DC-Protect Circuitry

[parsecaudio]

4fun
I agree with you.
A simple fuse is not an adequate protection for a loudspeaker, perhaps in case of use a particularly strong speaker with a low power amp, but this is a nonsense.....

[anatech]

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,

A fuse outside the feedback loop to the speaker is a deliberate attempt to introduce distortion to the signal.

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

[Bgt]

Originally posted by latala
To me the best method is to bite the bullet and use a high power Relay/Contactor.
I

Toatally agree, never had probs. with relays for speaker prot.
Use a double finger 1 so you minimize failure.

[AKN]

Anatech,

Crowbars will work but I have seen them cause fatal damage to the circuit board in large amplifiers.

That damage itself would indicate a design error more than the choice of protection circuit I think.

[Fritzell]

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/u...tLayout1.1.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/datashe...L/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?

[AKN]

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?

[anatech]

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

[AKN]

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.

the (large) triac was also transformed into a fancy bit of wire

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.

[Fritzell]

Hey.

But in protect mode collector is free floating and therefore base on transistor after is not defined. Maybe a pulldown from collector would help?

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/u..._Dcprotect.jpg

edit: I think i found the error. Will update soon.

[Fritzell]

Ok the problem was that I had shifted Source and Drain on the PCB layout . The circuit works just perfect now!