| Alcaid |
Can anyone help me with information on how to build a softstart circuit? I'm going to use a trafo that's 400VA (or maybe 600VA) and was adviced that a softstart circuit was needed.
I've searched the forum and haven't found anything yet. |
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| Pedja |
| quote: | Originally posted by Alcaid
I'm going to use a trafo that's 400VA (or maybe 600VA) and was adviced that a softstart circuit was needed. |
Needed for what?
Pedja |
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| li_gangyi |
| To limit the inrush current I guess... |
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| Pedja |
Yes, that’s the purpose of the soft start, but what is the problem to be solved?
Pedja |
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| li_gangyi |
that hguy needs a softstart circuit for his trafo...I suppose it would be a few limiting resisotrs and a timed relay to kick in at a determined time to give full power....with the curent flowing through the resistors for a few seconds before the whole full power is applied...check out this site
http://sound.westhost.com/project39.htm
Should be good for him... |
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| Pedja |
Li_gangyi, I am also trying to help. I was interested to hear what he considers as a reason to use it. Under normal circumstances he'll not need any soft start for 400W transformer (I know this 'cause I am using it), so it will be better for him not to use anything like that. We can, of course, simply exchange the schematics here, but that is not the most effective way to deal with audio. :o
Pedja ;) |
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| li_gangyi |
| Yeah...I was also wondering why he would need it...just use a slow blow fuse in the range of 1-2A...shou;ld be more than adequate for him.... |
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| Alcaid |
Sorry for the late reply (I've had an exam today).
So i don't need a softstart circuit for a 400VA toroid and two 22.000uF caps?
Whats the limit before you need one? |
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| li_gangyi |
| once you have something over ha;f a kilowatt then things begin to look serious...may I ask why you need such a big smoothing cap?? 10 000uF would be considered too much here for me...hehe |
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| peranders |
| quote: | Originally posted by Alcaid
Whats the limit before you need one? |
When the fuses blows too often!
Toroids over 500 VA needs soft start if the fuse is 10 A fast /230 volts. My 2 x 300VA PS has 77 A inruch current. 6 A slow blows one time of 10. Where I used to live had only 6 A max in the outlets.
My solution (one of them) is to use a 230 V 11-pin relay + integrated delay-on timer + 100 ohms Dale resistor without cooling (5-7 watts I'll guess). My second solution is my own design, very good but needs a bit polishing before publishing I'll suppose. Both circuits have very short reset time which is important in order to get an good protection (including short glitches on the mains).
The basic idea is to create a 24 V voltage with help from a capacitor (capacitive voltage divider) and then a regular RC-link to a transistor connected to the relay coil.
The delay time needed is only 0.3-1 second only. It's just a couple of periods on the mains.
Maybe I'll update this design if the interest is big enough.
I'll see that Lars Clausen also has a soft start circuit, may I ask what the basic idea is?
Let me also point out that a naked unconnected (at the secondary side) transformer itself creates this huge current. |
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| peranders |
| quote: | Originally posted by Alcaid
Sorry for the late reply (I've had an exam today).
So i don't need a softstart circuit for a 400VA toroid and two 22.000uF caps?
Whats the limit before you need one? |
10 A fast in your wall may take this most of the time but this is just on the edge. Beware of the huge wear of your power switch! |
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| Alcaid |
| I haven't buildt the amp yet, but I just wanted to be on the safe side. Better to ask BEFORE i plug in the amp and blow up the trafo because of the inrush-current. ;) |
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| li_gangyi |
| u wun blow ur trafo even with a big inrush current...provided u did put a fuse.. |
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| Alcaid |
| If the fuse is slow-blow, wouldn't the trafo be able to draw more current than it can handle for a short period (seconds)? |
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| peranders |
If you put it out of safety perspective it's better to have a softstarter and a fuse which isn't chosen for inrush currents.
400 VA, 3.15 AT + softstarter is better than 400 VA , 5 AT (maybe 4 or 3.15 AT) but it's OK to have only a fuse. But the poor mains switch must work harder. |
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| UrSv |
| One can also use smoething else for switch like the Pass A75 thyristor switching method. |
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| li_gangyi |
| I guess some of us might say the SCR might put some noise into the circuit...but I wouldn't really care about it...most of the time with a properly designed circuit..it's above the "normal" audiable frequency range...and will probably not affect the performance what so ever...but for the purist...I guess a battery source would be best for them... |
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