Opinions on adding fuses to power amp

"msg" Phil Allison

** Piss off - you snipping IMBECILE.

** Total bollocks.

** But obviously YOU have never studied how typical audio power amps are made or have the tiniest a clue why.

PISS OFF

- BLOODY FOOL

........ Phil

Wonderful. You're arguing with people who are agreeing with you.

Quite entertaining.

You're not trying to approach the notoriety of Rod Speed are you? He might be different, but he still makes sense.

You on the other hand make no sense whatsoever. Still entertaining though...

--
Linux Registered User # 302622

"John Tjerkshis Fuckwit "

( snip autistic, verbal drool )

** Those with the IQ of a donkey only understand donkey speak.

Hee - haw, hee - hawww......

..... Phil

fuses

without

I'm just trying to engineer a more respectable equivalent of fuses for trace rupture. Eventually one DC rail ruptured as a fuse. I've now decided to do some calculation to determine what the rupture current for that trace was. Then as the common return almost fused and is the same dimension but in the even worse case then carries twice the current , halving this trace rupture current to use as the fuse rupture current and reduce that for current carying capacity , by half again ? If all output trannies fused short circuit all round so loading both DC rails the combined common return would have ruptured at whatever that rupture current is for the trace that did burn up. Anyone know the term for laying double traces of solder along copper traces to increase current carrying , so I can research it ? I'm intrigued how you lay quite accurate molten solder runs and parallel over and with existing traces.

Anyway measurements: Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major axes of .15mm and .8mm. From tables of fusing currents for copper, also tin and lead (NB in the form of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps Lead+Tin elipse then 6 amps (not as much as I would have intuitively thought) Total 18 amps so 18/4 = 4.5 amps conventional fuse rating. Presumably the circular to sheet allowance would up this 4.5 amp figure , but by how much ? What sort of correction factor for thin sheet/non-circular heating then rupture allowance?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Beefing up!

When we used to build classic 'Hitachi' mosfet amps, we would lay a thickness of solder along the DC rails to the outputs transistors and to the output terminal. It`s a bit like welding, once you get the hang of it, you can lay down a respectable run of solder that looks quite professional. I spose you could beef up with tinned copper wire, but it wouldn`t look so neat. Some amps have hard wired connections directly to the output trannies, or, the DC feed from the PSU go directly to the pcb lands upon which the outputs are soldered/bolted.

Did you ever tell us what make and model of amplifier this is?

Is it really worth all this effort to redesign? You're never going to be able to protect against stray metal object clattering around inside, and I`m sure the original designers never gave it a thought.

Ron(UK)

trace

the

rupture

traces

then

major

form

,

These beefed-up traces are far too regular to be done by hand, also precise

45 degree X+Y plot lines so some sort of plotter type delivery system following the traces/etch artwork co-ordinates. But its not just the solder lines but the amount of solder delivered that is so uniform.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Ah you're talking abput the hatched (scotched) type thing, I`m sure that`s solder paste screen printed on during manufacture of the pcb.

Ron(UK)

precise

solder

None of that on this one but I know what you mean. But some extrusion of solder paste process via a vinyl sign cutter type plotter mechanism in place of the cutter would be more likely than molten solder. Then fused onto the board before populating with components but would that take solder bath soldering of the components afterwards ? The "beefing" is not applied after the main component soldering operation.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

whatever fuse the toroidal gets needs to be antisurge. Toroids are known for their sometimes heavy inrush.

This will give no protection to the output devices of course, it really will be nothing more than basic fire protection.

You'd get much better fuse response if you put them after the reservoirs, because they dont then need to handle high peak cap charging currents.

worst case is 1x not 2x. The common psu rail carries near zero if similar fault currents flow in both + and - rails.

then there would be near zero current in the common rail.

I dont think I've ever seen any attempt to do it to accurate dimensions. Neither flow soldering nor hand soldering work like that.

No, because cu and slobber blow at very different temps. 18A probably wont even be close.

I'm left wondering precisely what youre trying to achieve. If youre trying to protect output devices, none of the above will do it. If youre trying to protect speakers, the above are not whats wanted. If all you want is to prevent charring of pcb due to repeated blowing, its going to be cheaper in the end to prevent the repeat blowing. Really whats propsed so far doesnt seem to solve any real world issue. The amp sounds roughly designed, intended for a limited service life, and to change that you'd need to add a full set of current and safe area protection for the output devices, almost none of which can be achieved with fuses.

What are you really looking to achieve?

NT

I'll repeat it without the clutter.

I'm just trying to fashion a more respectable equivalent of fuses instead of trace burning and rupture.

He's *repairing* an amplifier not redesigning it.

A decent amplifier has some element of overcurrent protection built in anyway.

Graham

You don't have much experience of high power audio amplifiers do you ?

Graham

You'll find it was probably getting on for 100 amps or so.

Graham

Don't waste time.

The customer's not paying for you to redesign the amp. Fit some wire mesh or whatever to stop things being dropped inside it in future as per safety regs.

Graham

Quite !

In which case it shouldn't have a CE sticker.

Graham

No.

You've gone completely berserk.

Graham

You can't.

Fix it and tell the owner not to drop metal bits inside it.

Graham

I would imagine that the stray object originated inside the amp, probably a panel screw or captive nut never secured in the first place.

How often you you pick up an item of gear and hear something loose rolling around inside

Ron(UK)

As I understand it, in modern pcb production, the majority of the solder is 'silk screened' as a paste, onto the board as part of the process, then once the majority of components have been positioned, the whole assembly is passed between radiant heaters to melt the solder paste and fuse the whole lot together.

Of course, I could be wrong.

It's a miracle that any electronic stuff works at all these days!

Ron(UK)