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Old 12th Mar 2015, 9:55 am   #1
Goldie99
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Default Mains Switch - Caps - Which (UK) Class ?

Hi - could someone please clarify which safety capacitor class(es) are required for the following 2 situations in UK ? e.g., X1, X2, Y1, Y2, etc.

In both cases, the circuit diagram extracts are from 30-40 year old solid state Hi-Fi amplifiers: (1) is from a Sony, with 2 wire mains connection, (2) is from a NAD, with 3 wire mains connection (so includes Earth wire to chassis).

Do the different wiring approaches require different safety capacitor classes ? or am I just confusing myself (also a strong possibility !) ?

Thanks for any insight.

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Old 12th Mar 2015, 11:01 am   #2
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Default Re: Mains Switch - Caps - Which (UK) Class ?

These can be X class caps. Y class are only needed when they protect the user from mains.

However, I think it is a mistake to put caps across the switch. Better to put them across the supply just after the switch (i.e. across the transformer primary). Two reasons for this:
1. it makes the caps last longer - they only see mains when the equipment is on
2. it means that off really means off - no cap leakage current to surprise people
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Old 12th Mar 2015, 11:49 am   #3
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Thanks - I assumed they were there primarily to protect the switch contacts, e.g., prevent or minimise any internal contact arcing ?

Also, would moving them not constitute a change to the mains wiring, in terms of regulations, so instead of eventually replacing them like for like, which I think (?) is permitted, wouldn't relocating them mean having to bring the equipment up to present regulations ?

I'm not sure if that's any different, and it probably doesn't matter for my own use, but I'm trying to learn / understand.
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Old 12th Mar 2015, 12:03 pm   #4
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Arc suppressor capacitors fitted across switches normally are accompanied by a series resistor of about 100 Ohm used to 'snub' the inrush/discharge current of the capacitor when switching takes place, helping to increase life of the switch and reduce 'clicks' in the speaker.

I have seen these series CR snubber components which quite often have the resistor included inside the capacitor case drawn simply on the circuit diagram as capacitors without the resistor being shown.

Yours may be of this type, check the marking on the components for any indication of resistance value, it is virtually impossible to tell by measurement.

Adrian.

Last edited by RF Burn; 12th Mar 2015 at 12:14 pm.
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Old 12th Mar 2015, 12:40 pm   #5
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Thanks Adrian - I think in this case both examples are just capacitors. Not sure when the 'snubber' type became more common, but these date back to '80 - '85 timeframe.

Photo shows the item in question in the Sony example - the manual just lists it as "ceramic 0.01uF / 400V". The '103' I can follow, but I don't know the other symbols on it.

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Old 12th Mar 2015, 1:28 pm   #6
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Most of the markings on there are safety standard recognitions, so unless the part is faulty, there's no reason to replace it.

This is an interesting read:

http://www.farnell.com/datasheets/103697.pdf
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Old 12th Mar 2015, 2:01 pm   #7
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Excellent - I figured they were various safety 'standard' markings, but I didn't know where to find the information.... the datasheet helps a lot.

If I'm understanding everything now, both of the original examples would be the same in terms of safety caps, and I'd be looking for either X2, or X1/Y2, rated caps, with the same uF values, and at least the same AC voltage spec. - e.g., the following item for the Sony (0.01uF / 400V) example ?

http://uk.farnell.com/vishay-bc-comp...rad/dp/1902236

Is there any advantage in using the combined Cap + Resistor 'snubber' types ? and would they be rated in the same way ? I'm thinking more in terms of switch longevity for the NAD example, where the 2 section combined mains switch can be a pain to replace if it's damaged.

Sorry for all the questions - I like to study anything I'm not sure of, until I'm comfortable I have a good understanding.
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Old 12th Mar 2015, 2:23 pm   #8
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Default Re: Mains Switch - Caps - Which (UK) Class ?

If you are repairing something (especially for someone else) then you need to leave it as it is, unless there is a glaring error which you can correct. Replace faulty components only. I wasn't suggesting that you change the circuit, merely commenting on what I believe was a minor mistake made by the original designer.

Older equipment may just use a normal cap here, with a sufficient DC voltage rating to cope with the normal AC peak. This would now be regarded as bad practice. X caps are fail-safe, and can cope with both continuous AC voltage stress and short-term spikes. However, they often cope by gradually losing capacitance so eventually need to be replaced.

Putting the cap across the primary instead of across the switch still suppresses switch arcs. In most cases there is no need for a resistor in series, as the transformer primary resistance is sufficient for this purpose. Some people seem to get excited about calculating exact resistor values to get critical damping but there is no need for this and in most cases the information needed for the calculation is not available anyway.
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Old 12th Mar 2015, 2:39 pm   #9
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Default Re: Mains Switch - Caps - Which (UK) Class ?

If a capacitor which would ordinarily require an X rating has a metal film resistor in series with it, it needn't be X rated; as the resistor will act as a fuse in the event of the capacitor failing short-circuit.
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Old 12th Mar 2015, 2:43 pm   #10
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Quote:
Originally Posted by Goldie99 View Post
Also, would moving them not constitute a change to the mains wiring, in terms of regulations, so instead of eventually replacing them like for like, which I think (?) is permitted, wouldn't relocating them mean having to bring the equipment up to present regulations ?
Technically you are correct. In short, a piece of equipment only has to comply with the regulations in force at time of manufacture. If you change circuit configuration (in this case by putting a cap across the mains), then technically you have to ensure that it meets modern requirements since you have then altered the original design.

For your reference, X capacitors are designed for use across Live and Neutral so direct across the mains. Y capacitors are designed to connect between Live and earth and neutral and earth NOT across the mains.

As for connecting across a switch, probably X is the best to use since they are designed to fail O/C. My old Akai amplifier has a capacitor across the switch. I used to get loud switch-off pops and I discovered that the original cap had gone O/C. When I replaced it with a new 0.047uF X cap, there were no more pops so it proves that they are necessary in some cases.
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Old 12th Mar 2015, 3:07 pm   #11
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Default Re: Mains Switch - Caps - Which (UK) Class ?

As background - I only repair for myself (and close family), certainly not commercially - I dabbled with an interest in DIY amps when I was a teenager, but have only recently come back to it in earnest..... after a break of 40-50 years ! My comments re. regulations were purely so I understood the situation if I ever sold anything on in the future (there's a limit to how big the collection can get).

Specifically, I don't think my Sony caps need changing yet, with no obvious evidence of switch or cap damage, but it was a convenient example that I had to hand (it's the amp I'm working on at the moment). The NAD example relates more to a couple of older NAD's that may well have some burnt switch contacts, but that I haven't got to yet. Both illustrated a topic I wasn't sure of, so I thought they'd be good examples, and indeed everyone's answers have been very helpful, and to the point - so my thanks to all for the advice
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Old 12th Mar 2015, 3:16 pm   #12
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Y caps are used where a fault in the capacitor could result in the operator being exposed to mains so they are designed to fail o/c.

Y class must be used for suppression capacitors connected between live or neutral and earth when earth is wired to the chassis of a set with a mains transformer.

Y Class capacitors should also be used for aerial coupling capacitors in AC/DC sets for the same reason.

These capacitors are being used for switch suppression so an X class will do fine. They will prevent back EMF when the switch is turned off which would otherwise create a mighty click in the speakers.

Last edited by PJL; 12th Mar 2015 at 3:21 pm.
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Old 12th Mar 2015, 3:22 pm   #13
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Thanks also to AJS Derby & Sideband - both very useful & interesting comments, made while I was typing my last post.

I particular appreciate both of your logical explanations - I've read hundreds (maybe thousands) of vintage electronics related forum posts, but very few actually explain the logic behind particular choices. I was a research scientist for more years than I care to remember, and still much prefer to study something until I understand why it's done that way, what can go wrong, what are the alternatives, etc.... maybe I spent too many years in research ?!
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Old 12th Mar 2015, 4:54 pm   #14
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Goldie99,

Re: post no. 2 by G8HQP Dave.

The snubber type has the advantage over a plain capacitor when used "across the supply just after the switch (i.e. across the transformer primary)", as the series resistor limits the switch on surge that may happen if the mains voltage is just at its peak when the switch is closed. This is because the capacitor is charging up from zero to the peak voltage - and this current surge may cause the switch contacts to pit.

I always connect snubbers in this circuit "mains on/off switch" position because as Dave says, "Off really is Off".
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Old 12th Mar 2015, 5:18 pm   #15
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Is a few nF enough to cause switch pitting? 2nF across 340V stores 127uJ of energy. This energy gets there via the switch.
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Old 13th Mar 2015, 12:15 pm   #16
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Dave, the instantaneous current can be high - even though it of of short duration. By the way, I was thinking of 0.01uF (or greater) capacitor use in this context.
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Old 13th Mar 2015, 12:45 pm   #17
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Are we talking 2 different aspects here ?

Inrush current when the mains switch is first turned on, which I didn't think was the primary cause of contact pitting ?

And secondly, a 'bypass' route for instantaneously back EMF when the switch is turned off, and any contact arcing needs to be stopped / avoided - which I thought was how the caps directly on the switch were intended to help (or the snubber across the primary)..... maybe a rather overly simplistic view, but you'll have gathered, I'm (still) out of my depth here...
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Old 13th Mar 2015, 12:54 pm   #18
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Default Re: Mains Switch - Caps - Which (UK) Class ?

Goldie99,

You are right, there are several issues here and I think we are straying off the topic of capacitor classes...
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Old 13th Mar 2015, 12:57 pm   #19
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Default Re: Mains Switch - Caps - Which (UK) Class ?

A few nF is enough to stop arcing, but larger caps are often seen.
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Old 13th Mar 2015, 1:36 pm   #20
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Default Re: Mains Switch - Caps - Which (UK) Class ?

I agree Steve - I know what's used now, have a better idea of why, and know what & where to search for more depth and information as I need it. Thanks to all.
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