output transistor failure

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ed
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output transistor failure

Post by ed » Fri May 25, 2018 8:02 pm

here's one from the dumb fat boy at the back......

Is a fuse in both rails enough to protect the loudspeaker in the event that one half of a complementary pair fails?

I've tried for sometime to find any info on the interweb....and failed.
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Re: output transistor failure

Post by Dave the bass » Fri May 25, 2018 8:12 pm

Is the output device DC coupled to the speaker our-Ed (at the back of the room)?
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Re: output transistor failure

Post by ed » Fri May 25, 2018 8:25 pm

Is the output device DC coupled to the speaker our-Ed (at the back of the room)?
no.or yes, not sure what you mean by dc coupled...I beleive the term used is direct coupling..no capacitors sometimes called OCL.....standard push-pull complementary output pair......

I was reading up on all the methods for dc detection and disconnection of speaker in the event of detection...it all gets super complicated.....

logic says when one of the devices fails(usually short circuit), then you get the full force of that rail, neg or pos, up your speaker's jacksie, so to speak...this must be avoided.....

this full force scenario would put large load on the rail concerned which would trip a fuse...question is, would it be quick enough to save the speaker from the rail voltage.......with age my logic comes into question, feel free to chastise or laugh at me.
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Re: output transistor failure

Post by chris661 » Mon May 28, 2018 4:40 pm

If an output transistor fails short, yes, you'll usually get rail voltage (or close enough) on the output.

In theory, a low-rated fuse will be able to save a speaker by opening before thermal damage occurs.
Depending on the intended usage for the amplifier, however, you might find a fuse on the rails will blow on musical peaks.

Now, a speaker's thermal rating is based on pink noise with a 6dB crest factor. ie, if you want the long-term actual power handling of the speaker, you need to divide the power rating by four. That gives you the DC power handling of the speaker, give or take - if the coil is moved so it's mostly out of the gap, power handling will decrease further.

We'll say that a fuse that blows within a couple of seconds at a quarter of the driver's power rating is enough to save it.

Lets work through an example, then.
We'll say we're using an Alpair 10.3, which has a 30w nominal power rating. That's 7.5w of signal that has 100% duty cycle (DC, square waves)
Lets also say the amplifier we're using has 20v rails. If we put that 20v DC across the voice coil, 2.77...A of DC current will flow, so it'll be dissipating 55w, which will burn the coil pretty quickly - it'll have enough thermal mass to suck it up for a few seconds before the heat is too much.
To put exactly 7.5w of DC power through the driver, you need a smidge over 1A (driver's DC resistance, Re, is 7.2ohm). In that case, I'd probably pick a slow-blow 1A fuse. A fast-blow might pop under normal listening conditions, but a 1A fuse that's trying to pass 2.77777...A will blow quickly enough to protect the driver IMO.

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Re: output transistor failure

Post by ed » Mon May 28, 2018 8:04 pm

thanks for that, I was beginning to think I was asking a silly question, perhaps I still am..

You've duplicated exactly what my common sense theory was telling me.
the particular amp I was playing with which peaks at about 15 watts was showing 880ma RMS on each rail at about 12 watts delivered on the sim....I was going to do some real world testing but I thought somebody might have some empirical experience. I was also hoping somebody might offer the info on what real world SS designers do.

my current feeling(excuse the pun) is that 1A is going to be a bit near the mark...
btw the rails are 24v so there would be 3A DC or thereabouts on the speaker at point of failure.
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Re: output transistor failure

Post by Nick » Mon May 28, 2018 8:38 pm

I use a DC detector and a relay
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Re: output transistor failure

Post by Mike H » Tue May 29, 2018 9:32 am

Bear in mind a fuse rating is its max. current. To actually melt it, typically requires double, roughly.
 
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Re: output transistor failure

Post by ed » Tue May 29, 2018 10:17 am

Mike H wrote:
Tue May 29, 2018 9:32 am
Bear in mind a fuse rating is its max. current. To actually melt it, typically requires double, roughly.
how does that relate to slow-blow vs fast-blow fuses?
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Re: output transistor failure

Post by Nick » Tue May 29, 2018 10:24 am

From Wiki
The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuses have different characteristics of operating time compared to current. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow.
You need to look at the charts from the makers for each fuse.
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Re: output transistor failure

Post by ed » Tue May 29, 2018 11:30 am

you've probably guessed...I'm trying to back you into a corner here.....

I'm guessing you've looked at all this and decided......'fuses on the rail not for me'....'I'm going with dc detection and hang all the sq compromises'.......

how am I doing?

snigger

afterthought......It obviously is influenced by the proviso that a commercial approach needs to be more robust.
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Re: output transistor failure

Post by Nick » Tue May 29, 2018 12:08 pm

ed wrote:
Tue May 29, 2018 11:30 am
you've probably guessed...I'm trying to back you into a corner here.....

I'm guessing you've looked at all this and decided......'fuses on the rail not for me'....'I'm going with dc detection and hang all the sq compromises'.......

how am I doing?

snigger

afterthought......It obviously is influenced by the proviso that a commercial approach needs to be more robust.
Well, I didn't notice any sound compromises, and I wanted to at least look like I was trying to protect the speakers. Also I wanted a quiet and pain free start up, so I use the relay to allow the amp to wake up, make sure there isnt a problem in the output stage, then connect the speakers. I also detect HF as well as DC. I don't think its possible to get a rail fuse that meets the needs for speed and large headroom, and a fuse in the output is too intrusive for me. If it wasn't for the need to check on startup, then opto coupled mosfets in the supply rails and a DC detector would be my choice.
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Re: output transistor failure

Post by Mike H » Tue May 29, 2018 9:11 pm

I have used fuses in the supply of the power amps to protect the components (as much as is practicable) and the power supply. My germanium o/p amp has them. However is not DC outputs.
 
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