The most elegant and uncomplicated inrush current "soft start" is to use an Amatherm NTC product.
Inrush Current Limiters - Thermistors | Ametherm
Inrush Current Limiters - Thermistors | Ametherm
Thanks a lot Mooly!
I found this one:
MCMET 4A 250V - MULTICOMP - FUSE, RADIAL, SLOW BLOW, 4A | Farnell UK
is this kind of fuse what you mean?
Cheers.
I found this one:
MCMET 4A 250V - MULTICOMP - FUSE, RADIAL, SLOW BLOW, 4A | Farnell UK
is this kind of fuse what you mean?
Cheers.
That's a "Wickman" style fuse designed to be soldered into a PCB. I was thinking more traditional 20mm glass fuse and fuse holder.
0213004.TXP - LITTELFUSE - FUSE, ANTISURGE, 4A | CPC From This Range
More like this. (CPC is part of Farnell. Free delivery on orders over £5 I think)
0213004.TXP - LITTELFUSE - FUSE, ANTISURGE, 4A | CPC From This Range
More like this. (CPC is part of Farnell. Free delivery on orders over £5 I think)
A few hundred milliseconds doesn't deplete the tank circuit's charge enough to produce the same need required from cold start, (which is why you want inrush protection) when load to the transformer is at it's greatest. In fact, low voltage (brown out condition) can be more of a problem.
During my time at Altec-Lansing, prior to Mark IV Industries selling off the professional line, both Altec-Lansing and Electro-Voice amps were produced here in OKC, even in the multi-KW variants, this proved to be the best, most elegant and protective method, even over stepped-start.
Have a grerat day...
I can believe it with brown outs. In the UK we have (generally... I know some outlying areas can suffer more) very stable mains but other markets are not so fortunate. I like any design to be able to cope with pretty much anything, even someone going on.. off... on/off/on/off rapidly etc. It has to cope.
I am not an expert but for calculating a transformer fuse it should be VA/Primary Voltage which is in my case 300VA/230Volts equal 1.30 Amps. Is that correct? so the suitable fuse should be this one:
021301.6MXP - LITTELFUSE - FUSE, ANTISURGE, 1.6A | CPC
Because a 4Amp was the option with a conventional fuse to avoid it blowing on the start but I assume that the time delay fuse will hold the Inrush...
Cheers.
The 1.6A would cover actual running of the transformer but wouldn't cope with the inrush. The inrush current "varies" depending at what instant the mains switch closes with regard to where the incoming AC voltage actually is in its cycle. Non intuitively, the worst case scenario can be applying power when the mains voltage is at a zero crossing. Another variable is the point in the cycle you remove power because that precise point influences how much resisdual magnetism is left in the core. That residual magnetism becomes another factor when power is next applied.
So the 1.6 time delay fuse wouldn't hold out for long enough when feeding a 300va toroidal transformer. The inrush would last for longer than the fuse delay time.
So the 1.6 time delay fuse wouldn't hold out for long enough when feeding a 300va toroidal transformer. The inrush would last for longer than the fuse delay time.
you have confused the soft start (for the transformer) with slow charge (for the capacitors and the rectifier).
The transformer flux drops to remnant value almost as soon as power fails.
A restart after this very short period will invoke the transformer start up current. A thermistor that has been bypassed to allow it to cool and reduce the supply impedance will be ready to work again as a soft start device.
a motor or transformer fuse can be close rated or not close rated.
Close rated fuse is VA/Vac
ordinary fuse without soft start is 3*VA/Vac
a 300VA will run a 100W+100W stereo ClassAB amplifier all day every day with a T1.25A fuse and probably on a T1A as an alternative
Without a soft start, the Fuse should be around T4A.
This will allow the amplifier to start up repeatedly over many years.
Close rated fuse is VA/Vac
ordinary fuse without soft start is 3*VA/Vac
a 300VA will run a 100W+100W stereo ClassAB amplifier all day every day with a T1.25A fuse and probably on a T1A as an alternative
Without a soft start, the Fuse should be around T4A.
This will allow the amplifier to start up repeatedly over many years.
Wow! Thanks a Lot!! Mooly and AndrewT,
I've learnt why now!
. I put there a high inrush switch (like this one 1835.3112 - MARQUARDT - SWITCH, OFF, 2 CONTACT | Farnell UK) and a 4Amp time delay fuse and that will be it!
Again, thanks a lot guys!!
Cheers.
I've learnt why now!
. I put there a high inrush switch (like this one 1835.3112 - MARQUARDT - SWITCH, OFF, 2 CONTACT | Farnell UK) and a 4Amp time delay fuse and that will be it!Again, thanks a lot guys!!
Cheers.
How fast/slow will the T4A blow when you overload the transformer?
try looking up the fuse tables/plots issued by your fuse manufacturer.
How long is a T4A guaranteed to NOT blow when passing 8Arms? (quite a short period)
How long is a T4A guaranteed to blow when passing 8Arms? (quite a long period)
These two times are usually quite different, sometimes by a factor of 10.
How much heat is being generated in the faulty/overloaded equipment during the time that the T4A fuse has not yet blown?
Once you have these numbers, take note of how quickly a fuse blows when very slightly loaded (say 1.5times it's rating) and compare to how long it takes to blow when passing many times it's rated current, (try 5 times Irated and 20times Irated).
Once you have frightened yourself with the numbers, you might realise why we use close rated fuses.
try looking up the fuse tables/plots issued by your fuse manufacturer.
How long is a T4A guaranteed to NOT blow when passing 8Arms? (quite a short period)
How long is a T4A guaranteed to blow when passing 8Arms? (quite a long period)
These two times are usually quite different, sometimes by a factor of 10.
How much heat is being generated in the faulty/overloaded equipment during the time that the T4A fuse has not yet blown?
Once you have these numbers, take note of how quickly a fuse blows when very slightly loaded (say 1.5times it's rating) and compare to how long it takes to blow when passing many times it's rated current, (try 5 times Irated and 20times Irated).
Once you have frightened yourself with the numbers, you might realise why we use close rated fuses.
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There is no conflict because you should be fusing the secondaries as well, and quite possibly each power amp fed from the rails.
You can't expect a single primary fuse to cover all failure modes and provide the required protection for each. Hard and fast rules are hard to come by, I'll be the first to admit that, although in the end good design tempered with a little experience goes a long way to providing a satisfactory solution that works reliably year in, year out.
A diy amp that just pops a fuse once every couple of years may not be such a tragedy, if it was a commercial amp it would be a disaster. That PSU not just has to withstand normal use/customer abuse but also such things as the wonky double adapter at the end of an extension lead plugged into an equally wonky adapter into the wall. Its all typical of many users out in the field.
Well, I am not expecting a simple solution, just a solution.
I have no wanky adaptors double or single and this PSU is not for an Amplifier, it will feed several Modules of a Modular Synth so this is a regulated PSU.
I have no problem if I have to place fuse in every connector feeding every single module, also in primary and secondary and on top of that adding a soft start, the troble here is that I don't know if all of that will work or will be just a pain more than a soultion, do you understand what I am trying to say? If all of that will work just fine, no problem, I'll do it.
Thanks a lot for your detailed explanation anyway
.Cheers.
Oh nooo! should I post my concerns in confuse dot com?
soft start, no soft start, resistors, thermistors.... seems like there is no solution at all! it is just about try something and cross your fingers, isn't it??
I guess I'll turn on my PSU in 2024...
Nope
Fuse the primary so you don't get nuisance blowing. Fuse each secondary so that a drastic failure such as a reservoir cap going short or a diode in the bridge failing will annihilate the secondary fuse but have no problem with the cap charging currents in normal use. In practice that means a fairly large fuse again, perhaps a 4 or 6.3 amp (slow blow) depending how much capacitance you have and the va rating of the tranny. The fuses to the amplifier realistically only protect against failure of the output transistors (or chip if a chip amp) so should be scaled to suit. Probably in the 2 to 4 amp range for a typical domestic amp, or if you are testing at full power perhaps a little higher. No fuse will save a transistor, they just ain't quick enough. The fuse just keeps it all safe.
Well, I am not expecting a simple solution, just a solution.
I have no wanky adaptors double or single and this PSU is not for an Amplifier, it will feed several Modules of a Modular Synth so this is a regulated PSU.
I have no problem if I have to place fuse in every connector feeding every single module, also in primary and secondary and on top of that adding a soft start, the troble here is that I don't know if all of that will work or will be just a pain more than a soultion, do you understand what I am trying to say? If all of that will work just fine, no problem, I'll do it.
Thanks a lot for your detailed explanation anyway
Cheers.
Honestly... a 300 va transformer (was it 300 va you mentioned ?) doesn't really need a soft start (unless you want the satisfaction of implementing one as a technical refinement).
i get my NTC parts for free from discarded ATX psu's, that round green thing, sometimes black disk....in series with the fuse, these are 4 to 12 ohms cold, but once hot, drops insignificant ac voltage, i never even bother to short them out...
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