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Old 22nd April 2005, 11:16 PM   #11
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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?
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Old 23rd April 2005, 08:03 AM   #12
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Default Re: Toroid soft start

Quote:
Originally posted by officeboy
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/...or_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?
A comment about Promitheus. Interesting to copy a schematic from Elektor and then put his name on it with copyright 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.
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Old 23rd April 2005, 09:30 AM   #13
AndrewT is offline AndrewT  Scotland
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Hi,
Peranders cannot advertise his own wares/site, but I strongly suggest you go there.
The design and build info is superb with pics & schematics par excellence.
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Old 23rd April 2005, 08:25 PM   #14
sek is offline sek  Germany
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Default Re: Re: Toroid soft start

Hi,

Quote:
Originally posted by peranders
A comment about Promitheus. Interesting to copy a schematic from Elektor and then put his name on it with copyright unless he actually did this design.
I've had this question raised to him before, that's why I remembered the topic and the schematic. He didn't respond!


Quote:
Originally posted by AndrewT
Peranders cannot advertise his own wares/site, but I strongly suggest you go there.
Yep, that's what I also meant. No reason not to do it in a proven way.

Cheers,
Sebastian.
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Old 23rd April 2005, 08:33 PM   #15
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Quote:
Originally posted by AndrewT
Hi,
Peranders cannot advertise his own wares/site, but I strongly suggest you go there.
The design and build info is superb with pics & schematics par excellence.
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.
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Old 23rd April 2005, 08:47 PM   #16
sam9 is offline sam9  United States
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Quote:
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?
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.
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Old 24th April 2005, 09:10 AM   #17
AndrewT is offline AndrewT  Scotland
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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.
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Old 24th April 2005, 10:24 AM   #18
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Andrew, check out No Volt Release switches used on machine tools.
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Old 24th April 2005, 02:36 PM   #19
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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/showt...390#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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Old 24th April 2005, 04:31 PM   #20
sam9 is offline sam9  United States
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A useful resource on this topic might be Joeseph Carr's book on "DC power Supplies". My copy went missing some time ago but I think there was coverage of soft start systems and also latcing switch circuits.
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