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Old 2nd Jun 2021, 8:03 am   #1
TonyDuell
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Default Evil Mains Filter

Most of us know those metalised paper filter capacitors which have a habit of going short circuit and emitting horrible-smelling smoke. It's normally recommended to replace them with a plastic dielectric capactior.

I've just had an IEC mains inlet plug/filter fail in that way. Seems to have failed live-earth, it tripped the RCD in my consumer unit. I've temporarily replaced it with a plain IEC plug, I can't believe this bit of digital test gear with a linear PSU is going to put that much hash on the mains, at least not compared to all the little SMPSUs springing up everywhere. But sometime I'll fit the right part.

The original was Belling-Lee and according to the date stamped on the side is almost 40 years old. So I can't complain.

But looking at RS, etc, the type of capacitor in new filters is not specified. Do I just buy one and hope (if it lasts 40 years it'll see me out I guess) or is there a brand/type that is known not to fail in this way?
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Old 2nd Jun 2021, 8:27 am   #2
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Default Re: Evil Mains Filter

A few years ago a Rifa capacitor did its thing in the boss' pet 2465 scope, and I had a look at it. On our usual supplier's website they had Kemet capacitors of the right ratings. Good, I thought, anything but Rifa. What turned up? Rifa. Kemet had bought the brand and were operating it.

Even things with linear power supplies can need filters. The cause of the offense is switching noise from slow rectifiers like the higher voltage members of the 1N4007 family. They store charge and turn off with a nasty snap. You have to design-in mains filters to pass EMC tests. Sometimes you have to beef-up the usual Schaffner and Corcom types with an added capacitor in the X position because that rectifier noise goes all the way down to mains frequency and has harmonics to many tens of MHz.

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Old 2nd Jun 2021, 8:08 pm   #3
factory
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Default Re: Evil Mains Filter

The Schaffner filtered inlets used in some 1980's HP & Tek test gear also have a bad habit of failing in the same way, some say they contain RIFA madness , but the one I dissected from a Tek 24xx series scope actually contained WIMA MP3 paper capacitors instead, plus one unidentified white capacitor.
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That one got replaced with a Corcom one as that was the lowest priced equivalent from RS, I figured as the paper caps are usually more expensive to buy individually than plastic film types, that it hopefully won't use them.

But I wasn't aware of other brands failing, think I have seen a Belling Lee filtered inlet in my Solartron 7075 DVM, probably need to evict it now.

David
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Old 3rd Jun 2021, 4:44 am   #4
TonyDuell
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Default Re: Evil Mains Filter

Yes, linear PSUs can put noise on the mains although I suspect one piece of test gear used infrequently can really be worse than the thousands of SMSUs in use all the time nowadays. So, given that I have no idea which mains filters are not likely to fail in this way I either leave it as it is or fit something from RS and hope it lasts.

The fact that RS are out of stock of many types until November (!) is another matter.
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Old 3rd Jun 2021, 7:31 am   #5
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Default Re: Evil Mains Filter

It doesn't matter how many SMPSUs are out there fugging-up the ether, and the product you're doing has a linear power supply, you still have to get it to pass conducted and radiated emissions tests to get a legitimate CE mark. So it needs an effective filter.

One problem is that there is plenty of equipment around which does not meet the requirements, and that there is no enforcement.

A second problem is that back when the emissions regulations were formulated, no-one had a clue about the incredible surge in the number of noisy things the planet was going to contain in a couple of decade's time.

If we want to use radio, we need to revisit the permitted levels and drop them significantly, then we need enforcement with teeth.

David
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Old 3rd Jun 2021, 11:34 am   #6
TonyDuell
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Default Re: Evil Mains Filter

I am not building, I am repairing/restoring. The repaired unit is not going to be 'put on the market'.

As such I believe the legal requirement is not to cause harmful interference to other devices. I would be very surprised if anyone outside my house could tell if this unit is turned on or not, and as such it is not causing harmful interference.

That said, I do intend to replace the mains filter. The fitting of an unfiltered IEC plug is a temporary measure to enable me to carry on with the repair.

Now can we get back to my original question. Are there any makes/types of IEC chassis plug with mains filter that are known not to contain metalised paper capacitors? I'd rather fit such a unit if possible.
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Old 3rd Jun 2021, 1:09 pm   #7
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Default Re: Evil Mains Filter

Bulgin can be contacted at the UK office ;
https://www.bulgin.com/us/about/contact-us/

products;
https://www.bulgin.com/products/pub/...ns_filters.pdf
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Old 3rd Jun 2021, 2:49 pm   #8
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Default Re: Evil Mains Filter

Had a Rifa style capacitor commit suicide in the usual way. My boss asked me to look at this urgently needed power supply and I told him I would order a replacement capacitor. He glared at me and in a gruff voice suggested it would surely be more than that?
He snatched the power supply from my hands and sent it out for repair. Now this was in the early 70's, the power supply came back with a repair bill ticket of £135.
I kept thinking, that'll teach you boss man, you big know all.
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Old 3rd Jun 2021, 4:22 pm   #9
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Default Re: Evil Mains Filter

Bosses eh think they are in charge oh wait they are unfortunately. Back on topic another reason to refit the filter is the amount of RF noise that comes in from other people here at least if you hold a radio near the mains input to the house the noise level goes thru the roof whether it would affect your bit of equipment I don't know but why risk it
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Old 3rd Jun 2021, 8:07 pm   #10
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Default Re: Evil Mains Filter

I was just giving the reason why even linear supplies got mains filters in them from new.

Had a capacitor go up inside a Corcom filtered IEC inlet. It burned through the casing. Well it had to, to get rid of the foul smoke I suppose.

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Old 5th Jun 2021, 12:56 pm   #11
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Default Re: Evil Mains Filter

We had a bunch of integrated filter/IEC-connector/on-off-switch/fuse units release their magic smoke on some server power-supplies a decade or so back. After a couple of them let-go we realised there was a congenital fault and pre-emptively had all the offending power-supplies replaced.

Much cheaper than having a data-centre's automatic fire-detection go off at 3AM and yours-truly having to meet the disgruntled firemen to explain the false-alarm then spend the rest of the rather-tiring day calling clients to explain why there was a 15-minute service-outage.
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Old 13th Jun 2021, 4:22 pm   #12
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Default Re: Evil Mains Filter

I had a few minutes spare today so I took a hacksaw to the can of the old filter (no I don't intend to repair it, but I like to know just how things have failed). After peeling off the can and breaking away the potting compound I found the expected class X metalised paper capacitor. I couldn't see a brand on it, but they all fail the same way.

I assume the values of the capacitors and inductors in such a filter are not critical, you just buy one which will stand the load current. For example the class Y capacitors in the original are said to be 3n3 on the can, the replacement I've found has 2n2. But I can't believe the original designer did anything more than picking a suitably-rated filter from the catalogues.

The original is 6A which is overkill here, but I might as well fit the same rating. It is not a 'low leakage' one with no capacitors to ground, and I'll get one with the discharge resistor between L and N, even though I am not sure the original had that. Other than that I can't see any reason to choose a particular one.
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