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Old 22nd Aug 2015, 7:30 pm   #1
akuram1
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Default Suppression Capacitor advice

It is a long time since I had to deal with capacitors so although this is not a TV problem all I require is for someone to give a little advice.
We have a Singer sewing machine which is vintage ( about 1955 ) and the fort control has two 0.04uf capacitors one of which has gone short-circuit and so I need to replace both.
Maplin have 0.047uf Wima Polypropylene Metal foil 1250v capacitors and my simple for you experts is would they be suitable.

Thanks
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Old 22nd Aug 2015, 7:44 pm   #2
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Default Re: Capacitor advice

They will probably be fine, but bog standard X2 caps will be just as good. The value isn't critical. When I had to change the suppressor caps in the foot pedal of my 1934 Singer 201K I just used scavenged X2 caps from the junk box. Any scrap switch mode power supply will contain several.
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Old 22nd Aug 2015, 7:45 pm   #3
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Default Re: Capacitor advice

99 % that they will be ok.
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Old 22nd Aug 2015, 7:51 pm   #4
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Default Re: Capacitor advice

Thank-you very much appreciated. I have been away from the trade since 1972 and lost a lot of simple knowledge in that time although I really enjoy following the threads on this site.
Again many thanks
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Old 22nd Aug 2015, 8:51 pm   #5
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Default Re: Capacitor advice

It's not just about the working voltage - it's about the failure mode of the capacitors when used as suppressors. They should be X Class if they go across the mains, or Y Class if from Line to earth. Metallized film x or Y Class capacitors can repair themselves after a voltage spike - ceramic caps can’t - their failure mode can be a short circuit, with risk of shock in the case of a Y capacitor. With metalized film X & Y Class caps, if breakdown occurs after a voltage spike the thin metallization of metalized film X & Y Class capacitors heats up, melts, and the area around the breakdown is isolated, so a short circuit doesn’t arise.

.047uF X & Y Class caps are widely available on e-bay and from Farnell etc. (Ebay offerings are the same brand as from Farnell).

Hope that helps.
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Old 23rd Aug 2015, 3:30 am   #6
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Default Re: Capacitor advice

Y caps are often ceramic. I'm not sure how they are constructed to fulfill the "fail open" requirement, but apparently they are.

A 1250V polypropylene foil capcitor will mostly exceed the requirements of X2 or Y2, but to be on the safe side it would be best to fit Y2 capacitors (assuming they go from motor houding to brushes or something similar.
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Old 23rd Aug 2015, 7:35 am   #7
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Default Re: Capacitor advice

The "smokin' RIFA" capacitors definitely seemed to contain two capacitor structures. I'm guessing they were in series, so if one failed short the other would still have a safely high impedance. Prob'ly the same with ceramic ones; two layers of ceramic, metallised on both sides, the connection to the middle metallisation not being brought out, so it just looks like a slightly fat ceramic disc capacitor.
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Old 23rd Aug 2015, 10:20 am   #8
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Default Re: Capacitor advice

I was asked to mend one of those singer foot pedals a few years ago, it only had one capacitor connected across the switch. It had also failed short circuit resulting in the motor running continuously, a X2 capacitor of similar value was used to replace it.

I have also seen ceramic capacitors with both X and Y markings.

David
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Old 23rd Aug 2015, 11:25 am   #9
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Default Re: Capacitor advice

I think mine had two in series and I replaced like with like, though a single X2 would have been fine. Nothing was connected to earth - the foot pedal is 2 wire throughout. The machine itself is just floating despite the vast expanse of exposed metal, and does produce a very, very faint tingle.
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Old 23rd Aug 2015, 11:50 am   #10
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Default Re: Capacitor advice

1934 vintage, vast expanse of exposed metal, tingle? Definitely a case for an RCD plug, I think.
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Old 23rd Aug 2015, 12:08 pm   #11
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Default Re: Capacitor advice

Or an earth connexion.
 
Old 23rd Aug 2015, 1:12 pm   #12
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Default Re: Capacitor advice

No part of my body is earthed when I use it, so if the worst happens I'll just get a nasty belt. An RCD adaptor is a sensible precaution though, thanks for the suggestion.
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Old 23rd Aug 2015, 1:54 pm   #13
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Default Re: Capacitor advice

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No part of my body is earthed when I use it
Someone else moves it near a radiator to do some sewing in the winter?
 
Old 23rd Aug 2015, 2:10 pm   #14
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Default Re: Suppression Capacitor advice

This is a good explanation. Some Transatlantic terminology needs to be taken into account.

http://www.justradios.com/safetytips.html
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Old 23rd Aug 2015, 2:18 pm   #15
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Default Re: Suppression Capacitor advice

Few domestic electric sewing machines from the external motor era (basically, mid 60s and before) were earthed. Many people used them plugged into the light fitting in a bedroom. The mains enters the pedal and leaves using two circuits, one direct to power the machine lamp and the other from the pedal rheostat to power the motor. The motor is bolted to the machine metalwork.

It's normal for such machines to give a very slight tingle in the absence of any fault because of capacitive leakage within the motor. Obviously earthing would suppress this, but that would require replacement of the pedal and motor, which would destroy the originality of the machine quite apart from costing about £40 for a lower quality motor.
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Old 23rd Aug 2015, 2:35 pm   #16
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Default Re: Suppression Capacitor advice

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Obviously earthing would suppress this, but that would require replacement of the pedal and motor
Why not an external earth lead, to maintain a bit of originality use the hole in the top of an older style 13A plug. It probably was OK in the days the machine was made with not many earthed things about but these days with the whole house bonded to 'earth' I wouldn't take the risk especially for a domestic machine where I am not thinking "electricity" let alone other users not versed in the subject.
 
Old 23rd Aug 2015, 2:51 pm   #17
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Default Re: Suppression Capacitor advice

I take your point, but I'm the only person who uses this machine so I don't have any concerns. Many thousands of vintage machines of this type remain in regular use, some with extremely ropey wiring arrangements, and I don't recall anyone ever being killed using one. I suspect you are much more likely to receive a serious injury by dropping one on your foot or even trying to pick one up (they are hard work for one person to lift).

It would certainly be possible to wire a single earth connection from the metalwork to an additional BS1363 plug if anybody is worried about this, but it would be a bit of a lash up. The technically correct solution is to fit a modern motor and electronic control pedal, where the motor is isolated from the metalwork and everything is either double insulated or wired with an earth return.
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Old 23rd Aug 2015, 3:08 pm   #18
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Default Re: Suppression Capacitor advice

Like you Paul
I don't worry about such things when it comes to my own equipment, the odd tingle lets me know it is an electrical appliance, when I worked on site I would actively seek out non safety mains plugs, the only ones I took that were safety were some MK 13 amp with spring loaded shrouds on them.
Arrogance ? I don't know for me it is a bit of too much health and safety you know, though I cannot construct a case for my attitude

But hey ho enjoy your machine with its lively feel I know I would

Gary
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Old 23rd Aug 2015, 6:35 pm   #19
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I'm the only person who uses this machine
So far... and proper earthing easy to do.
 
Old 30th Aug 2015, 5:59 pm   #20
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Default Re: Suppression Capacitor advice

be interesting to measure the actual leakage current available to earth on that one
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