Soft Start and Preventing Relay Arcing - diyAudio
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Old 31st May 2005, 05:04 PM   #1
lgreen is offline lgreen  United States
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Default Soft Start and Preventing Relay Arcing

Here is a question on implementing a relay based soft start and trying to avoid arcs within the relay. I'm switching the AC voltage coming out of the wall, so want to do this correctly. I've got a delay circuit and will implement the soft start as shown on the attached schematic....I think, so I wanted everyone's input.

Here is the schematic:

Click the image to open in full size.

Its intended to work like this-

1. When the amp is turned on the relay is connected as shown with a resistive soft start element (resistor or thermister or combo of both) in line with one primary of the transformer to prevent a surge.

2. After a time delay, S1 is activated and the 12VDC circuit provides 12VDC to the relay, causing the relay to switch, which connects the primary of the transformer directly to the AC inlet, bypassing the resistive element.

I'd like to prevent arcs within the relay when this switching happens. I heard you all use capacitors for this, which is why I'm thinking of using C2 and C3. C2 would prevent the initial arc, and C3 would prevent an arc when the amp is turned off and the relay returns to its initial state....I think.

D1 is to prevent voltage from relay actuation from interfering with the control circuit. I'm not sure what C1 is for, but I've seen people use a cap here.

3. My question is -- do I have this right? Are D1, C1, C2 and C3 necessary, will this setup prevent relay arcing? Are the C values correct? If not, what will?
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Old 1st June 2005, 02:30 PM   #2
lgreen is offline lgreen  United States
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Default bump

anyone?
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Old 1st June 2005, 02:43 PM   #3
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If you eliminates C2 and C3, reduce C1 to 100nF and put a NTC ( negative Term. coef. Resistor) in place of C2 have not arcs and works all.

Ciao

Mauro
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Old 1st June 2005, 03:25 PM   #4
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C1 is not needed. You need to change your relay switching so that the resistor is shorteD out or not, not two different lines are switched. Youy way you will have a momentary interruption in power as the relay switches, and the consequent arcs.
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Old 1st June 2005, 04:18 PM   #5
lgreen is offline lgreen  United States
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Quote:
You need to change your relay switching so that the resistor is shorted out or not, not two different lines are switched. Youy way you will have a momentary interruption in power as the relay switches, and the consequent arcs.
OK I think i get what you are saying. You are saying that the relay should just short out the resistor or thermister. What about this:

Click the image to open in full size.

The schematic shows the initial condition. After a time delay the switch will flip causing the relay to change states. Thus, when the time delay is over, the relay simply shorts out the resistor or thermister, leaving it in the circuit. Is this design better? Is C1 required or harmful (what value should it be?)
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Old 1st June 2005, 04:37 PM   #6
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Default Re: bump

Quote:
Originally posted by lgreen
anyone?
You should first ask yourself: Is this a problem?

My answer is:
A normal 8 A/250 AVC relay will last more 15 years if you switch on the amp once a day.
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Old 1st June 2005, 04:45 PM   #7
lgreen is offline lgreen  United States
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Quote:
You should first ask yourself: Is this a problem?
One of the reasons I take the trouble to do DIY is to interact with everyone and try to learn concepts that seem interesting, such as this one. I don't care if it lasts 20 years, I'd still be interested to learn about how to construct a soft start that doesn't arc.

Besides, I can't tell if you are indicating that the 1st or 2nd one would last 15 years, as its not stated.
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Old 1st June 2005, 05:13 PM   #8
SY is offline SY  United States
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Why not just use a zero crossing relay and avoid the problem in the first place?
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Old 1st June 2005, 05:19 PM   #9
lgreen is offline lgreen  United States
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Default because

Quote:
Why not just use a zero crossing relay and avoid the problem in the first place?
ahhh....because I never heard of it?
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Old 1st June 2005, 05:20 PM   #10
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The circuit is now spot on. Just reduce C1 to a few tens of nF so that you can use a cap better suited to snubbing. It actually wouldn't matter if you left it out as you won't be totally breaking any large currents, but it won't hurt to leave it in.
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