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Old 20th April 2011, 06:16 PM   #621
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Quote:
Originally Posted by Baztien View Post
Is the operating current given by the manufacturer? I read 3A on my trafo
That's it. In your case the 3 A is the secondary current per rail.
In other cases you have the power rating, e.g. 72 VA, divide that by the amount of secondaries which gives 36 VA per winding and divide that by the nominal output voltage, e.g. 12 V which also leads to 3 A.
For a transformer secondary you can use a slow-blow fuse with the nominal current rating, so T3A.

Quote:
Originally Posted by Baztien View Post
what do you mean by "see"? look up on a graph like the one you described? or test what faults can rupture that fuse?
You usually need those graphs, if you want to protect semiconductors.
Some manufacturers make your life easier by stating an integral rating in A≤s for fuses which you can also find e.g. in the datasheets of rectifiers. Then you only need to choose a fuse with a rating lower or equal to the rectifier's. Or a rectifier with a higher or equal rating to the fuse's.[/QUOTE]
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Old 20th April 2011, 06:59 PM   #622
AndrewT is offline AndrewT  Scotland
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Pacific,
would you care to estimate how many times a T3A will charge up 10mF of smoothing capacitance before fatigue failure causes nuisance blowing?
What if 20mF or 4m7F were used instead?
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Old 20th April 2011, 07:00 PM   #623
infinia is offline infinia  United States
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Isn't a single SB fuse on the primary good enough, to protect against fire hazards. Shorts on the secondary will be reflected back. The XFMR should also have overtemp protection in case of smoldering shorts. haha
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Old 20th April 2011, 07:09 PM   #624
AndrewT is offline AndrewT  Scotland
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Originally Posted by infinia View Post
Isn't a single SB fuse on the primary good enough, to protect against fire hazards. Shorts on the secondary will be reflected back. The XFMR should also have overtemp protection in case of smoldering shorts. haha
I agree, but Pacific is trying to persuade us to do otherwise.
Let's hear what he has to say.
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Old 21st April 2011, 07:10 PM   #625
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Originally Posted by AndrewT View Post
Pacific,
would you care to estimate how many times a T3A will charge up 10mF of smoothing capacitance before fatigue failure causes nuisance blowing?
What if 20mF or 4m7F were used instead?
I have been working as an electrical engineer for several decades now and I have yet to see a fuse blow from fatigue, although I know the theory behind it. I have however several times seen old fuses carry much more than their nominal current for long times without blowing. Another fatigue mechanism?

But I will try an estimate. We have a 12 V 3 A transformer. A 25 V 22 mF capacitor has around 30 mOhms, the 72 VA transformer has probably 0,6 Ohms. 12 Veff corresponds to 16,92 Vpk (letís ignore the reduction due to the rectifier), which gives a peak inrush current of ~27 A for less than 10 ms (first half cycle) which is 9 times the nominal fuse current. The datasheet from Eska for 5x20 mm T 1-3,15 A fuses specifies the pre-arcing time limit for 10 times the nominal fuse current to be 10-150 ms. So even the 22 mF capacitor will leave a 10 % safety margin for the worst case fuse, while the average fuse will withstand that current for much longer and will never blow due to aging.

In real life you will find that the voltage reduction due to the rectifier and the additional resistances from wires, traces and the fuses themselves will even reduce the actual inrush current to below the recommended 25% average temperature derating margin.

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Originally Posted by infinia View Post
Isn't a single SB fuse on the primary good enough, to protect against fire hazards. Shorts on the secondary will be reflected back. The XFMR should also have overtemp protection in case of smoldering shorts. haha
Shorts in the secondary circuit with low enough impedance will cause a primary fuse to blow. Some shorts with high impedance will not cause the primary fuse to blow. Overload will only trip the primary fuse if you use a soft-start arrangement that allows you to use a tightly fitted fuse rating.

You can only skip the secondary fuses when you can guarantee that the transformer cannot be overloaded. Commercial amp manufacturers do that by including current limiters.
Chip amps also have current limiters, so you could use a transformer with a current rating greater than or equal to the current limit threshold times the amp channels. E. g. use an LM3886 stereo amplifier. Each of the channels has a current limiter at 8 A, so you can skip the secondary fuse, if your transformer has a secondary current rating of 16 A. For the common 2x18 V transformer that means, you need a 576 VA transformer to skip secondary fuses for a 2x30 W into 8 Ohm amplifier.

Transformers with built-in overtemperature protection are a fine thing, but rarely used. You will rather find them in lighting transformers than in DIY audio.
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Old 22nd April 2011, 09:40 PM   #626
infinia is offline infinia  United States
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naah there are no such things as guarantees concern overloading! No one can garantee a current limit circuit (chip amp or otherwise) will work under any fault or other conditions! Sorry Pacific that's s really a silly story about current limits and XFMR sizes, we are talking about PS faults after all here, right. The problem is not so much big transformers but with the small cheap ones ie the winding resistances are high enough not to blow the fuse ie smoldering shorts. The safety approval process, at least for UL listed products is to short the secondary winding on the transformer sample and wait till temps stabilize for the prescribed amount of time (hours) with no appearance of flames, molten metals etc. Think about all the wall worts out there!
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Old 23rd April 2011, 08:18 PM   #627
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Originally Posted by infinia View Post
No one can garantee a current limit circuit (chip amp or otherwise) will work under any fault or other conditions!
All the more reason to use fuses for overload protection.

Quote:
Originally Posted by infinia View Post
we are talking about PS faults after all here, right.
Depends on who "we" are. I was posting about transformer protection against shorts and overload, no matter where they come from.

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Originally Posted by infinia View Post
The problem is not so much big transformers but with the small cheap ones ie the winding resistances are high enough not to blow the fuse ie smoldering shorts.
Smoldering shorts won't happen with a right-sized overload protection by secondary fuses rated for the transformer's nominal current.

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Originally Posted by infinia View Post
The safety approval process, at least for UL listed products is to short the secondary winding on the transformer sample and wait till temps stabilize for the prescribed amount of time (hours) with no appearance of flames, molten metals etc.
Most transformers will not work for several hours with their secondary shorted. One of the windings will assume the role of a fuse, melt and go open circuit.
Such an unreasonable procedure does also not sound at all like anything UL engineers would do, at least not the ones I have dealt with up to now. Or maybe you witnessed the test of a welding transformer?
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Old 23rd April 2011, 10:28 PM   #628
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Quote:
Most transformers will not work for several hours with their secondary shorted. One of the windings will assume the role of a fuse, melt and go open circuit.
Such an unreasonable procedure does also not sound at all like anything UL engineers would do, at least not the ones I have dealt with up to now. Or maybe you witnessed the test of a welding transformer?
yes most, but not all
A good outcome with a shorted secondary would be fusing open of the normally designed in primary safety fuse. or failing that, the transformer wire itself , hopefully quickly! There are UL tests that require this, and under any condition there cannot be combustion . Sometimes the shorts do not fuse open a transformer winding very fast at all, these are usually the very small ones. Even if you decide to use external secondary fuses you are not exempt from the testing. full stop AFAIK XFMRs wth built-in internal fuses w/ approvals are the exception.
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Old 24th April 2011, 12:15 PM   #629
AndrewT is offline AndrewT  Scotland
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I am still reading Pacific's replies.
He has not convinced me that inserting T rated fuses between the secondary and the smoothing capacitors offers any additional safety.

I still think that fuses in this location need to be so big that they will not rupture quickly enough when a fault does occur. In the meantime any one or more of the Primary fuse and the supply rails fuses and the mains fuse will have ruptured when the abuse and/or fault occurs.
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Old 26th April 2011, 07:40 PM   #630
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Originally Posted by infinia View Post
Even if you decide to use external secondary fuses you are not exempt from the testing.
I never claimed the opposite, but it is also true the other way round. Even if you have tested the transformer for UL listing, you are not exempt from installing adequate protection measures which include secondary fuses most of the time.

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Originally Posted by AndrewT View Post
I am still reading Pacific's replies.
He has not convinced me that inserting T rated fuses between the secondary and the smoothing capacitors offers any additional safety.
The fuses go between the secondary and the rectifier.
I am not trying to convince you of anything. I am describing the way transformer protection is taught at professional schools and the way it is done in industrial environments.
The following link is from a transformer manufacturerís homepage http://www.block-trafo.de/assets/dat...__0_48_mb_.pdf. Please note the hint on page 4 of the document:
Quote:
If only one short circuit protection is possible on the primary, the overload protection on the secondary should be carried out acc. to the information given on type/rating plate.
Dimensioning shall be effected considering the tripping characteristic, in general, on secondary rated current:
If you leaf through the document, you will find that the primary protection leads to a much higher power rating than the secondary protection. That is because
- the primary protection must be big enough to take transformer losses into account.
- the primary protection must be big enough to take reactance into account.
- the primary protection must be big enough to withstand the inrush current.

Secondary fuses avoid those three pitfalls.

Quote:
Originally Posted by AndrewT View Post
I still think that fuses in this location need to be so big that they will not rupture quickly enough when a fault does occur. In the meantime any one or more of the Primary fuse and the supply rails fuses and the mains fuse will have ruptured when the abuse and/or fault occurs.
You can always test it.

How can rail fuses according to your layout blow when a short or overload occurs in the power supply?
They cannot, because the fault is upstream and they can only react to downstream faults.

How can mains fuses blow before primary fuses?
How can primary fuses blow before secondary fuses?
The same answer applies to both questions. You ignored selective fuse coordination in your design.
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