Opinions on adding fuses to power amp

400 watt RMS amp for use on 240 V mains. Destructed due to metal dropping in and shorting the amp . The only fuse is on the mains rated at 8 amp, maker's design rating, which shows no sag , discolour or anything like that, ie untouched. Shorted power trannies, burnt low power trannies and even a piece of 3mm trace from the main bridge rectifier burnt through and that had 2 runs of added solder over the track for current carrying, the other polarity trace overheated but not ruptured I'm thinking of bridging that gap with a fuse and another on the other rail after cutting it. The mains transformer is rated at 2x 47V,5 amp. So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of what rating ?

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

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Reply to
N Cook
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How did that happen ? The 'cabinet' is supposed to prevent such things, it's an IEC safety requirement.

Sounds like an incompetent manufacturer to me. 8 amps will provide a continuous

2kW almost ! Given typical fusing characteristics, consider that up to 4kW for maybe 10 seconds.

IEC 60065 failure very likely.

Probably something like 3.15A but you may need to add a surge-gard type NTC in the mains input.

What make / model was it ?

Graham

Reply to
Eeyore

The power supply to your amp probably has massively oversize filter capacitors.

Average current draw at 400W out might be only two amps, but because of the rectifier-filter construction it doesn't draw that two amps continuously. It draws it in very brief gulps 120 times a second. Typical conduction angle might be 20 percent in a well designed supply; with massively oversize filter caps it could be as small as 5 percent. Massively oversized filter caps have been all the rage in consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average current (2 amps) but the root-mean-square (RMS) current (which could indeed be 8 amps at full load).

And remember, fuses aren't really there to protect your electronics, they're in there to prevent fires.

If you want a supply that doesn't have such a small conduction angle, you go to choke-input supplies, which for some bizarre reason aren't nearly as popular as they were 50 or 70 years ago :-).

Tim.

Reply to
Tim Shoppa

So what you are saying is that this catastrophic failure of a 400 watt amp didn't blow an 8 amp fuse? I find that really hard to believe not that you are making this up mind you.

Reply to
Meat Plow

which

of

trace

rail

fuses of

The 2 reservoir caps are 6800uF, 80V rating with presumably about 65V on each , when all is in working order.

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

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Reply to
N Cook

which

of

trace

rail

fuses of

Well semi-catastrophic, the positive side in a sorry state but the negative rail side cold-tests ok so far. I was wrong when I said about the overheated trace it was the ground return not the negative rail, the + trace burned through as a "fuse", no deliberate necking at those points.

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

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Reply to
N Cook

Hmmmm well I suppose anything is possible. I haven't gotten into these new fangled Chinese amps though. My experience was with good old Crown, Soundcraftsman (yuk) Peavey, BGW and all those that had a dozen or so devices like MJ-15024/25 pairs on each rail. I have a 20 year old long defunk SCS (Sound Code Systems) 750 WRMS MOSFET amp down in my basement that still works great after I repaired it. I used to use it for bass guitar in the bridge mode. Has an infinite variable speed fan that adjusts with the heat sink temp. Uses now obsolete J-50's and K-134's. I bought up a bunch of them years ago when I was luck enough to find some so I would have spares if the amp decided to toss an output which it hasn't in the

15 years since I repaired it. I use the amp now for my PA in the basement where I practice.

Anyway, I think fusing the rails is feasible.

Reply to
Meat Plow

Absolute utter complete and total rubbish.

'Oversized' caps will only slightly change the conduction angle. It's readily shown by calculation. Certainly no way whatever will the rms current be 8A !

Graham

Reply to
Eeyore

It's good practice IMHO.

Unfortunately it's unlike to *prevent* further 'burn ups' although it might help mitigate them slightly.

To actually prevent burn-ups you need to design for that and the small Chinese companies I've come across who offer cheap OEM products don't have that level of design skill. With bipolar output amplifiers you'll normally need to use a number of fusible resistors (or else rate the output devices more conservatively - which cheap designs never do of course).

Graham

Reply to
Eeyore

That's a reasonable size. They won't be causing excessive switch on current.

Is the power transformer toroidal or EI ? That's where the big difference is. Toroids will take a considerable switch-on current (it's largely due to their magnetic characteristics).

Graham

Reply to
Eeyore

That shouldn't be terribly hard to calculate, assuming a conduction angle of your choice. So how about adding some support for the assertion?

--
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Reply to
CJT

current.

is.

their

Toroidal , not E-I lamination transformer I'm coming around to thinking 5 amp anti-surge in the mains fuse-holder and a 5 amp fuse shunted in the + and - rail traces from the DC side of the bridge rectifier before the reservoir caps, but undecided whether quick-blow or anti-surge ones there.

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

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Reply to
N Cook

Fix the amp, put the fuses of your choice in and put it through its paces. You'll probably want to run some music through it at high volume preferably some Hip Hop . See which fuses blow and which don't.

Reply to
Meat Plow

10% conduction angle gives peak current of 20A, RMS current of 6.3A, not too unreasonable to choose an 8A fuse in that case. 6.25% conduction angle gives 8A RMS, but that'd blow the fuse way too often.

It takes big capacitors and toroidal transformers with really stiff windings but many folks are building audio power supplies that way. I personally don't like razor-thin conduction angles like 10% but that's the style of other folks, not me!

Tim.

Reply to
Tim Shoppa

"N Cook"

** Is that 400 watts per channel or 200 per channel ?
** Then the AC supply current did not exceed 8 amps by much for more than a fraction of a second.
** Err - " trannies " = transistors ??
** Say it is rated at 500 VA, for simplicity.
** From another post I see it is a toroidal type.

Couple of facts:

  1. With secondary shorted ( before or after the bridge) primary current will rise to circa 42 amps rms - so bye bye to any 8 amp fuse real quick.

  1. At switch on, the peak supply current will regularly exceed +/- 100 amps for the first half cycle - diminishing to the idle value over the next 10 - 15 cycles or so.

  2. At 400 watts output, the AC current draw will be around 4 amps rms - assuming this is a typical, low bias, class AB amplifier design. With hard clipping the figure will rise to over 6 amps rms.

So - the maker's choice of an 8 amp AC fuse is not unreasonable, given the above facts.

The knee jerk reaction of matching the fuse rating to the VA rating of the AC tranny does not work in practice - just try using a 2 amp fuse if in doubt.

Maybe try a 6.3 amp " anti-surge " fuse - the kind with spiral wound fuse wire.

Keep a few spares handy.

...... Phil

Reply to
Phil Allison

"N Cook"

** Be very wary of adding +/- DC rail fues to any power amp designed without them.

Very likely if one or other DC fuse blows or is removed, the amp's output will swing fully to the rail with its fuse still intact.

Recipe for fried speakers.

You have been warned.

....... Phil

Reply to
Phil Allison

One could add overcurrent shutdown circuitry on each rail designed to shut down the entire supply, or for antique simplicity use 'indicator fuses' which have a spring-loaded plunger which makes contact with an endstop terminal when they blow -- the endstop is often connected to a crowbar which blows main fuses upstream. These are small cartridge fuses with the same form factor as standard varieties; I had to replace lots of them in old line printers where they were used in the hammer drivers -- in this application they did not crowbar but instead were wired to status circuits that reported the failure.

Regards,

Michael

Reply to
msg

** How idiotic !

** Or simply use no DC rail fuses and let the AC supply fuse blow when overloaded - as originally intended by the designer.

Those power amps that DO use +/-DC rail fuses, almost invariably have them fitted in line with the output devices but NOT in line with low power, voltage amplification circuitry.

It must be possible to remove either DC ( or both) rail fuse with the amp under load and drive and have no circuit damage occur. Any competent amp designer can arrange things so this is the case - but not likely the OP.

....... Phil

Reply to
Phil Allison

Evidently the designer did not account for the sort of catastrophic failure described by the OP. The large capacitive reservoir in the supply is very much like that in the line printers that employed the sorts of protection I described, including the "idiotic" independent supply rail overcurrent protection scheme with very fast response times (faster than any fusible link).

Indeed that was implicit in my response; the rails in question are the power amp rails, _not_ low-power rails.

Yes, but the OP did not design the amp and short of re-engineering it he would need workable solutions.

Regards,

Michael

Reply to
msg

"msg" Phil Allison

** Nonsense.

Output stage S/C failure is intended to blow the AC supply fuse.

The circuit damage is already done when it failed.

( snip drivel)

** No - you have got it WRONG again !!!

It is the same two DC rails, split to service the output stage devices via fuses.

YOU have NO clue whatever about power amplifier circuitry.

Better you learned to shut the f*ck up.

** Well, he ain't gonna get any from a posturing ASS like you then.

The only *problem* the OP ever had was due to dropping a metal object inside the amp - to which the solution is damn obvious.

...... Phil

Reply to
Phil Allison

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