A fuse in series with the output is a bad idea for both protection and fidelity. If that is all the designer can come up with, it's time for him to go back to school, or give up design work.
Andrew,
I think the first thing to do is mute the input (works for a DC fault out of a preamp) and disconnect the energy source from the load. This may take the form of a crowbar, relay or shut down (as in a Carver). Carver's scheme worked very well, and is my favorite for being able to preserve sound quality while limiting damage to both the load (speaker) and the amplifier circuit. Crowbars will work but I have seen them cause fatal damage to the circuit board in large amplifiers.
-Chris
Andrew,
I'll second your "Don't". Inside the feedback loop, you now have to fight an additional non-linearity.
I think the first thing to do is mute the input (works for a DC fault out of a preamp) and disconnect the energy source from the load. This may take the form of a crowbar, relay or shut down (as in a Carver). Carver's scheme worked very well, and is my favorite for being able to preserve sound quality while limiting damage to both the load (speaker) and the amplifier circuit. Crowbars will work but I have seen them cause fatal damage to the circuit board in large amplifiers.
-Chris
Ok hello again. I have now built this circuit, but i got some problems. I forgot camera so no pictures yet 
Schematics again:
http://files.upl.silentwhisper.net/upload4/Amp_protectLayout1.1.pdf
First of all. The "Sensing" circuits works fine. It doesn't react to 5Hz 30Vampl. (sinus). That was the lowest i could test. But to DC voltage it reacts instantly, LED lights up and the 12V that was over the the zenerdiodes goes away. It keeps in this mode for about 10sec after DC is cut from input. Then the LED fades out and 12V goes up again.
The problem is that the mosfets never cuts when voltage over zeners goes away. It's still voltage on the Drain. When I measure voltage Between Drain and Source in normal mode, it was about 0.4mV (With 1kR to ground) but in protect mode (when voltage between D->S should be V+->Gnd) It's instead 0.6V and current still flows.
Can this be because there is a diode forward biased inside the mosfet?
The datasheet for IRF9540 http://pdf1.alldatasheet.com/datasheet-pdf/view/67507/INTERSIL/IRF9540.html
It shows a diode there..
Hmm. Can i swap the 9540 with the 540 to get rid of this problem? or must P-mosfets be used with positive voltages?
Schematics again:
http://files.upl.silentwhisper.net/upload4/Amp_protectLayout1.1.pdf
First of all. The "Sensing" circuits works fine. It doesn't react to 5Hz 30Vampl. (sinus). That was the lowest i could test. But to DC voltage it reacts instantly, LED lights up and the 12V that was over the the zenerdiodes goes away. It keeps in this mode for about 10sec after DC is cut from input. Then the LED fades out and 12V goes up again.
The problem is that the mosfets never cuts when voltage over zeners goes away. It's still voltage on the Drain. When I measure voltage Between Drain and Source in normal mode, it was about 0.4mV (With 1kR to ground) but in protect mode (when voltage between D->S should be V+->Gnd) It's instead 0.6V and current still flows.
Can this be because there is a diode forward biased inside the mosfet?
The datasheet for IRF9540 http://pdf1.alldatasheet.com/datasheet-pdf/view/67507/INTERSIL/IRF9540.html
It shows a diode there..
Hmm. Can i swap the 9540 with the 540 to get rid of this problem? or must P-mosfets be used with positive voltages?
Hello!
I am a little worried about output from that last transistor whose emitter is connected to V+
In nomal mode everything is fine, transistor after is conducting.
But in protect mode collector is free floating and therefore base on transistor after is not defined. Maybe a pulldown from collector would help?
I am a little worried about output from that last transistor whose emitter is connected to V+
In nomal mode everything is fine, transistor after is conducting.
But in protect mode collector is free floating and therefore base on transistor after is not defined. Maybe a pulldown from collector would help?
Hi 4fun,
This has happened in some large commercial amps in the 600 W range. Each was a mono block, not switching. Conventional amp.
Big, wide areas of foil were reduced to scorch marks and sputtering. Legs were blasted off transistors. Evidently, it had a really good power supply.
The only thing one can say is that at really high power levels, when something goes wrong, it goes really, really wrong! Quickly, I might add. If it's any conciliation, the (large) triac was also transformed into a fancy bit of wire.
-Chris
This has happened in some large commercial amps in the 600 W range. Each was a mono block, not switching. Conventional amp.
Big, wide areas of foil were reduced to scorch marks and sputtering. Legs were blasted off transistors. Evidently, it had a really good power supply.
The only thing one can say is that at really high power levels, when something goes wrong, it goes really, really wrong!
Quickly, I might add. If it's any conciliation, the (large) triac was also transformed into a fancy bit of wire.-Chris
Hi anatech!
But crowbar circuit on this powerclass that FritZell is making should work if properly sized I think.
Yep, I can agree that high currrent can make big mess.
Btw, Remember Crest amps from VS series that leaked elecrolytic on pcb, damage could vary from somewhat damaged pcb to total disaster.
But crowbar circuit on this powerclass that FritZell is making should work if properly sized I think.
Yep, I can agree that high currrent can make big mess.
At least speaker saved
Btw, Remember Crest amps from VS series that leaked elecrolytic on pcb, damage could vary from somewhat damaged pcb to total disaster.
Hey.
Aha this is true. I shall put, say a 100k to earth from collector. But this can't be the problem since the 12V Vgs goes away when the last Bc556 shuts off. There is something weird with these mosfets. They don't shut off when there is no Vgs. Instead there is a voltagedrop of 0.6V from Drain to Source (with load)
I'm quite lost here...
Picture:
http://files.upl.silentwhisper.net/upload0/Amp_Dcprotect.jpg
edit: I think i found the error. Will update soon.
Aha this is true. I shall put, say a 100k to earth from collector. But this can't be the problem since the 12V Vgs goes away when the last Bc556 shuts off. There is something weird with these mosfets. They don't shut off when there is no Vgs
. Instead there is a voltagedrop of 0.6V from Drain to Source (with load)I'm quite lost here...
Picture:
http://files.upl.silentwhisper.net/upload0/Amp_Dcprotect.jpg
edit: I think i found the error. Will update soon.
Oh yes!
Thanks for all the help also!
Updated Schematics:
http://files.upl.silentwhisper.net/upload5/DCprotectLayout.pdf
Thanks for all the help also!
Updated Schematics:
http://files.upl.silentwhisper.net/upload5/DCprotectLayout.pdf
Hi!
First of all I must do a correction to the scheme because I discovered that the circuit works like this: When the amp sends out DC the Voltage to it cuts, but then when DC has gone this circuit very quickly turns on Voltage to the amp again which results in a "blopping" loud sound which is quite bad.
However this problem were solved putting a 47uF cap (series with a low ohm resistor to prevent from large incurrents) parallell with the LED (before Leds resistor). This makes the voltage to the base of the transistor to keep on a little longer. Therefore this one i guess its better to be high hFE.
I don't believe have to the other transistors has to be high hFE though. But this should be quite easy to calculate.
I must go now but i shall write more when i get home.
First of all I must do a correction to the scheme because I discovered that the circuit works like this: When the amp sends out DC the Voltage to it cuts, but then when DC has gone this circuit very quickly turns on Voltage to the amp again which results in a "blopping" loud sound which is quite bad.
However this problem were solved putting a 47uF cap (series with a low ohm resistor to prevent from large incurrents) parallell with the LED (before Leds resistor). This makes the voltage to the base of the transistor to keep on a little longer. Therefore this one i guess its better to be high hFE.
I don't believe have to the other transistors has to be high hFE though. But this should be quite easy to calculate.
I must go now but i shall write more when i get home
.Ok I found the site which inspired me to build this circuit. Here it is: http://www.geofex.com/Article_Folders/ampprot/dcprot.htm
If you're running high voltagerails, you should consider other transistors I think. Also change the 3.3k resistors to the zeners. They're running on 6.3mA in my circuit. For +-60V something like 16kR 1W would be better. And for the transistors, BC640 looks suiable. Remember, most of the voltagedrop on the "mosfet-drivers" goes to the 3.3k resistors. The two lowsignaltransistors acts just like a gate.
Schematic: http://files.upl.silentwhisper.net/upload4/DCprotectLayout.pdf
If you're running high voltagerails, you should consider other transistors I think. Also change the 3.3k resistors to the zeners. They're running on 6.3mA in my circuit. For +-60V something like 16kR 1W would be better. And for the transistors, BC640 looks suiable. Remember, most of the voltagedrop on the "mosfet-drivers" goes to the 3.3k resistors. The two lowsignaltransistors acts just like a gate.
Schematic: http://files.upl.silentwhisper.net/upload4/DCprotectLayout.pdf
The schematic has swedish text so here is a translation:
Överhettningsskydd = overtemp protection
Till slutsteg = to power amp
Från slutsteg = from power amp
Tillslagsfördröjning = Startup delay
Monterad på kylfläns = Mounted on heatsink
Till högtalarrelä = To speaker relay
The design is old so take it as a source of inspiration. Maybe one day I'll update it.
The circuit can be seen below at the bottom of the picture.
http://home5.swipnet.se/~w-50674/hifi_pics/hifi_100pr/qro_pcb_top.jpg
More info (not much) here:
http://home5.swipnet.se/~w-50719/hifi/qro/index.html
Överhettningsskydd = overtemp protection
Till slutsteg = to power amp
Från slutsteg = from power amp
Tillslagsfördröjning = Startup delay
Monterad på kylfläns = Mounted on heatsink
Till högtalarrelä = To speaker relay
The design is old so take it as a source of inspiration. Maybe one day I'll update it.
The circuit can be seen below at the bottom of the picture.
http://home5.swipnet.se/~w-50674/hifi_pics/hifi_100pr/qro_pcb_top.jpg
More info (not much) here:
http://home5.swipnet.se/~w-50719/hifi/qro/index.html
Hi Mikeks,
tell me your opinion.
Your reply is ambiguous.
Are you supporting protection or not?
Are you in favour of fuses or not?
Are you for or against fuses in the speaker feed?
Are you for or against a comment on the location of a protection fuse?
Leave it the way you have and I for one will never know.
tell me your opinion.
Your reply is ambiguous.
Are you supporting protection or not?
Are you in favour of fuses or not?
Are you for or against fuses in the speaker feed?
Are you for or against a comment on the location of a protection fuse?
Leave it the way you have and I for one will never know.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.