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Spark suppressing cap

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Spark suppressing cap

Postby Adam Inglis » Fri Feb 15, 2019 6:08 am

I'm repairing a Farfisa Compact organ, circa 1968.
In the PSU, there is a piece of charred goo that used to be a capacitor, btw the mains switch and the fuse. The schematic tells me it was 4.7 nF 630 volts (non polar).
I assume this was a spark suppression cap?
In similar situations I've used a modern "X2" type cap, rated 275 volts. (I don't have anything rated at or near 630 volts!)
I reckon this would be fine - any thoughts?
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Re: Spark suppressing cap

Postby DGL. » Fri Feb 15, 2019 8:12 am

I would have thought that a mains rated class X2 capacitor should be fine, that 630V rating would probably be for DC current and like you thought would be a suppression capacitor, which is what the class X2 capacitor is designed for (primarily the fact that it fails open, as you don't want a capacitor across the mains to fail dead short!).

Jut one bit of advice, if the capacitor that you are using is made by RIFA then probably find a different brand (i.e. Panasonic), when RIFA capacitors fail they fail quite spectacularly.
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Re: Spark suppressing cap

Postby Folderol » Fri Feb 15, 2019 8:58 am

That seems rather small for mains suppression. They are normally about 100nF - the better ones also have a low value series resistor built in. I'm inclined to think it's more an RF suppression one, in which case you would want the higher voltage rating, and probably a ceramic type.

Was the original a flat plate shape, rod or block? That can give a bit of a clue as to what it was.
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Re: Spark suppressing cap

Postby Adam Inglis » Fri Feb 15, 2019 11:56 am

Thanks DGL, the X2 caps I have are labelled "Suntan" (?!!)

Folderol wrote:Was the original a flat plate shape, rod or block? That can give a bit of a clue as to what it was.

It was literally a small pile of ash, sitting in the bottom of the metal PSU enclosure!
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Re: Spark suppressing cap

Postby Folderol » Fri Feb 15, 2019 2:52 pm

Hmmm prolly ceramic then. The other types usually leave more evidence.

You say it was between the switch and the fuse. This is a bit ambiguous. Can you describe exactly where each end of the component goes?
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Re: Spark suppressing cap

Postby Adam Inglis » Sat Feb 16, 2019 12:32 pm

Oh sorry, of course, it goes to ground, from a point btw mains switch and fuse, all on the primary side of the transformer.
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Re: Spark suppressing cap

Postby Hugh Robjohns » Sat Feb 16, 2019 1:10 pm

Sounds like a standard X2 would do the job then. It would appear to be there for simple switch suppression/noise filtering duties.

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Re: Spark suppressing cap

Postby DGL. » Sat Feb 16, 2019 8:26 pm

and so long as the capacitor is AFTER the switch then a bleed resistor should not be required as the device itself should bleed capacitor itself, if it's before the switch then a 1/2w resistor of something like 680k should do, connected across the terminals of the capacitor.
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Re: Spark suppressing cap

Postby Folderol » Sat Feb 16, 2019 10:40 pm

Hmmm. Post got eaten... again :frown:

Sorry to be picky again, but when you say to 'ground' do you mean Neutral? If not, and it's to Earth then it should be one of a pattern of three L>N, L>E, N>E.
No need for any bleed resistors regardless, at that capacitance, even if it was holding the peak of 300V you'd barely feel a discharge.

Personally, I'm still not convinced it's a spark snubber. The value is too small, and these are usually put across the source of the spark - which in this case would be the switch contacts themselves.
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Re: Spark suppressing cap

Postby Adam Inglis » Sun Feb 17, 2019 1:23 am

Thanks all

Folderol wrote:Sorry to be picky again, but when you say to 'ground' do you mean Neutral? If not, and it's to Earth then it should be one of a pattern of three L>N, L>E, N>E.

Will, your "pickiness" is always appreciated in these circumstances.

Here's the schematic:
http://www.adambaby.com/tempdownloadfil ... mboPSU.pdf
See top left.
So it appears to be chassis ground.
I'll once again display my ignorance: what's the difference, effectively, between those scenarios?
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Re: Spark suppressing cap

Postby Folderol » Sun Feb 17, 2019 2:23 am

Well, with that wiring, it certainly has nothing to do with spark suppression! But then I suspected that from it's value. I'm not even sure how much RF filtering it can do, as it doesn't stop any crap coming in on the neutral.

I would suggest the simplest thing would be to just replace it with a high voltage ceramic type. RS do them (up to about 1kV). I don't much like this, but at least you are as close as possible to the original design.
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Re: Spark suppressing cap

Postby Adam Inglis » Mon Feb 18, 2019 10:45 am

excellent, will do, cheers
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