Help Guide Me About Fuse

[Agung Kurniawan]

Hi everyone..
In my latest post I have a problem about transformer temperature. But the guy's overhere were help me fix the problem. Thanks to them

But there's still some noob question (wich I don't know the answer) flying over my head. It's about Fuse.
I have seen many guitar amp design and I see Fuse in the power supply section. The same thing I have seen in some pedal power supply.
So, my question is:

In power supply teritory, what does fuse do?
How to pick and chose one of them to be used in the circuit?

Since they said we need fuse for 'safety', Im planing to use some for my DIY guitar amp and my pedal ps.
Thanks...

[GibsonGM]

It is a good question, Agung!   

Well...you know that if you exceed the carrying capacity for a wire, it will melt, right?   So, the wire inside a transformer or part of your circuit can melt if you allow too much current to flow through it.   This happens because of a defect, something broken, or bad design (while we are experimenting!).

A fuse is a very small piece of wire inside a glass envelope.  It is "rated", so we know how much current will make it blow, or create an electrical "open" in the circuit.  This can save the circuit, or your power supply, or both.   A short circuit or other problem can try to draw so much current that things light on fire!!   If selected properly, the fuse will blow before you damage your equipment.  Do not count on a fuse to save YOU, though!   It is not fast enough.

A fuse is chosen by taking down the data about your circuit and power supply and considering what you are doing with the components.   If your transformer is rated to deliver 1A on the secondary, a fuse will be chosen to protect it from exceeding the maximum allowable current.  This may be well below the 1A rating, or close to it, depending on the circuits that follow.   

If your guitar effects is expected to draw 100mA, for example, you may choose a fuse that will blow at 150mA to protect it if something is shorted internally.   If you look up "how to select a fuse" on the internet, there are formulas to help you do this, and they are easy to use.   It is about finding a balance that protects your circuit, but also allows the power supply to be flexible enough for multiple use.   You may find a fuse for the power supply on the primary, and another one on the secondary to protect the circuit it is powering...

I hope this gives you some help!

[key-bored]

Hey, and while the topic is open, I'd like to know something about fuses, also.

Can ordinary automotive fuses be used?  I mean, an Amp is an Amp, right?  So if I want to use a 1.5A auto fuse, is there some problem with that?

KB

[Rob Strand]

Can ordinary automotive fuses be used?  I mean, an Amp is an Amp, right?  So if I want to use a 1.5A auto fuse, is there some problem with that?

For the low voltage side it's probably OK but it is not suitable for mains.  Main fuses needs to rated at the correct voltage.

[Agung Kurniawan]

thanks sir, that really helping

You may find a fuse for the power supply on the primary, and another one on the secondary to protect the circuit it is powering

So then the main sourch that may causing a problem is the transformer, right?
is that any certain circumstance that may causing a transformer to deliver much bigger current than it should?

[Rob Strand]

The details of a rating a fuses are quite complicated - beyond this group really.

- Protecting AC side, DC side or both.   Usually the safety rules of a country require a mains fuse on the AC side.

- Operating current  and long term operation:   Obviously you need a fuse which does not blow with the normal load current.   However if you have a 1A load a fuse rate at 1A will blow.   It might take a long time to blow but it will probably low.   Take a look at some fuse data sheets (say littlefuse or bussman), the fuse rating is not a single number, it depends on both time current and is represented as a set of curves.

- Fault current:  If your circuit limits the current and you want to protect against a fault then there is a minimum fuse rating which will blow.   If you short circuit the secondary of a transformer it will not pull infinite current and blow the fuse it might pull 10 or more times the normal current.

- Load Type: When you turn on a piece of equipment there is a surge.  If you have a fast fuse with a surge load it will blow instantly.   There is another rating of a fuse called I^2 t (I squared t) which represents when it trips with pulse currents.   You need to know the shape of the start-up waveform to compute the integral of i(t)^2.

- There's different speed fuses:   F = Fast "Quick Blow" ;  T = Thermal "Slow Blow".
The speed is not well defined and you need to consult the specs for a fuse.
Generally you will use slow blow fuses in places where there are large surge currents.

- nuisance tripping; Regardless if the fuse is fast or slow if the fuse is underrated it will fatigue over time and eventually blow (this is called "nuisance tripping").     You will find yet another table in the datasheets showing the operating i^2 t vs the number of times you can apply the pulse before the fuse fatigues.

Sometimes you have to use a larger rating to prevent false tripping but then you must check it will actually blow with the fault current.

I have seen some transformer which say "must be used with fuse rating xxx speed yyy" to comply with national standards.


[Edit: Look a:
http://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf
]

[PRR]

Welcome.

> Can ordinary automotive fuses be used?

Why? Are they cheaper?

> I mean, an Amp is an Amp, right?

Only until the fuse must BREAK the current. Then it has to stand-off the entire voltage available and stay broken.

Automotive fuses are usually rated 50V.

Wall-power fuses are rated 250V.

There is also a difference in Available Power. In a car you have your alternator and your battery. You can pull many thousand watts for a few seconds. In household electricity you have potentially the entire power of your electric utility company, a whole dam's-worth of power (except these days a big coal or gas fire). While the wires and pole-transformers limit the power, you can have MEGA-watts of power for a few seconds. (240V, at 20KVA fault-rating, 5MW until something explodes.)

I'm not saying a car fuse won't break 120V, most days. But we can't be SURE it will break, without re-welding itself, or throwing hot metal around the room. The wall-power fuses are tested for such work.

The 50V fuses will be fine for 99% of our low-voltage work.

On the 120V/230V side, we *must* use the 250V fuses.

There are many other considerations, as Rob explains. But Voltage rating is important.

[Rob Strand]

This might be more as a guide:
http://www1.cooperbussmann.com/pdf/b3cfa54a-aad1-4db3-ac58-64403d067f29.pdf

[GibsonGM]

thanks sir, that really helping

You may find a fuse for the power supply on the primary, and another one on the secondary to protect the circuit it is powering

So then the main sourch that may causing a problem is the transformer, right?
is that any certain circumstance that may causing a transformer to deliver much bigger current than it should?

The others have really provided VERY good information on fuses...but if I understand you correctly, you are asking what circumstances may cause a transformer to deliver more current than it should?

A short circuit anywhere from INSIDE the transformer all the way to your circuit output could place a load on the transformer that is nearly infinite, which of course could cause it to melt.   In fact, internal arcing, overheating, burning odor (burning insulation) in transformers are signs that this has happened and has damaged the transformer.     An internal fault may ruin the transformer, or too much of a load on the output may ruin it. 

When you ask a transformer to deliver more current than it is rated for, its voltage output will drop as its current goes up.  It becomes warm ,as you found out!    Then, the insulation around the wire inside of it begins to melt.  It shorts to its own turns of wire, gets hotter, begins to arc, and then may light on fire if the mains breaker does not trip in time. 

So - if you have chosen a fuse in the right way, if the transformer is asked to provide current above the fuse rating, instead of the transformer possibly causing a fire, the fuse 'opens' just like a switch.  It will be destroyed, but it is a very inexpensive component that saves lives and property. 

[Agung Kurniawan]

The others have really provided VERY good information on fuses...but if I understand you correctly, you are asking what circumstances may cause a transformer to deliver more current than it should?

A short circuit anywhere from INSIDE the transformer all the way to your circuit output could place a load on the transformer that is nearly infinite, which of course could cause it to melt.   In fact, internal arcing, overheating, burning odor (burning insulation) in transformers are signs that this has happened and has damaged the transformer.     An internal fault may ruin the transformer, or too much of a load on the output may ruin it. 

When you ask a transformer to deliver more current than it is rated for, its voltage output will drop as its current goes up.  It becomes warm ,as you found out!    Then, the insulation around the wire inside of it begins to melt.  It shorts to its own turns of wire, gets hotter, begins to arc, and then may light on fire if the mains breaker does not trip in time. 

So - if you have chosen a fuse in the right way, if the transformer is asked to provide current above the fuse rating, instead of the transformer possibly causing a fire, the fuse 'opens' just like a switch.  It will be destroyed, but it is a very inexpensive component that saves lives and property.

thanks a lot sir, that is helping.
this is my second time found smoke out of the transformer. maybe I will trying to use toroidal from now

[R.G.]

You're learning isolated bits and pieces of a complex problem. This can lead to wrong ideas.

Fuses are short sections of wire. What is special about them is that the makers of the fuse have carefully designed these short sections of wire to have a somewhat predictable melting characteristic. Notice that I said "somewhat" predictable.

Fuses are designed to melt and open a circuit before "Something Bad" happens. Exactly what that "Something Bad" is can vary widely.

In most countries, it is a requirement to have a fuse in series with the AC mains power going to any device that uses AC mains power. The purpose of this fuse is to prevent the device from starting a fire if something fails inside that pulls too much current. It is NOT to protect the device itself.  Some countries allow special "inherently safe" devices to not use fuses. This special classification is set up for devices that can prove that by the nature of their design, they cannot allow dangerous currents to flow or enough heating to start a fire. Some AC power transformers qualify for this classification, but never, ever assume that your transformer does until you're expert enough not to know what this means without asking.

Fuses are thermal devices. They work by getting hot. Get them too hot, and they melt and open the circuit. Exactly how hot they get as a result of how much current flows in them versus the external temperature and their construction is a very complicated problem in thermodynamics and heat transfer. Over long periods of time (seconds to minutes or hours, depending on the time-delay nature of the fuse design) they will tend to open ("blow") based on their average current. A one-ampere fuse does NOT blow at one ampere; in fact, that is the definition of a one-ampere fuse: it will carry 1A, but not substantially more than that, forever. At some higher current, maybe 1.5A, it will open in a few minutes to an hour. At higher currents, maybe 2A to 5A, it will open in a few seconds. At yet higher currents, it will open very quickly indeed.

The exact time to open depends on the time delay rating of the fuse and the energy delivered to the fuse over a certain time interval. For long periods of time, the temperature of the fuse wire depends on the balance of the heating of the fuse and how fast heat is conducted away from it. The temperature of an object depends heavily on how heat can get out of it. Heat leaves an object by conduction, convection, and radiation. Conduction and convection can be great removers of heat. Radiation sucks as a heat transfer method. If the current in a fuse wire is exactly the rating, the manufacturer will have designed it to carry that current forever; that is, the conduction, convection, and radiation from the fuse wire will get out exactly as much heat as the current generates, and the temperature rise in the fuse wire will stop before it melts the fuse. At very slight overcurrents, the temperature will gently rise up the melting point and sometime it will sage and open.

If there is a big pulse of current, maybe 5-1000 times the fuse current rating, so much energy is being pumped into the wire by current-squared-times-resistance heating that it cannot get out. So the temperature of the fuse wire depends on its mass and the thermal capacity of the wire for heating. If this energy results in a temperature rise over the melting temp, the fuse opens quickly. Energy into a fuse is directly related to the square of the current in the fuse and the time, so there is an "I-squared-T" rating on each fuse to tell you roughly how fast it will open for some amount of instantaneous overcurrent.

This is where the time-delay rating of the fuse is designed in. Time delay ("slow-blow") fuses have a high I-squared-T rating compared to their nominal carry-current rating. So it takes a BIG pulse of current to trip it in a short time. That allows power-on transients to get through without nuisance tripping, but still protect against seconds-long overcurrents.

Copper wire is rated for how much current it can carry, including when it hits a temperature that melts its insulation. Wires should always be current limted to some current below where their insulation melts. Where high power and current is available from AC mains or a DC supply, you should either use big enough wire to not melt the wire's insulation, or limit the current somehow, with other circuit components, or a fuse. Fuses are the dedicated weak link, in line with the engineering commandment that if you can't make it work forever without failing, make it easy to fix.

So now to some short answers.
If your transformer is not "inherently limited", you should use an AC power fuse in front of it. This stops fires from burning down the house you - and perhaps your family - lives.
The right size of fuse to use is one that just barely and RELIABLY lets the power-on current surge happen without tripping, and also supports the maximum current used by the circuit for an unlimited time. Anything more compromises the fire protection.

[Rob Strand]

If your transformer is not "inherently limited", you should use an AC power fuse in front of it. This stops fires from burning down the house you - and perhaps your family - lives.
The right size of fuse to use is one that just barely and RELIABLY lets the power-on current surge happen without tripping, and also supports the maximum current used by the circuit for an unlimited time. Anything more compromises the fire protection.

Because that ends up being a fine balance, many modern devices (including transformers) have given up with with wire fuses and now use non-resettable thermal fuses.   These sense the problem directly - the problem being heating and catching fire.

I know you are across this stuff RG because you worked in power supply design.    The whole safety thing seems easy on the outside but it's actually quite a profession in itself.     If you look inside a CRT monitor (and any consumer goods for that matter)  there is a lot of fine detail in the type of parts chosen throughout the device - certain areas have "blow" open resistors for example, capacitors that cannot fail short (cautious design even put two of those in series).  All parts, including the wire need to comply with safety standards.  Despite being cheap made in china junk, the big companies pay a lot of attention to safety.    When you buy "crap" over the internet it gets into the country under the radar, it may not meet the standards (even if it says so) and that's when you see people getting electrocuted and things catching fire!