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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Jul 2018, 7:50 am | #1 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 0.047µF, 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 |
1st Jul 2018, 10:50 am | #2 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
Posts: 2,801
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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 |
1st Jul 2018, 11:54 am | #3 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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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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1st Jul 2018, 12:36 pm | #4 |
Hexode
Join Date: Apr 2008
Location: Ramsgate, Kent, UK.
Posts: 252
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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 |
1st Jul 2018, 2:47 pm | #5 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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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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1st Jul 2018, 9:32 pm | #6 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
1st Jul 2018, 9:40 pm | #7 | |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Snubber resistor advice please.
Quote:
John |
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1st Jul 2018, 10:06 pm | #8 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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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. |
1st Jul 2018, 10:40 pm | #9 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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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2nd Jul 2018, 6:47 am | #10 | ||
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Snubber resistor advice please.
Quote:
Quote:
Thanks John |
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2nd Jul 2018, 9:13 am | #11 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,099
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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. |
2nd Jul 2018, 2:21 pm | #12 |
Rest in Peace
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Location: Solihull, West Midlands, UK.
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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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2nd Jul 2018, 3:29 pm | #13 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
2nd Jul 2018, 3:36 pm | #14 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
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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. |
2nd Jul 2018, 6:12 pm | #15 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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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If I have seen further than others, it is because I was standing on a pile of failed experiments. |
2nd Jul 2018, 6:20 pm | #16 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Snubber resistor advice please.
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 |
2nd Jul 2018, 8:38 pm | #17 | |
Rest in Peace
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Location: Solihull, West Midlands, UK.
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Re: Snubber resistor advice please.
Quote:
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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2nd Jul 2018, 10:08 pm | #18 |
Hexode
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 419
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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 Last edited by G4_Pete; 2nd Jul 2018 at 10:16 pm. |
2nd Jul 2018, 10:55 pm | #19 | ||
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,099
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Re: Snubber resistor advice please.
Quote:
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 |
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3rd Jul 2018, 7:55 am | #20 | |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Snubber resistor advice please.
Quote:
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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