DC-Protect Circuitry

[mikeks]

.........the 47uF cap will prevent this.

For how long before the MOSFET switches reconnect the supply to 'speakers?

 

[Fritzell]

A few seconds.

It can be rebuilt with a triac or a thyristor to keep the protection on until a reset is activated. However I leave this to the individual designer. I'm happy with this circuit. Also, "connect PSU to speaker", this is only for a very very short time until mosfets brakes again. This is not worse than a high power output signal like a square wave signal.

 

[AndrewT]

Hi,
latching means once triggered, it locks in the triggered state.
Then the user has to take action to unlock it.

 

[Fritzell]

Aha. I thought mikeks ment that the circuit sucked hehe.. sorry for my bad english

Ah anyway. Yes, i agree then.

 

[mikeks]

Aha. I thought mikeks ment that the circuit sucked hehe.. sorry for my bad english

Ah anyway. Yes, i agree then.

AAHHH SO!

You now realise that, once invoked, this circuit will re-connect the DC to your 'speakers-possibly destroying them?

 

[AndrewT]

Hi Mikeks,
nice to be tolerant, even better if you took a little longer (used more words) to avoid that ambiguity.
Any response to my questions?

 

[mikeks]

Hi Andrew

Sorry...missed your Qs....fuse in output line? Bad...although advocated by no less a personage than JLH..

In feedback loop? Even worse!

If such design in your possession, consign to skip-sharpish!

 

[AndrewT]

Hi,

This is not worse than a high power output signal like a square wave signal

this could be a lot worse than a square wave signal.
If the reason the DC detect triggered was a shorted output device then re-closing the rail protection devices will apply full rail voltage to the speaker terminal.
Worse, the smoothing caps have had time to recharge during each off cycle.

 

[AndrewT]

Hi Mikeks,
thanks for the clarification.

Now re-read what you posted

Purveyors of such excreta should be lined up against a wall and shot!

what excreta are you referring to? There's the ambiguity.

 

[mikeks]

excreta=amps with fuses in output rail.

 

[AndrewT]

You are a (wo)man of few words.

 

[mikeks]

 

[jacco vermeulen]

This reminds me of the story of the Scot who accidently wound up in the trans bar.
Please do shave your legs, Michael.

 

[anatech]

Hi Mike,
I strongly agree with you with regard to fuses in series with the output of an amplifier.

-Chris

 

[AKN]

Has anyone measured distorsion introduced by a fuse on output?

 

[anatech]

Hi 4fun,
It will vary as the value and power level varies. Frequency as well I imagine (thermal time constant, not inductance).

Imagine you have a variable resistance in series with your speakers. It's constantly changing and you have no real control over it.

-Chris

 

[AKN]

Hi anatech!

I think most crowbar output circuits will have a fuse rated much higher than normal output, fuse value just below crowbar capability. In some cases not, I have seen it many times.

I don't disagree, a fuse is not good. But I am interested how bad by measurement.

 

[anatech]

Hi 4fun,
I'd rather see the fuses in the supply rails. The crowbar will cause those to blow. There is no reason the voltage amp section couldn't be taken off before this so there is no danger in the output going to the other rail if only one fuse blows (unlikely). Even if it tried, the crowbar short is still applied, so the other fuse blows as well.

Works for me. Mike, you like?

-Chris

 

[AKN]

That short silver wire inside fuse will heat up by current and change resistance. How much, anyone has graphs?

Lets not forget load eg speakers and their voice coils, plenty of wire that's heats up (and down), but that is somewhat inevitable of course.

 

[anatech]

Hi 4fun,
That information should be available from Littlefuse or Bussman. Any other fuse manufacturer too I would think.

There might be something in here for example.

-Chris