm and the Phase Linear 700B I was using caused the voice coil in the JBL4311b woofer to jump the gap. I had to have it reconed.
 
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