How to calculate the amperage of fuse

[Jefnando]

Hello people of Diy paradise!

I have a rather simple question here :
how to calculate the amperage of the fuse for DIY amps?

I know roughly how to calculate for home appliances using wattage, Eg: P= IV
but how bout DIY projects that uses power trannies?

Eg: does a 15-0-15: 12VA tranny require a fuse rated slightly bigger than 0.05A? if the primary voltage would be 240V?

Also, what type of fuse should i use for amps? Slow-blow or fast-blow? and normally, fuses aren't rated much other than the voltage and the amperage, so how to determine whether a fuse is a fast or slow blow type?

[assendor]

Hello Jefnando,

Slow blow types are the ones you MUST use if you fuse the transformer on the primary(240V) side, this cause the transformers have a rather high in-rush current.
The amperage size of the fuse on primary is normally calculated in order to protect the transformer from possible overloads.
In some cases only a fuse on primary side of the transformer is sufficient for a total protection, but Personally I like to have a fuse ONLY at the Secondary side of the transformer, due to the high In-rush currents present.
(It is not recommended to have only a fuse after the rectifier and capacitor bank, because if the rectifier or a capacitor shorts and fails, the transformer then will melt down and may cause a fire.)

Now there are more things to think of, even if you fuse only the secondary side, you still have to think of the possible high currents present while charging a capacitor bank from 0v to operating voltage.

Often we then use "Soft Start" systems especially when the transformer and capacitor bank is rather huge stuff, this is often just a resistor in series with the transformer on primary side, till the caps are charged.
A time delay circuit and a proper rated relay or two will do this job nicely.

This solution also removes the problems with transformers high in-Rush currents.
On top of this, you may then also not anymore be bothered with selecting the Fuse either.

BTW, fuses are normally marked with Voltage rating, Ampere, and (T = Slow) (F = Fast)
There may also be used other types of markings, Z = Superfast, A = Fast, B = Standard Speed, C = Slow, D = Super slow.
If there are no markings of the speed of the fuse, it most likely is something in between slow and fast.
If we want to get even more complicated, some fuses are more or less designed only to give a good protection against
short circuits. (These are not so good for protecting against overloads)
Typically used to protect cables and limiting risk of fire when powered with a battery, or battery bank.

Other fuses are specially made for being good at disconnecting at rather small overloads over time.
Typically more used for protecting transformers from overheating.

Of course there are also combination's of these types too.
OR Over heating protection on Primary of the transformer, and Short circuit protection sized fuse for your connected equipment on the secondary.

How much can be written about a fuse?? HA HA..

-Jonas

[Jefnando]

Jonas, thanks for the quick reply..

I've done some research online before creating this post and i found that this website http://www.allaboutcircuits.com/vol_1/chpt_12/4.html provide some quite clean information bout fuse.

Jonas, i really appreciate your reply but i am afraid i have more questions now . about the "unmarked fuse that is in between T and F", is it  safe to use that for the primary of the transformer? Or should i just stick strickly to the slow-blow type? your debate about using the fuse only in the secondary of the transformer, is that before or after the rectifier or both, and if i do that should i use a T or F fuse? Also about the caps charging, should +20% of the expected current rating be enough? Final question: How to calculate the amperage of fuse???

Currently all my amps are unfused, there is one that i did about a year ago that gets it 250V B+ directly from the rectified and smoothed mains without going through an isolation transformer. Guess its time to think bout safety? 

[assendor]

Hello again Jefnando,

If your Fuse is too fast it will blow when you don't want it to, so it is really not any problem other than that.

About fusing, It depends.
If you have an enormous capacitor bank, then it is very desirable to have a Fuse also after the CAPACITORS,
because if you then get some fault in your circuitry everything will blow into pieces and smoke before the capacitors are discharged.
So in this case, Primary Fuse, and After the CAPS..
(I love using insanely HUGE capacitor banks, Not just because, but because I like the sound better.)
NOW we can discuss how much the FUSE will degrade the Power supply quality when added after the capacitors, OK USE GOLD FUSES; HA HA .. )

In Smaller transformers like in the 100VA and less size, it will most often be OK just to have a fuse on Primary or secondary for protecting the transformer.

The current you run in a circuit for charging up can be rather small, since the capacitors don't really has to be fully charged to reduce the current enormously.
In-Rush for Transformers are just the magnetic field buildup when accidentally connected when the AC is on top, close to top of sine wave. This of course happens often.
So a tiny current would do here to stop this extreme 10+ X Rated Current.

NOW it is not needed to have these delay relay systems if it is no problem, this is most often on bigger transformers,
if it works without such a thing, well then keep it simple.

The formula you used, P=UxI  I=P/U  100VA/240V  100/240V=0.42Ampere 400mA Should be OK, or maybe even 500mA.
2KVA/240V=8.32Ampere. 8Amps Or 10Amps. Depends.

Safety, YES it is important, especially if you leave your equipment on all the time, or a lot of time when you are not there, OR SLEEPING... 
We always hear about having several barriers to break before a disaster is possible.
Now if you always are there, and always awake, No Fuses needed if you have fire detection systems as well...
IN CASE you fall asleep anyway.. HA HA.. .

I had a disaster with a Tube amp that had no fuses whatsoever.
Almost a fire, A terrible smoke and smell, Amplifier totally burned out, a nice lesson to be learned.
The room is rather thought of to be fire proof as well, so there where ONE BARRIER and that was that..
I'll use Fuses more often after that.

If a tube fails you may fry your equipment. If a capacitor dies the same. If a rectifier fails the same.
There are many things that can happen even if everything looks safe.

I've thought of trying going directly from the mains to an amplifier, but this is one of the few things in my life that has not be tested out.
Now it has to be said, you may like the sound better if you get yourself a transformer.
AT least you should use a very very good HOME MADE mains filter.

NOT to speak of the possible danger of getting shocked.
It is not preferable other than as an experiment to run such equipment directly from the mains without a transformer.
Whatever you do, do never try to make two devices powered directly and connect them together, UNLESS you really know what you are doing, and knows what is what of Neutral/Phase-1-2-3, and are SURE your don't mess this up.
You will most certainly be in high risk of killing yourself.
With other Grid types than TN- TNC - TNC-S you have even higher risks.. Please be careful.

Fuses, well, the mains will most likely be secured with fuses, so you are OK when it comes to total failures.
You will not have any transformers melting down.. HA HA HA .-.

-Jonas

[Jefnando]

Jonas, thanks for replying.

AAAAAAHHHHHHHH NNNNOOOOOOOOO!!!!!!! Jonas, after reading what your reply i think i am gonna be a safety freak from now on; you've pointed out so many possibilities of frying the amp and the owner in case any of the many parts fails. Thats scary alright. How i wish i am still in the ignorant side where safety is something that only others need. Gosh, now i think i am gonna load my amps with fuses! So a primary, a secondary and one after the caps?

About the calculation of amperage, so it really depends on the transformer? How about trannies with a few secondaries? especially those that are made for tube rectifiers, how are tose calculated? Do i just calculate the amperage of each secondary and sum them up? Also, if i am gonna use fuse on the secondary and one after the caps, how is that calculated? should i also use a 250V rated fuse or go lower or higher depending on the circuit voltage?

I learnt the hard way; The amp without isolation transformer fried 5(FIVE) of my sound cards(I listen mainly through my com). Apparently both the amps and my com were electrically connected because the PS of the com isn't isolated from the mains...

[assendor]

Hello Jefnando,

Most likely the non transformer amp is the reason for your fried things, like you said.
You must be glad you haven't fried yourself.

You don't need loads of fuses, normally one or two is plenty.
It has to be considered from the amounts of energy you have in the system.

Like if you are playing with rather high voltages plus high currents, then you must be more careful.
IF the total POWER P is high, like in the 1000WATT, then it is a good choice to be on the safe side.
 
Good luck.

-Jonas

[Jefnando]

Jonas, thanks again for replying.

I guess i'll stick to the slow blow fuse before the transformer primary. I think it is better that was because a transformer is one of the expensive parts in an amp, plus the fuse at that position will be able to provide protection to all the components in case of a surge or short circuit. Not long ago, i blew a 6V, 2A transformer due to miswiring of the rectifier diodes, don't wish that to happen again.

Jonas i hope it wouldn't be too much if i am to ask for your help once more in solving the following problems/situations:

How to calculate the fuse amperage for the following(fuse positioned before the primary):

1) Transformer with multiple secondaries, eg: 350VAC, 100mA + 6.3V, 2A

2) multiple transformers connected in parallel, eg: one 350VAC,100mA transformer and one 6.3V, 2A transformer in parallel

3) Improvised isolation transformer(series) eg:two 6V 500mA transformers wired back to back

4) multiple transformers connected in parallel and series, eg: situation (3) and one 6.3V, 2A transformer in parallel

Thank you..

[KenC]

Your 4 questions are actually one questions.

Ask yourself a few questions and you should know the answer
1) what type of energy is passing through the fuse.
2) what are we protecting, the power cord, the transformer or whatever. Then put a fuse right in front.
3) use the energy figure to find the current using ohms law

2 methods,
The detail, no nonsense, no compromise, nick picking method
Use PSUII to check how much is your in rush current. if its too high, reduce the cap size until you are happy. just keep simulate the figures until you get reasonable damping factor.

The backyard electrician method,
Use the lower of total VA of either your transformer rating or total VA consumption expected. If it still doesn't work, increase the fuse size until they don't blow.

BTW, fuse does have impact on the sonic characteristic....

Cheers

KenC

[Jefnando]

KenC, thanks for the reply

Yeah, they are actually very much one question, but i just wanted to be sure how to really calculate, so i dont have to keep on asking if i am to use any of those combinations.

Anyway, since you've put it that way, i guess being a trial and error person, i'll just do it like how you say; buy a bunch of fuse and start blowing them smaller ones until i reach the suitable fuse. Wahahah. Never thought of that, Thanks.

Fuse does have an impact on the sonic, but is the change so much that a fuse should be avoided? i don't think so..