3A fuses are capable of passing ~700W to the transformers for ever, ~1500W for many seconds.
What VA are they?
What VA are they?
Hi Andrew, just my calculations, 2 of 300 va this total 600 va divide into 230(primary voltage) equal to 2.6 , I rounded it off to 3. I use fast blow fuse.
300VA should run for ever on a 1A fuse powering a heavily used music amplifier.
That will require some form of soft start.
Using 3A for 300VA is a cop out that uses the "normal" Fuse rating = 3times VA / Vac rating for motors and transformers. But this offers little protection, because the "big" fuse takes so long to rupture. You can have a serious overheating fault drawing double the power rating for many minutes and even approaching an hour.
That will require some form of soft start.
Using 3A for 300VA is a cop out that uses the "normal" Fuse rating = 3times VA / Vac rating for motors and transformers. But this offers little protection, because the "big" fuse takes so long to rupture. You can have a serious overheating fault drawing double the power rating for many minutes and even approaching an hour.
Hi AT, I did used 1A Fast blow but it did not survive for 5 seconds. I thought the primary fuse rating is calculated by total VA of transformers divide by primary voltage but I only have 3amp Fastblow fuse left thus.
Quan
Andrew, am I correct in my understanding that a 2A slow blow fuse would suffice for a 500VA transformer, powered from 240VAC mains, through a soft start circuit?
VA / Vac is close rated fusing.
3times VA /Vac is normal fusing.
Normal fusing is not very effective in preventing severe overheating.
I always try to use close rated fusing. That requires all transformers bigger than 50VA to have a soft start.
Look at the fuse graphs given by the manufacturers.
Look at the typical rupture times at 1times rated current. It's off the log scaled graph.
Now look at the two times and ten times rated current. How long does the fuse that is three times too big pass current that will cause severe overheating?
How much shorter is the time for a close rated fuse?
Take that 500VA as an example.
Look up T2A fuse data. How long will it pass 4A (1kW) and 20A (5kW).
Now look up T6A fuse data. How long will it pass 4A and 20A(5kW) 40A (10kW) !!!!!!!!
3times VA /Vac is normal fusing.
Normal fusing is not very effective in preventing severe overheating.
I always try to use close rated fusing. That requires all transformers bigger than 50VA to have a soft start.
Look at the fuse graphs given by the manufacturers.
Look at the typical rupture times at 1times rated current. It's off the log scaled graph.
Now look at the two times and ten times rated current. How long does the fuse that is three times too big pass current that will cause severe overheating?
How much shorter is the time for a close rated fuse?
Take that 500VA as an example.
Look up T2A fuse data. How long will it pass 4A (1kW) and 20A (5kW).
Now look up T6A fuse data. How long will it pass 4A and 20A(5kW) 40A (10kW) !!!!!!!!
Very interesting. I didn't realise that fuses were this slow to react to large increases in current. The datasheet I looked at showed that a T2A fuse would pass 5-6A for more than 10 seconds!
I will use close rated fusing in all my designs from this point onward.
I will use close rated fusing in all my designs from this point onward.
That's why we see that semiconductors are reputed to be the mechanism to protect your fuses.
The semiconductor blow first. Even when close rated fusing is used.
What is the MAXIMUM time till rupture of a T2A passing 5A? It can be double the typical value and can be four times the guaranteed MINIMUM time. You could be talking 20 to 40seconds @ 1.2kW.
If it was a T6A fuse !!!!!!!
The semiconductor blow first. Even when close rated fusing is used.
More than 10seconds typical, or more than 10seconds guaranteed?
What is the MAXIMUM time till rupture of a T2A passing 5A? It can be double the typical value and can be four times the guaranteed MINIMUM time. You could be talking 20 to 40seconds @ 1.2kW.
If it was a T6A fuse !!!!!!!
Last edited:
I see many Members stating that EI transformers do not need soft start.
I also see many Members stating and repeating advice that ONLY transformer BIGGER than 300VA need soft start.
To my mind and researches these Members are wrong. They are only considering how much their lights dim and whether their distribution board breakers drop out. It seems they have not considered the amount of heat that can be generated inside equipment with large or no fusing when something serious goes wrong.
I want all my equipment to run without any fuses blowing, nor breakers dropping out, while everything is operating correctly.
If I am stupid while swapping speaker leads, or hot swapping interconnects, then I won't mind a safety breaker or safety fuse opening. It will remind me not to be stupid.
Do I want my equipment to keep running when I parallel connect a second pair of speakers and thus increase the output dissipations (and stresses) above normal loud operating conditions/temperatures?
I also see many Members stating and repeating advice that ONLY transformer BIGGER than 300VA need soft start.
To my mind and researches these Members are wrong. They are only considering how much their lights dim and whether their distribution board breakers drop out. It seems they have not considered the amount of heat that can be generated inside equipment with large or no fusing when something serious goes wrong.
I want all my equipment to run without any fuses blowing, nor breakers dropping out, while everything is operating correctly.
If I am stupid while swapping speaker leads, or hot swapping interconnects, then I won't mind a safety breaker or safety fuse opening. It will remind me not to be stupid.
Do I want my equipment to keep running when I parallel connect a second pair of speakers and thus increase the output dissipations (and stresses) above normal loud operating conditions/temperatures?
AndrewT,
There are millions of amps produced by the famous companies that use transformers much bigger than 50VA and does not have soft start circuit.
Ideally, all amps should have soft start but considering added complexity and very small possibility of problems, this advice seems like exaggeration. We do not need paranoia among diy-ers. How many amps you know that failed because they did not used close rated fusing and soft start? If there is some problem no fuse will prevent amp failure. The fuses are extremely unreliable components. As far as fire hazard is concerned I always presume that we build amps with metal enclosures, not wooden. Also, I hope that we all keep electrical home installation in good condition, with sufficient cross section of conductors, etc.
Last edited:
I bought two broken amps from Ebay last week to salvage the cases and heatsinks. Not high end units just "run of the mill" stuff you find in average households. Both had small 170W transformers and both had soft start circuitry comprising thermistor and bypass relay.
But what really surprised me was when I disassembled my wife's old "mini system". Despite the fact that it it had a tiny transformer and cheap parts, it still had the soft start circuitry on the power supply board.
Hi guys, i 've tried 2Amp slow blow fuse and it blew straight away after switching on. Does this means a soft start is in order as per AT?. It sounds then ideally each individual transformer should have its own primary fuse to avoid overating total fuse value ?.
Quan
Quan
Hi Quan
Andrew (and others) have advocated seperate fusing in other threads. It gives better protection in case one transformer has a problem but not the other. Its pretty unlikely both would fail simultaneously.
If the only reason you use a soft starter is to stop the main breaker tripping, you can get away without it until >300VA. Rod Elliott says as much on his pages. I guess if you had 2x 300VA you could implement some sort of stagger start circuit (better yet just use a soft starter).
The argument AndrewT successfully makes is that a fuse can pass current far greater than its rating for very long periods of time. For short periods it can pass enormous currents that far exceed its rating. In the interests of safety (a fuse is a safety device, if we aren't concerned about safety why even use one?) then the fuse should be close rated.
I guess the eye opener for me was pulling apart these old broken amps. All were obviously built to a price with cheap components, small filter caps, flimsy heatsink, etc. But all of them had a soft start circuit comprising small dedicated board mounted transformer, thermistor, bypass relay and trigger logic. If it wasn't an important safety feature then I have no doubt the penny pinching engineers and accountants would have omitted it from the design.
Andrew (and others) have advocated seperate fusing in other threads. It gives better protection in case one transformer has a problem but not the other. Its pretty unlikely both would fail simultaneously.
If the only reason you use a soft starter is to stop the main breaker tripping, you can get away without it until >300VA. Rod Elliott says as much on his pages. I guess if you had 2x 300VA you could implement some sort of stagger start circuit (better yet just use a soft starter).
The argument AndrewT successfully makes is that a fuse can pass current far greater than its rating for very long periods of time. For short periods it can pass enormous currents that far exceed its rating. In the interests of safety (a fuse is a safety device, if we aren't concerned about safety why even use one?) then the fuse should be close rated.
I guess the eye opener for me was pulling apart these old broken amps. All were obviously built to a price with cheap components, small filter caps, flimsy heatsink, etc. But all of them had a soft start circuit comprising small dedicated board mounted transformer, thermistor, bypass relay and trigger logic. If it wasn't an important safety feature then I have no doubt the penny pinching engineers and accountants would have omitted it from the design.
What size were the fuses? If the fuse is small, relative to the size of the transformer and the expected inrush current, you have your answer. If the fuse is large, then it was there for another reason, such as protecting a weak component against a peak current at startup, etc. (UL, CSA, and their equivalents mandate some interesting tests for home electronics.) Or it could be there to eliminate startup noise from the speakers, from a poorly designed circuit. It would be interesting to know what happens when you bypass one of those commercial soft-start circuits... 😀
What I wanted to say is that fuses are not great protection in the first place. Fuses will blow for no apparent reason and will pass destructive currents that will cause malfunction of amp. Nobody can predict how particular fuse that you put in your amp will behave.
It seems that mains fuse is there not to protect the amp itself but to prevent fire hazard according to direction for each country. Therefore, I doubt the usefulness of mains fuses for the protection of amps. HT fuses are somewhat better protection for the transformer secondaries but also unpredictable and therefore not really that good protection. Some kind of (magnetic) circuit breaker would be much better than fuse, in my opinion.
But, in the diy context building soft start circuit just to be able to choose close rated fuse is not very practical. So much complexity just to suit an fuse, component that is unreliable in the first place. You need space to put such circuit, you have problem to place additional conductors, etc. Is not it much simpler to choose slow blow fuse of sufficient rating? Think for the moment about vacuum cleaners. How big is inrush that 1500W motor produces?
It seems that mains fuse is there not to protect the amp itself but to prevent fire hazard according to direction for each country. Therefore, I doubt the usefulness of mains fuses for the protection of amps. HT fuses are somewhat better protection for the transformer secondaries but also unpredictable and therefore not really that good protection. Some kind of (magnetic) circuit breaker would be much better than fuse, in my opinion.
But, in the diy context building soft start circuit just to be able to choose close rated fuse is not very practical. So much complexity just to suit an fuse, component that is unreliable in the first place. You need space to put such circuit, you have problem to place additional conductors, etc. Is not it much simpler to choose slow blow fuse of sufficient rating? Think for the moment about vacuum cleaners. How big is inrush that 1500W motor produces?
Just want to add 'a something' to the above posts (talking 'bout fuses) that I've come across over the years -
Firstly, as Andrew has mentioned above, they're not the most accurate item regarding designed power failure/burn tolerances and as most of the commercially available ones are built for ac power use, they're designed for mains 110, 120, 220 Hz use, not dc rails that we're looking at here.
Then, they do NOT age gracefully, without failing - a generally accepted procedure by most tech's with a unit in for repair is to initially replace the fuses. Also, in a number of amps and preamps, when replacing the mains fuse, even tho it's not blown, sometimes produces an improvement in the sound - not a significant 'thing' like those expensive fuses, for example, but more of a 'good procedure' (I've not noticed any improvement in any equipment with those expensive fuses, but assume it's my lack, as other people seem to have good results).
Low power Circuit Breakers seem to have similar characteristics as fuses, IMO - the electronic Fet switches set up as over current limiters are about the best way of doing things but they're not simple things, (or cheap either), and there are plenty of threads here over the years to look them up.
In Peter/Miles design here, the fuses are after the main filtering caps so don't have to suffer the high current charging pulses and it also acts as a soft start for the following circuits but it's still good practice to use an initial current limiting 'soft start' cct for the primary windings, particularly with the Torroidal transformers.
I generally use the simple Thermistor 10 Ohm ones in series with the primary windings for power amps and the 40 - 100 Ohm ones for lower power ccts, plus the usual input line filters, dc traps, snubbers, etc (this is just my particular way of doing things, not any 'rule' or anything!)
... my 2 cents, to a particularly useful project - my thanks to Peter, Miles and everyone.
Firstly, as Andrew has mentioned above, they're not the most accurate item regarding designed power failure/burn tolerances and as most of the commercially available ones are built for ac power use, they're designed for mains 110, 120, 220 Hz use, not dc rails that we're looking at here.
Then, they do NOT age gracefully, without failing - a generally accepted procedure by most tech's with a unit in for repair is to initially replace the fuses. Also, in a number of amps and preamps, when replacing the mains fuse, even tho it's not blown, sometimes produces an improvement in the sound - not a significant 'thing' like those expensive fuses, for example, but more of a 'good procedure' (I've not noticed any improvement in any equipment with those expensive fuses, but assume it's my lack, as other people seem to have good results).
Low power Circuit Breakers seem to have similar characteristics as fuses, IMO - the electronic Fet switches set up as over current limiters are about the best way of doing things but they're not simple things, (or cheap either), and there are plenty of threads here over the years to look them up.
In Peter/Miles design here, the fuses are after the main filtering caps so don't have to suffer the high current charging pulses and it also acts as a soft start for the following circuits but it's still good practice to use an initial current limiting 'soft start' cct for the primary windings, particularly with the Torroidal transformers.
I generally use the simple Thermistor 10 Ohm ones in series with the primary windings for power amps and the 40 - 100 Ohm ones for lower power ccts, plus the usual input line filters, dc traps, snubbers, etc (this is just my particular way of doing things, not any 'rule' or anything!)
... my 2 cents, to a particularly useful project - my thanks to Peter, Miles and everyone.
Oh, forgot to mention this piece of essential 'feel good' circuitry, even tho it's a bit off the subject -
It's called a dc protection cct - even with CMx in power supplies, sometimes things 'go wrong' and that 'dreaded dc' appears on the speaker terminals with unpleasant results - I only leave these simple protection devices out with single rail amps that have a dc blocking capacitor (hey, they work for me quite well!) or the electronic current sense/limiting circuits on the power rails to the o/p stages.
It's called a dc protection cct - even with CMx in power supplies, sometimes things 'go wrong' and that 'dreaded dc' appears on the speaker terminals with unpleasant results - I only leave these simple protection devices out with single rail amps that have a dc blocking capacitor (hey, they work for me quite well!) or the electronic current sense/limiting circuits on the power rails to the o/p stages.
Ivan,
you barking up the wrong tree.
It seems you are not reading what I have said.
I never said anywhere, nor at any time, that fuses prevent failures.
Fuses are there to help prevent you setting fire to your house.
They are a Safety feature and as such act "after an incident".
AndrewT,
I admit that I misunderstood your post. Now it's clear what we are talking about. We certainly need some kind of protection against fire hazard but I am not sure what is the best method. I think that we need a separate thread about safety in audio amps, including user safety, not only home safety. There was some comments by Ian Finch regarding the use of open SMPS modules recommended by Lazy Cat. It is my impression that user safety is somehow neglected aspect of general safety topic.
Is there an official safety directive for audio products available on the net?
I admit that I misunderstood your post. Now it's clear what we are talking about. We certainly need some kind of protection against fire hazard but I am not sure what is the best method. I think that we need a separate thread about safety in audio amps, including user safety, not only home safety. There was some comments by Ian Finch regarding the use of open SMPS modules recommended by Lazy Cat. It is my impression that user safety is somehow neglected aspect of general safety topic.
Is there an official safety directive for audio products available on the net?
- Status
- Not open for further replies.