When to use softstart

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Hey all.

I'm planning on building an F3 power amp, and am considering going dual mono (why not !!!).

Anyways, I know that when your transformer requirements are beyond 300VA, a softstart is recommended, but is that per transformer, or overall ?

I'm considering 2 250VA transformers (with 66000uF after each of them). In this scenario, since the inrush current is split over two transformers, I'm not sure if the softstart is required.
 
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lordvader said:
Hey all.

I'm planning on building an F3 power amp, and am considering going dual mono (why not !!!).

Anyways, I know that when your transformer requirements are beyond 300VA, a softstart is recommended, but is that per transformer, or overall ?

I'm considering 2 250VA transformers (with 66000uF after each of them). In this scenario, since the inrush current is split over two transformers, I'm not sure if the softstart is required.


it's not just VA rating of xformer , but also No. of kilouF you have after ;

in any case - use CL60 as Papa does and enjoy
 
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Zen Mod said:



:rofl:


that's why you don't need soft start .......

you can also put there smith nail instead

:clown:


Sure, but the 10A fuses were cheaper than nails of the same size :D

Well, the fuse is there to stop things from turning real bad in case the trafo goes south. On the other side of the PSU, the fets will blow, take out the source resistors, and put an end to any further events.

Magura :)


EDIT: Just checked, the fuse is 8A slow blow, not that it makes any particular difference.
 
A softstart circuit not only prevents the fuses from blowing, it is also going easier on the expensive caps... Although I have no proof, I would bet that it lengthens the useful live of the PSU components.

A softstart circuit can be as simple as a thermistor, or, if you don't like the idea, a power resistor and relay (driven from the secondary side of the transformer) shorting it some ms later.
 
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Magura said:



Slow blow 13A ceramic fuses.


Magura :)


Ahh this is stone age... Try modern automatic circuit breakers.

Doesn't the lights dimm and the computers reset when hard starting this 200.000uF power supply? I bet that if you move the amplifier some day to a place with a slightly weak mains line, this is going to happen.

A smart amplifier intended to work everywhere should include soft start.
 
Re: Re: Re: Re: Re: Re: Re: Re: When to use softstart

Eva said:



Ahh this is stone age... Try modern automatic circuit breakers.

Doesn't the lights dimm and the computers reset when hard starting this 200.000uF power supply? I bet that if you move the amplifier some day to a place with a slightly weak mains line, this is going to happen.

A smart amplifier intended to work everywhere should include soft start.

Yeah, you're right as far as the stoneage fuses, but they happened to be there when I bought the house.

In Denmark we have very nice mains, hardly any unwanted stuff and hardly any fluctation, compared to most other countries I know of. So as long as I stay here in DK, this should cause no problems.


Magura :)
 
Reminds me of the days when I took my DIY PA system to parties... At one place the circuit breaker would consistently trip when powering on the last of the six amplifiers. We had to "simulate" soft start by manually resetting the breaker 10 or 20 times within some seconds - until the capacitors had enough charge to not trip it again.

Lesson learned: Made a small soft-start PCB, bought twenty sets of relays and power resistors and installed one in every amp I came across ;)
 
Hi,
your 250VA operating on 220/230Vac mains will run all day at full power from a 1A fuse.
But as with all transformers and motors it needs a much larger, or slower, fuse to start up. It is usual to fit a fuse rating of about three times the full power current rating.
This would require a 3A fuse for each transformer.

If you fit a soft start you can reduce the fuse back down to or maybe even less than the full power rating and still operate the amplifier without nuisance blowing of the mains fuse.

Remember, the fuse is there to stop the downstream cables and equipment setting the house on fire if a serious fault causes excessive heating.
A close rated fuse provides better/safer protection than taking the easy (big fuse) route.
 
peranders said:
Magura, I'll guess you have a robust mains switch also? You must have inrush currents of 100 A or more.


Reality is something like 30A inrush, so no particular issues there.

I though have wondered in the past if such could be a problem, so I opened a mains switch that had been in service for a few month. No harm what so ever to the contacts.


Magura :)
 
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