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Old 1st Jul 2018, 7:50 am   #1
John M0GLN
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Default Snubber resistor advice please.

My TV and it's surround sound equipment are supplied via an Eon Intelli-plug and a 4 way extension lead, this all works as one press on the remote control and it switches everything on or off, the only problem is when switching off there's a loud pop in the speakers, this presumably is from the relay coil in the plug, after a couple of attempts to cure this, a surge protected extension lead didn't help, I searched around and found an Iskra 0.047F, 275VAC, X2 capacitor which I fitted into a 13A plug top next to the other plugs and this works fine, however after further reading it seems that I really need a snubber and that means fitting a resistor in series, this I can do if it's not too big, can anyone give me some idea of a suitable value or perhaps more importantly what wattage and voltage rating please?

Thanks

John
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Old 1st Jul 2018, 10:50 am   #2
Restoration73
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Default Re: Snubber resistor advice please.

As your C is in parallel with all the mains devices, I doubt whether a resistor will make any difference. Ideally in the AV amp there is a speaker muting circuit that means that
device should be switched off first, this would necessitate a delay circuit which would be complex to implement. For those who need snubbers a source is http://uk.farnell.com/roxburgh/xe120...099?st=snubber
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Old 1st Jul 2018, 11:54 am   #3
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Default Re: Snubber resistor advice please.

If it works.......

Putting a X class cap across the line won't upset anything except power factor (infinitesimally) so why bother?
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Old 1st Jul 2018, 12:36 pm   #4
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Default Re: Snubber resistor advice please.

My age old cure for eliminating 'clicks' caused by switch/relay contacts opening on 240V AC is to use a snubber consisting of a 0.1uF 275V+ X2 in series with a 100 Ohm 1/2W resistor placed across the switch/relay contacts. (I guess that a 0.047uF should work too).

The idea is to prevent the voltage across on the opening contacts from rising too quickly and therefore cause an arc, hence the term 'arc snubber' or just 'snubber'.

Used this fix many many times over the past 40 odd years and it usually does the trick.

That reminds me, when I was an apprentice I was working on installing contactor (very large relay) switching panels for large electric motors on overhead cranes. There was a supply of compressed 'quench' gas routed to a valve on each contactor which shot a spurt of gas onto the 'contacts' as they opened and closed in order to quench arcing, IIRC the gas was SF6 (Sulphur Hexaflouride?). It certainly did its job as I discovered when a panel was tested that had a non functioning quench gas supply!

Adrian
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Old 1st Jul 2018, 2:47 pm   #5
G8HQP Dave
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Default Re: Snubber resistor advice please.

In most cases of interest to this website a small capacitor is sufficient, as that will prevent the voltage rising too far or too quickly. Snubbers are needed when there is significant inductance in the circuit (so significant stored energy) but not enough resistance to dissipate the energy quickly.
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Old 1st Jul 2018, 9:32 pm   #6
John M0GLN
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Default Re: Snubber resistor advice please.

Thank you all for your replies which are what I had hoped for, although commercial snubbers have a resistor in series, for my purpose I'll keep it as it is seeing that it works.

John
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Old 1st Jul 2018, 9:40 pm   #7
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Default Re: Snubber resistor advice please.

Quote:
Originally Posted by Restoration73 View Post
Ideally in the AV amp there is a speaker muting circuit that means that device should be switched off first, this would necessitate a delay circuit which would be complex to implement
I wonder if it has but won't work because I always have the power switch permanently on and it never goes into standby before going off.

John
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Old 1st Jul 2018, 10:06 pm   #8
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Default Re: Snubber resistor advice please.

For me, the obvious 'software' solution would be to put the individual devices into standby and leaving it at that, rather than subjecting them to the trauma of a hard power-off.

A hard power-off of 'freeview' TVs/PVRs/set-top-boxes can mean they miss out on over-the-air software updates, and they can also experience glitches in the on-screen program-menus if you don't give them 10 minutes or so of 'learning-time' to re-acquire the menus when powered-up.
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Old 1st Jul 2018, 10:40 pm   #9
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Default Re: Snubber resistor advice please.

Class X capacitors are supposed by definition to be able to withstand being connected directly across the mains line and neutral indefinitely, without a series resistor; and not fail in such a way as to set fire to anything else.

As far as adding a resistor goes, post hoc, ergo propter hoc applies. If you have a working solution, leave it alone!
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Old 2nd Jul 2018, 6:47 am   #10
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Default Re: Snubber resistor advice please.

Quote:
Originally Posted by G6Tanuki View Post
For me, the obvious 'software' solution would be to put the individual devices into standby and leaving it at that, rather than subjecting them to the trauma of a hard power-off.

A hard power-off of 'freeview' TVs/PVRs/set-top-boxes can mean they miss out on over-the-air software updates, and they can also experience glitches in the on-screen program-menus if you don't give them 10 minutes or so of 'learning-time' to re-acquire the menus when powered-up.
That's what I have been doing and I was just trying to get away from using 3 remotes and having to manually switch the subwoofer off, I know it's not that arduous a task but it seemed a good idea, the Sky box is left in standby all the time when not used.

Quote:
Originally Posted by julie_m View Post
Class X capacitors are supposed by definition to be able to withstand being connected directly across the mains line and neutral indefinitely, without a series resistor; and not fail in such a way as to set fire to anything else.

As far as adding a resistor goes, post hoc, ergo propter hoc applies. If you have a working solution, leave it alone!
I will do and I've learnt a bit of Latin as a bonus.

Thanks

John
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Old 2nd Jul 2018, 9:13 am   #11
GMB
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Default Re: Snubber resistor advice please.

I think the reason for adding resistors in series with capacitors in this kind of situation is to control the inrush surge if the capacitor is switched on at the point of high voltage in the cycle. If that happens there will be contact damage and also an emc pulse that will have the opposite effect to that intended.

Many modern devices with mains emc filters do not have the resistors and you can always tell because you do see a blue flash at the switch some times when turning on. Inductive devices make the blue flash when you turn them off of course, and there the snubber resistor goes in parallel with the inductance to give the voltage somewhere to go.

This kind of thing is a counter-example to "working solution, leave alone" because the damage builds up over time.
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Old 2nd Jul 2018, 2:21 pm   #12
G8HQP Dave
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Default Re: Snubber resistor advice please.

A simpler way to control inrush is to use a smaller capacitor. 10nF is usually big enough to control switch arcs and small enough not to take too much inrush.
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Old 2nd Jul 2018, 3:29 pm   #13
John M0GLN
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Default Re: Snubber resistor advice please.

To reduce the inrush current that GMB warned about I've just tried the suggestion from G8HQP Dave, I didn't have a 10nF X2 but found a NOS Erie 10nF 1400VW ceramic disc for a quick trial and it works so that should be better than the 47nF, I'll order an X2 !0nF.

Thanks

John
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Old 2nd Jul 2018, 3:36 pm   #14
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Default Re: Snubber resistor advice please.

I should just point out that reducing the size of the capacitor does NOT reduce the inrush current at all, probably increases it I suspect. What it does reduce is the total inrush energy, so the spark that you will still get will be harder to see. So out of sight......

Why not do it properly?
Oh, I think you may be able to buy X class with built in resistors exactly for this reason.
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Old 2nd Jul 2018, 6:12 pm   #15
julie_m
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Default Re: Snubber resistor advice please.

A capacitor with an integral series resistor would not be Class X rated, by definition. Class X means direct connection between line and neutral, with no series resistor.
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Old 2nd Jul 2018, 6:20 pm   #16
John M0GLN
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Default Re: Snubber resistor advice please.

Quote:
Originally Posted by GMB View Post
Why not do it properly?
By which you mean use a resistor? That's no problem and I can just experiment to find the highest value which still works.

With regards to inrush current, which are we talking about? The current being passed by the continuous cycling of the capacitor by the 230VAC mains or the current surge from the relay which causes the popping in the speakers that I'm trying to suppress.

John
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Old 2nd Jul 2018, 8:38 pm   #17
G8HQP Dave
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Default Re: Snubber resistor advice please.

Quote:
Originally Posted by GMB
I should just point out that reducing the size of the capacitor does NOT reduce the inrush current at all, probably increases it I suspect.
OK, strictly speaking the instantaneous initial inrush current will be zero, assuming there is some inductance in the circuit. The current at the next instant depends on circuit resistance, inductance and capacitance. When we say 'inrush current' we probably mean the current averaged over some appropriate initial time period, which may relate to switch bounce time.

The damage done to a switch by a current surge is likely to depend on both the size of the surge and the length in time of the surge. A smaller cap means a shorter surge. Any blue spark is not caused by the switch closing but opening, possibly due to switch bounce during what we call closing. A capacitor will help prevent this by limiting the maximum voltage (Vmax = sqrt(L/C) x current broken).
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Old 2nd Jul 2018, 10:08 pm   #18
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Default Re: Snubber resistor advice please.

These are a couple of pictures I took of an X2 capacitor that had halved in value. My assumption was that current surges were gradually taking out bits of the foil. This one had been permanently connected as a mains dropper for a thermostat totaling several years, granted not exactly the same service but it is illustrative of the hidden damage. limiting the surges with a series resistor takes stress off the capacitor but may reduce the spike suppression effectiveness.
As mentioned in this thread snubbers are available ready made but from my industry experience on AC snubbers I would say that selecting the right one is always a compromise as you seldom know all the parameters including velocity of contact opening. Often they do not do what you expect! But by using a commercial RC snubber unit you will at least have something where design parameters R and C will have been thought about.

Delving off a bit into the deeper depths I worked with these people for a while ( No connection at all now) see lab note 103 under "get smart about the arc" for what you might be up against.
https://www.arcsuppressiontechnologies.com/

pete
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Old 2nd Jul 2018, 10:55 pm   #19
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Default Re: Snubber resistor advice please.

Quote:
Any blue spark is not caused by the switch closing but opening
Not correct. In fact you can often tell whether a circuit is capacitive or inductive by whether the blue flash is at switch on or off. The "infinite" inrush of a pure capacitor causes an arc as the first few atoms of the switch contacts touch across - and that small initial contact area is promptly vapourised and makes the spark you see, and so errodes the contacts.

Quote:
A capacitor with an integral series resistor would not be Class X rated
X class is a safety rating, not a quality rating of the capacitor.
By way of example, here is the exact thing to which I referred, which as you will see is X2 rated: http://www.farnell.com/datasheets/1504055.pdf
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Old 3rd Jul 2018, 7:55 am   #20
John M0GLN
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Default Re: Snubber resistor advice please.

Quote:
By way of example, here is the exact thing to which I referred, which as you will see is X2 rated: http://www.farnell.com/datasheets/1504055.pdf
My application is slightly different to one where a suppression device is fitted across the relay coil and switch contacts, as I don't have access to either of these my X2 cap' is directly across the whole ring main and as close as possible, adjacent 13A socket, to the surround sound system, the 10nF works in this position and from the Farnell sheet posted it gives me an idea of what resistors are used in commercial components.
When my new X2 caps arrive I'll try with various resistor values as although it works fine as it is, with a resistor it should reduce this inrush current.

Thanks

John
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