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Old 11th Dec 2019, 10:14 pm   #1
sparkymike
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Default Suppression capacitors.

Are radio mains supression capacitors any different to normal caps ?
I need to replace a .04uf cap, square shaped, and only one in the shed is a .1 600volt working and slightly larger, but same shape.(RS product )
Would this be ok ?
Mike.
ps. just made out that original also says ? 1oo ohms X
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Old 11th Dec 2019, 11:09 pm   #2
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Default Re: Suppression capacitors.

The X is important. X-rated capacitors are approved for use across the mains, line-to-neutral.

[Y rated (Y2 nowadays) are for line-ground and neutral-ground applications and must only fail open circuit and must survive flashovers without shorting.]

100 ohms suggest that it is an R-C combination which works as an absorber.

Best replace like with like as far as ratings and approvals go, or else you could wind up carrying the can.

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Old 12th Dec 2019, 5:20 am   #3
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Default Re: Suppression capacitors.

Most of the time you can just omit replacing these capacitors and leave them out unless there are major issues with RF interference on the mains. Most of the time these capacitors cause more harm than any good they do as they are so unreliable.

The main reason is that the manufacturers underestimated the voltage ratings they needed because of voltage transients that arise on the mains and on the primary side of power supplies when instruments are turned off, at certain times of the cycle when there is stored energy in the field of the transformers/inductors.

In any case if you replace these with another X2 rated capacitor you might have done your civil duty, but you can also expect it to fail again at least twice in your lifetime, especially if they are the metalized paper types.

To avoid failure now I replace these with 1500V DC rated parts, and this insulation rating makes it as safe or safer than many objects carrying insulated conductors in appliances connected across the mains.

It is important to have a fuse prior to the circuit point where the capacitor connects to protect the instrument wiring, PCB tracks etc, inside the instrument that lead to the capacitor, in the event the capacitor did short out.

A lot of the time I worry more about "carrying the can" for ignoring a mistake that somebody else made and repeating it without thinking, like re-fitting standard X2 capacitors according to some external rule, when I know the parts are not reliable and can smoke any time because they were made too physically compact with insulation that was too thin, with inadequate voltage ratings for the task. A little like the thin insulation issue in cell phone batteries.
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Old 12th Dec 2019, 8:30 am   #4
sparkymike
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Default Re: Suppression capacitors.

Thanks for the info. I will replace it with a 600 plus volt one.
Mike.
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Old 12th Dec 2019, 11:31 am   #5
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Default Re: Suppression capacitors.

Quote:
Originally Posted by sparkymike View Post
Thanks for the info. I will replace it with a 600 plus volt one.
Mike.
Don't go below 1000v rating, ideally use a 1250 or 1500v part. Some people have argued that 900V is ok, but 600 to 750 v DC rated parts (which is about what most 240V RMS X2 caps are rated at) can fail. I prefer a much larger margin, so I never go below 1250v for a capacitor wired across the mains to allow for practically every worse case scenario with voltage transients and don't forget a fuse if there is not already one there...

Last edited by Argus25; 12th Dec 2019 at 11:44 am.
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Old 12th Dec 2019, 8:53 pm   #6
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Default Re: Suppression capacitors.

And avoid paper capacitors like the plague. RIFA is the best known and worst quality brand, but WIMA also still manufactures the MP3 series.

Edit: I'm one of those who argues that 700-900VDC would theoreticaly be okay but I also advocate for a better margin and since in practice there isn't anything between 630V and 1000V I also end up advising 1000V or better, or just use a good quality 275VAC or higher rated X2 capacitor.
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Old 13th Dec 2019, 4:01 pm   #7
stainless
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Default Re: Suppression capacitors.

What type of 1000/1250/1500v capacitor is reccomended? Polyester, polypropyline.......?
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Old 13th Dec 2019, 4:13 pm   #8
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Default Re: Suppression capacitors.

what's the suppressor for? If its across the mains it needs, as already said, an X-type. Any X type will be made of the correct material, normally metallised polypropylene.

But if you mention '100R', is that a capacitor, or an encapsulated RC Network? These are more often used across switch contacts or thyristors etc.

Best not replace an RC network with a capacitor.
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Old 14th Dec 2019, 10:51 am   #9
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Default Re: Suppression capacitors.

Designers add RC networks for a variety of reasons. They are sometimes called 'Snubber' circuits.

They are often associated with inductive components such as transformers, motors and solenoids.
Voltage spikes from 'inductive kick' and unwanted oscillations can be created when current is disconnected from an inductive device. Snubbers help to dampen the voltage spikes.

I have seen them enclosed in a package which looks like a capacitor.
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Old 14th Dec 2019, 11:12 am   #10
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Default Re: Suppression capacitors.

Actually, any "snubber" is pretty useless if it has more than a few Ohms in series with it. So some of the "historical RC modules" that contained capacitors with series resistors of more than 5 to 10 Ohms were a total waste of time, or should I say money. If you are going to bother with a snubber, its series resistance needs to be at least lower than a few Ohms, or its effectivity is substantially reduced. Most of the time there is more than enough resistance in the copper wire that composes the primary winding of the transformer or the coil of the inductive device. The same applied in DC circuits switching inductors where the diode is a snubber, you do not have to add resistance in series with the diode.
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Old 14th Dec 2019, 12:34 pm   #11
Craig Sawyers
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Default Re: Suppression capacitors.

The diode is there to stop the inductive negative swing blowing up the transistor (as in a transistor driving a relay coil). It does not therefore require a resistor.

But for a transformer of say 10mH primary inductance, with a 0.1uF capacitor, the Q is around 300. So the voltage across the mains switch can be very high. Now you are right in that the primary resistance damps the Q somewhat. But a CR snubber *across the switch contacts* with R=100 ohms is a good belt and braces way of not burning the contacts or indeed arcing across the opening contacts.

If it is an X capacitor across L and N, then I agree - it should just be an X-rated capacitor with no series resistor.
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