It's time to finally get moving on another project.
I have a 750VA piltron that needs to make some beautiful (loud) music.
This will be my highest power project yet, and first off I need a soft/delay start.
I plan on using something simple like http://mitglied.lycos.de/Promitheus/delay_circuit_for_toroids.htm
But I have 2 worries/questions.
1) To me it looks like this circuit needs 2 legs of AC, will the 120 that we have around here do the trick? (With neutral going into the bottom of B1)
2) The 4 10ohm, 5W resistors, this will give 40ohms resistance, and at 120V this is 3A. A 750VA transformer can draw greater then 6A, is this the intent, to only allow about 1/2 of the needed power to get through? Or do I need to increase this rating to give my amp more power to be able to start up (some of this is what has me confused, specifically "To determine the power rating for the ballast resistor, which is 125% of the transformer power rating at full power ..."
Also why 4 resistors? I could see if they were parallel to help spread dissipation, but in a case like this wouldn’t a single 40ohm work just fine?
I have a 750VA piltron that needs to make some beautiful (loud) music.
This will be my highest power project yet, and first off I need a soft/delay start.
I plan on using something simple like http://mitglied.lycos.de/Promitheus/delay_circuit_for_toroids.htm
But I have 2 worries/questions.
1) To me it looks like this circuit needs 2 legs of AC, will the 120 that we have around here do the trick? (With neutral going into the bottom of B1)
2) The 4 10ohm, 5W resistors, this will give 40ohms resistance, and at 120V this is 3A. A 750VA transformer can draw greater then 6A, is this the intent, to only allow about 1/2 of the needed power to get through? Or do I need to increase this rating to give my amp more power to be able to start up (some of this is what has me confused, specifically "To determine the power rating for the ballast resistor, which is 125% of the transformer power rating at full power ..."
Also why 4 resistors? I could see if they were parallel to help spread dissipation, but in a case like this wouldn’t a single 40ohm work just fine?
I'm interested in this circuit as well. It seems to me that:
1. yes, it would be fine I think, but you could reduce c2 and 3 or r1/r2 to get similar timing.
2. I see your point. At 120v it would seem to be best at 20 ohms, not 40 ohms for the same rate of inrush.
Also, isn't using 4 resistors is series spreading dissipation as 4 resistor bodies are being used somewhat equally? If in parallel and not perfectly matched, wouldn't one current hog?
My question...
Why not put, say, 24v or so of zener just before the relay coil? Wouldn't this more decisively and consistently fire the relay closed?
please flame me if I'm wrong on any of this.
Marc
1. yes, it would be fine I think, but you could reduce c2 and 3 or r1/r2 to get similar timing.
2. I see your point. At 120v it would seem to be best at 20 ohms, not 40 ohms for the same rate of inrush.
Also, isn't using 4 resistors is series spreading dissipation as 4 resistor bodies are being used somewhat equally? If in parallel and not perfectly matched, wouldn't one current hog?
My question...
Why not put, say, 24v or so of zener just before the relay coil? Wouldn't this more decisively and consistently fire the relay closed?
please flame me if I'm wrong on any of this.
Marc
What happens if the 24VDC relay sticks "open?" Won't the voltage supplied to it continue to rise beyond 24VDC when C2 and C3 become charged?
If the relay sticks "open" what will be the DC voltage supplied by B1, assuming a 120 VAC house supply? Won't it eventually rise to 168VDC (1.4 X 120VAC)?
Am I wrong on this? Or, do R1,2,3 and R4,5,6,7 function as a voltage divider circuit?
If the relay sticks "open" what will be the DC voltage supplied by B1, assuming a 120 VAC house supply? Won't it eventually rise to 168VDC (1.4 X 120VAC)?
Am I wrong on this? Or, do R1,2,3 and R4,5,6,7 function as a voltage divider circuit?
Dick,
I see your point. However, I think voltage is limited at the coil by the resistor 'ladder' created by R1,2,3 and the relay coil. The resistance of the coil will be unchanged whether the relay sticks or not. R4,5,6,7 arent really a part of the circuit.
I think the voltage would spike if the relay coil went open..even then, no harm done.
Please notice I say "I think" a lot here
Edit: I have a feeling that large variations in the resistance of different brands/styles of relay coils could cause a problem..I THINK.
The more I look at this circuit, the more I like it. Thanks for bringing it up!
I see your point. However, I think voltage is limited at the coil by the resistor 'ladder' created by R1,2,3 and the relay coil. The resistance of the coil will be unchanged whether the relay sticks or not. R4,5,6,7 arent really a part of the circuit.
I think the voltage would spike if the relay coil went open..even then, no harm done.
Please notice I say "I think" a lot here
Edit: I have a feeling that large variations in the resistance of different brands/styles of relay coils could cause a problem..I THINK.
The more I look at this circuit, the more I like it. Thanks for bringing it up!
After a little more reading on the basics of resistors yes I’ve found that resistors can be beefed up by using them in parallel or in series.mpmarino said:
If the relay sticks, then I think you will have some very hot resistors on your hands.
But the relay sticking should have no effect on B1 as the electro magnet will still be drawing power and trying to close the circuit.
As to the voltage potential of B1 I wish I could tell you. I know that there is some voltage dropping happening, but I can’t seem to get this to work in LTSpice.
An elegant solution
Look at what Coulomb has done with Maxhawk's soft switch, found here:
http://www.diyaudio.com/forums/showthread.php?postid=624330#post624330
Pretty neat!
Look at what Coulomb has done with Maxhawk's soft switch, found here:
http://www.diyaudio.com/forums/showthread.php?postid=624330#post624330
Pretty neat!
Hi officeboy,
this topic has been discussed many times before. There is a multitude of alternatives available, please do the search.
You will find discussions about pros and cons of the circuit available at Promitheus' website. Also, circuits by Rod Elliot (recommended) and others, even by many forum members (e.g. peranders, available as PCB if I'm not mistaken).
Sebastian.
this topic has been discussed many times before. There is a multitude of alternatives available, please do the search.
You will find discussions about pros and cons of the circuit available at Promitheus' website. Also, circuits by Rod Elliot (recommended) and others, even by many forum members (e.g. peranders, available as PCB if I'm not mistaken).
Sebastian.
sek said:
There are a few threads here about soft starts, but none i could find with these specifics. My electronics knowledge is quite simple, and i like to learn as i go along. So i would not want to build this till i could as least see why things are doing what they do.
There are many better documented circuits available, but the cost is a bit higher, and my budget is about $5 (but I have lots of old broken electronic things to rob parts from)
Page 6 of this PDF has basically the same circuit, but with a little more discussion.
http://www.edn.com/contents/images/30702di.pdf
http://www.edn.com/contents/images/30702di.pdf
The simplest thing would obvious be the thermistor trick as you all know. When they heat up, the resistance falls and you have an instant soft start. But then you have these bloody hot things stuck in your cirquit and you don't want that. You conceive a brilliant plan to use resistors you switch on and off, basically stepping the input current.
How about you replace the resistors in your smart soft-start circuit with thermistors and get the best of both? At switch-on the thermistors will have about 10 ohms resistance each, dropping as the current increases. You measure the time delay it takes to get to full current, and set this delay in your soft-start relay. The relay shorts the thermistors, taking over the current softly, as the thermistors cool down again.
It's what I plan to do for my Aleph 2 1KVA trannies. I haven't implemented it yet.
Cheers,
Bouke
How about you replace the resistors in your smart soft-start circuit with thermistors and get the best of both? At switch-on the thermistors will have about 10 ohms resistance each, dropping as the current increases. You measure the time delay it takes to get to full current, and set this delay in your soft-start relay. The relay shorts the thermistors, taking over the current softly, as the thermistors cool down again.
It's what I plan to do for my Aleph 2 1KVA trannies. I haven't implemented it yet.
Cheers,
Bouke
The thermistor as a replacement for the power resistor is a good idea, and it's nice to have them out of the power feed for later when the amp stabilizes and maybe uses very little power playing something quiet, you don’t want that power protection to kick back in again.
I just wish I could find a little more info about these transformerless power supplies and how they work. If the power goes through the capacitor then what is the point of the resistor's in parallel with it.
And if increasing the cap size increases the amount of power (I) available, what is decreasing voltage? Or will it just keep rising if there is no load?
I just wish I could find a little more info about these transformerless power supplies and how they work. If the power goes through the capacitor then what is the point of the resistor's in parallel with it.
And if increasing the cap size increases the amount of power (I) available, what is decreasing voltage? Or will it just keep rising if there is no load?
A comment about Promitheus. Interesting to copy a schematic from Elektor and then put his name on it with copyrightofficeboy said:
unless he actually did this design.Anyway, don't forget why you will need a softstart. My main reason was to avoid fuse popping, nothing else.
With 50 W resistors you have a massive overkill. I have used 100 ohms/5 W (=max 3 A) wirewound with 0.4 sec. together with 600 VA/66000 uF. This have worked for 16 years with one switch-on each day in average. The only part which has gone broken was a series resistor for the relay drive. This is why I have a couple of resistors in series now. Back then when I designed it I didn't think of max transient capability of the 0.6 W metal film resistor.
So with my principle you will have a reliable softstart which is proven by age.
Re: Re: Toroid soft start
Hi,
I've had this question raised to him before, that's why I remembered the topic and the schematic. He didn't respond!
Yep, that's what I also meant. No reason not to do it in a proven way.
Cheers,
Sebastian.
Hi,
peranders said:A comment about Promitheus. Interesting to copy a schematic from Elektor and then put his name on it with copyright
I've had this question raised to him before, that's why I remembered the topic and the schematic. He didn't respond!
AndrewT said:
Yep, that's what I also meant. No reason not to do it in a proven way.
Cheers,
Sebastian.
AndrewT said:
Nor does he need to advertise. I've been there and the schematic for his soft start is in my pile of 4-5 designs to try and distill down to something i can understand.
But I don't like to just use something that someone else did.
It's hardly DIY then, more like paint by color.
As for the Elektor softstart, I know it's from there, and it's also copied on a lot of other websites too, But i picked the english one.
Current flows only for a couple of seconds so I doubt precion is very important. In any case the basic concept exists in many variations (*** www.ampslab.com to the list iof you want a kit, and R.Slone as well). Ampslab avoides you concern by using a single 20W resistor.
The potentially limiting aspect of using an in-rush termistor is that they have to cool down after each start cycle before they will function correctly. This mean that if you switch the amp on and then immediately off again, you have to wait before switching it back on or the current limiting won't ocurr. It's probably not a long wait, but it is something you should check out to avoid a surprise.
Hi,
for the short OFF then ON reason, I am considering series connection of half resistor & thermistor.
My power supply reliability is poor, I get many irregular short term power outages and a few longer term each year.
I am looking for ideas on LATCHING OFF when power fails as a foolproof alternative.
for the short OFF then ON reason, I am considering series connection of half resistor & thermistor.
My power supply reliability is poor, I get many irregular short term power outages and a few longer term each year.
I am looking for ideas on LATCHING OFF when power fails as a foolproof alternative.
I still don't understand all the need for discussion of various strategies for a soft start to protect ON/OFF switches, diode bridges, and handle inrush current to large toroids. Also, the use of a thermistor is not ideal. Plus I agree with AndrewT that quick latch off cycling and repeated time delay is imortant. See this:
http://www.diyaudio.com/forums/showthread.php?postid=608390#post608390
The AC time delay relays are simple to implement and not that expensive. I got mine from an eBay auction.
What am I missing?
http://www.diyaudio.com/forums/showthread.php?postid=608390#post608390
The AC time delay relays are simple to implement and not that expensive. I got mine from an eBay auction.
What am I missing?
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