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Old Yesterday, 06:10 PM   #1
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Default Solid State Rail Fuse Replacement

I'm trying to replace rail fuses with a current sensing circuit and a low RDS mosfet. This should eliminate the possibility of fuse distortion while better protecting the amplifier on overcurrent as well.

This will consist of two small daughter cards soldered in place of the existing fuse holders that are interconnected so if one protection circuit activates, it will signal the other circuit to activate, shutting down both rails.

I'd like to add provisions for interface with other protection circuitry in the amplifier to activate these as well. If a speaker DC detector activates, it would be nice to disconnect both rails from the amplifier before the supply caps dump through the shorting output device, for example.
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File Type: pdf DipTrace Schematic - Rail Fuse.dch.pdf (25.9 KB, 51 views)

Last edited by jwilhelm; Yesterday at 06:22 PM.
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Old Yesterday, 07:20 PM   #2
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Sorry, but this sounds a pretty crazy thing to try and do - making the amplifier a LOT less safe, and more likely increasing any imaginary distortion. A simple fuse is a VERY reliable and VERY safe component, and probably has no more effect on the sound than a piece of wire (which is all it is).
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Old Yesterday, 07:36 PM   #3
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Many are building amplifiers without rail fuses. This is a safer alternative to that.

Standard slow blow fuses properly sized to the current needed in an amplifier will start to melt on peaks of a sine wave under maximum load. You can see it as discolouration in the center of the fuse element. You can also see it on a scope. It looks like clipping. Normal amplifiers have too large of a fuse in them to properly protect output devices. How many times have you heard of output devices blowing without taking out a rain fuse?

I've already got a very reliable protection circuit running that will shut down my amplifier and open speaker connections faster than a fuse can blow on a short circuited output. This will enhance that protection circuit by opening connection to the supply caps on overcurrent or any other type of fault.
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Old Yesterday, 07:39 PM   #4
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Hi,
I do that but I using a microprocessor to control /regulate the rails voltage and a hall sensor to protect the speakers. If the current reached the set current high limit the rails voltage are shutdown to protect the speakers. I used it to control the rails voltage for an LM3886 amplifier. Attached it is a block diagram showing how it is done.
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File Type: doc speaker protection acs712.doc (75.5 KB, 17 views)
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Old Yesterday, 07:43 PM   #5
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Quote:
Originally Posted by tauro0221 View Post
Hi,
I do that but I using a microprocessor to control /regulate the rails voltage and a hall sensor to protect the speakers. If the current reached the set current high limit the rails voltage are shutdown to protect the speakers. I used it to control the rails voltage for an LM3886 amplifier. Attached it is a block diagram showing how it is done.
Hi Tauro0221

I remember your circuit from the 21 Century Protection thread. I believe it was you who recommended this current sensor I am using.
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Old Yesterday, 07:46 PM   #6
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Quote:
Originally Posted by jwilhelm View Post
Standard slow blow fuses properly sized to the current needed in an amplifier will start to melt on peaks of a sine wave under maximum load. You can see it as discolouration in the center of the fuse element. You can also see it on a scope. It looks like clipping. Normal amplifiers have too large of a fuse in them to properly protect output devices. How many times have you heard of output devices blowing without taking out a rain fuse?
That's NOT what the fuses are for, they are safety devices (mostly to protect the expensive mains transformer, and subsequent potential fire damage), NOT a crude attempt to save the output devices from been killed by misuse.

Your idea of 'properly sized' is most probably FAR lower than the fuses should be - which could explain why you imagine they might have some effect on the sound.

Interesting that you consider a tiny (and almost certainly inaudible) distortion as more serious that your method of 100% distortion - which killing the sound totally is
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Old Yesterday, 07:56 PM   #7
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Quote:
Originally Posted by Nigel Goodwin View Post
That's NOT what the fuses are for, they are safety devices (mostly to protect the expensive mains transformer, and subsequent potential fire damage), NOT a crude attempt to save the output devices from been killed by misuse.

Your idea of 'properly sized' is most probably FAR lower than the fuses should be - which could explain why you imagine they might have some effect on the sound.

Interesting that you consider a tiny (and almost certainly inaudible) distortion as more serious that your method of 100% distortion - which killing the sound totally is
I put a fuse on the mains side of my transformer to protect it. I put protection after the supply to protect the rest of my amplifier. There are many more parts to an amplifier than a transformer.

It seems I have offended you by trying to improve on an antiquated safety system. I use a microcontroller to control safety circuits as well. Some of us have left the stone age.
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Old Yesterday, 07:59 PM   #8
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Quote:
Originally Posted by Nigel Goodwin View Post
Sorry, but this sounds a pretty crazy thing to try and do - making the amplifier a LOT less safe, and more likely increasing any imaginary distortion. A simple fuse is a VERY reliable and VERY safe component, and probably has no more effect on the sound than a piece of wire (which is all it is).
Now , you comment. But , did not do the reading.

It is a valid patent - Patent US6587027 - Solid state fuse - Google Patents

I can see where your "crazy" statement comes from never being in an
industrial process environment.
They were using solid state breakers in the 70-80's for industrial
circuit protection. Those old pieces of silicon are most likely still working.

Now , with uOHM MOSFETS and better hall sensors , you get this-
Solid State Breakers | Solid State Relays | Perfect Switch, LLC .

These breakers protect "life and limb" out at the plant site , military
compliant - built to last 100 years. None of the old fuse's drawbacks.

"Increasing distortion" OMG - Don't you read ( u OHM !!! ) like a bloody
14ga wire with NO fuse.

A standard fuse is much more of a nichrome resistor element than
a 14 ga copper conductor - more reading needed !

At the 30 second overload rating of a 15A fuse , over 500mV drop. as it
gets incandescent , many ohms before it goes. the SS fuse goes
from uOHM to infinity instantly.

OS
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Old Yesterday, 08:07 PM   #9
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I read a little and read the hall sensor...........That's a awfully fancy way to do a simple job. Just the sensor must be more expensive than any of the power devices used.

In my book a simple rail fuse should do it IF you use the money to put in more pairs of power transistors. I am sure you save money having more power transistors. More transistors in parallel, the much better chance they survive long enough to burn the fuse first.

Looking at the circuit, I am not sure you even save pcb space compares to just putting more power pairs. AND not to mention the more pairs you use, the more forgiving is the choice of transistors, less requirements of constant Hfe at high current( as you have more pairs in parallel), Less requirement on SOA stuff. You can buy cheaper transistor instead of "the" transistor.

That's one thing I don't understand. reading a lot of the schematics here. Poeple try to get a lot of power out of one or two pairs of power transistor, then in turn, worry about the SOA, beta droop, resorting to "the" transistor. I started out with 5 pairs of output transistors. Don't tell me the rail fuse can out last all these transistors. You get better and cheaper transistors when you put less requirements on them.

AND, who cares about 1/10 of an ohm through the fuse? For one, this is at the collector, there is a lot of common mode rejection. These are secondary effect. Put by pass caps right at the collector to smooth out the voltage. I use a 10uF in parallel with a 0.1uF right on the collector of each of the power BJT. If needed to, I can put more.

Worry more on the layout. All the talks, then when I look at a lot of the layout posted, long power and ground trace, no bypass cap any where close. resistance is proportional to length/width. with long trace, the voltage drop can be worst than a fuse that is so short. I don't see anyone worry about this very obvious problem.....Then laugh at me using ground and power planes.

Last edited by Alan0354; Yesterday at 08:21 PM.
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Old Yesterday, 08:17 PM   #10
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Quote:
Originally Posted by Alan0354 View Post
I read a little and read the hall sensor...........That's a awfully fancy way to do a simple job. Just the sensor must be more expensive than any of the power devices used.

In my book a simple rail fuse should do it IF you use the money to put in more pairs of power transistors. I am sure you save money having more power transistors. More transistors in parallel, the much better chance they survive long enough to burn the fuse first.

Looking at the circuit, I am not sure you even save pcb space compares to just putting more power pairs. AND not to mention the more pairs you use, the more forgiving is the choice of transistors, less requirements of constant Hfe at high current( as you have more pairs in parallel), Less requirement on SOA stuff. You can buy cheaper transistor instead of "the" transistor.
Hi Alan.

OS and I both have amplifiers with output devices spread out over 16'' heat sinks. No more room for output devices.

What I'm thinking of would be around 1 1/4'' square board standing up off the main amp board on leads from where the rail fuse holder would normally be soldered. There's around $20 worth of parts there. It's considerably more than a fuse and holder cost, but this is DIY. Who cares?
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