john curl said:
Sounds good to me, John.
I'm actually happy that this thread has now touched on protection circuits.
I know that some are averse to putting a relay contact in the speaker signal path, but that probably depends an awful lot on the quality of the relay one uses. I did one design where I put the relay inside the negative feedback loop, and it seemed to work pretty well. I used a diode-based bypass scheme to make sure the FB loop remained closed when the relay was open. Have you ever tried anything like that?
Speaking of relays, another approach is to use the relay to deliberately short the output to ground under a fault condition in order to protect the speaker. This of course means that the output stage needs to be either shut down at that point or be able to tolerate a dead short. When my crowbar circuit trips and kills gate drive, it also activates a relay to short the output. That way, if the MOSFETs for some reason are fried to a short circuit (thus conducting lots of current even in the absence of gate drive) the speakers remain protected. In addition to all of that, I use rail fuses, but they should generally never go unless something really fatal in the amplifier has happened.
I agree that often enough paralleled output transistors are not used. Actually, compared to the cost of transformers and heat sinks and reservoir capacitors, output transistors are not terribly expensive, and skimping on them may be foolish. If enough paralleled transistors are used, virtually any output protection circuit can have a high enough trigger point to be not intrusive at all.
Cheers,
Bob
.suzyj said:
Suzy, I largely agree with you, but I have to admit that even I have accidentally shorted outputs of my amplifiers at times, and, from experience, it is a terrible feeling when you have just fried your own amplifier because you left out protection thinking that might never happen.
Bob
A simple solution remains using a MOSFET in every rail feeding the power stage. SOA can be detected easily and turn off the MOSFETS thereby cutting power completely to the output stage. Neith speaker or transistors are damaged. This type of protection is common in industrial aplications. The switching MOSFET dissipates very litlle heat since it is either on or off.
If a faulty condition is detected power is removed from the output stage and will only reset after the main power is turned off then on. It is that simple and very cost effective, Adding 20 or 50 mOhm into the feed would have little effect on sound quality. Moreover, you have a protection circuit that is capable of responding to the fault without delay.
I have implemented this in one of my amps that is used at club parties and no-one has ever destroyed it or the speakers. When the music suddenly stops and the red light illuminates someone has done something bad and the amp will stand back and say okay, when you cleared the fault, we can carry on.
I do not use fuse protection at all and the whole protection systems sits outside the amp circuit with monitoring points in the amp.
You should try this, it is most satisfying to see someone say OH S$%T I connected the two speaker wires together and look over his shoulder to observe the sparks and smoke, but nothing, just waiting untill you finish and then lets party.
If a faulty condition is detected power is removed from the output stage and will only reset after the main power is turned off then on. It is that simple and very cost effective, Adding 20 or 50 mOhm into the feed would have little effect on sound quality. Moreover, you have a protection circuit that is capable of responding to the fault without delay.
I have implemented this in one of my amps that is used at club parties and no-one has ever destroyed it or the speakers. When the music suddenly stops and the red light illuminates someone has done something bad and the amp will stand back and say okay, when you cleared the fault, we can carry on.
I do not use fuse protection at all and the whole protection systems sits outside the amp circuit with monitoring points in the amp.
You should try this, it is most satisfying to see someone say OH S$%T I connected the two speaker wires together and look over his shoulder to observe the sparks and smoke, but nothing, just waiting untill you finish and then lets party.
Bob Cordell said:
Hey, i never called you average !
But i'm the diyA fool who copies your words and Mr Curl's, finds most of it re-assuring and some of it a headache kickstarter.
Personally speaking, i rely more on the words of a few experienced EEs than on a whole lot of soap box gurus.
And you'd better have drivers on your list, you're not here for a freebee.
I know what a bad relay is, i'm still waiting to hear what some would consider a good relay.
Rodent said:
Yes, this is a good, workable idea. My latching crowbar circuit removes the gate drive from the output transistor itself, making it an instant "off switch", so operation is similar. As John points out, these approaches need the appropriate control circuit. In my case as well, of course, the amplifier must be power cycled to restore it to operation.
Bob
Rodent said:
Rodent said:
Let's have your schematic please.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=51732&highlight=
