Would you trust these caps?

[WaveyDipole]

I took a plunge and purchased components off eBay. In this case, the component was a replacement for a 4700pF blue ceramic capacitor rated at 2kv.

On first attempt, what arrived were capacitors marked '471 3KV', wrong value and physically much smaller and thinner than the item they were to replace. I contatced the seller and explained the situation. No problem - bin mis-labelled - I will send you the correct ones. You can keep the ones we sent for another project. Ok, I thought. Great!

What I received today is marked '472 3KV' yet surprisingly smaller (slightly shorter diameter disc and thinner) than the 470pF caps I received in error. I have attached a picture for comparison.

I checked them on the LCR meter. The first one I pulled out was around 3.3nF which although in the right ball park (i.e. 1000s of pF rather than 100s), for +/-20% or even -20%/+80% was well out of spec. A few others I checked ranged between 4.3nF and 4.9nF which at least capacitance-wise seemed to be what they claim to be. But in that case, would you trust the rated voltage?

Mea culpa - I purchased from eBay so lesson learned. I can't fault the supplier for their polite communication and willingness to replace the parts sent in error, but even so, I am not going to use these in a HV circuit. I will maybe keep them for use in non-critical and much lower voltage situations but I now ordered another set from a reputable supplier!

[HamishBoxer]

Not sure the hi kv are normally round but maybe I am being prejudice.Of course modern caps are a lot smaller these days.

[paulsherwin]

Your values seem plausible apart from the first one which is indeed out of spec. I take it you've double checked your measuring procedure?

If the actual applied voltage is under 1.5kV (likely if the original part was rated 2kV) then these parts should be OK unless they are out and out mislabelled fakes, which doesn't happen very often with commodity caps.

I buy components from eBay sellers all the time and have never had a bad experience, but I don't buy any exotic stuff (3kV caps are exotic by my standards).

[WaveyDipole]

Hamish, I take your point and I agree that due to better dielectrics and manufacturing methods, modern caps are often smaller than those manufactured years ago, but both caps in the picture were purchased this week and supplied by the same seller. Both are physically smaller than examples I purchased maybe a couple of years ago from a reputable source. I had no spares left to put side-by-side in the comparison photograph, which was the reason for the order.

Paul, I take your point as well regarding measuring procedure and what you say about commodity caps. I also appreciate that testing caps at different frequencies yields different results. The results I gave are with the LCR meter set at 100Hz. The capacitance does drop by a few pF as expected when you go to 1kHz and then to 10kHz and 100kHz, but the result is still broadly in line with the expected value of the cap. I should perhaps have mentioned that I also measured some of the caps marked 471 that I received in error and capacitance-wise these are approximately 470pF as expected. So when I see the one marked 472 (4700pF) is actually physically smaller than the one marked 471 (470pF) at allegedly the same voltage rating, I am naturally rather suspicious so, hence, before installing it and potentially subjecting it to such high voltage levels, I took the liberty of asking the experts on here.

Its possible, I suppose, that they will withstand the rated 3kv but I am guessing that the 472's are perhaps mis-labelled 1kv or 400v parts rather than outright fakes. Not sure how one would test that though? I suspect that a HV supply, loud popping noises and release of smoke might be involved.... along with a notepad and pencil to record at what voltage said popping noise and escape of smoke occurred. I don't have suitable HV kit for that kind of thing here though!

I too have occasionally purchased components off eBay in the past with no real issues, but this time I'm not so sure.

[John10b]

Reading these posts made me think why bye components from x and not other well know suppliers?
I did read the forum rules before I posted this, but just in case I was wrong I used x and not proper name.
I have never purchased any components from x and I just wonder why others use it?
Cheers
John

[paulsherwin]

I don't think the component size is an issue in itself. Caps vary a lot in physical appearance depending on the manufacturer and the technology used. Some modern caps are absolutely tiny.

I don't blame you for following your instincts though, especially if there are safety issues involved. There are certainly some dodgy components on eBay so you're right to be cautious.

John, are you saying you don't know if you can use the term 'eBay' here? You certainly can, though you can't discuss the company itself or individual auctions and sellers. We strongly discourage the use of euphemisms like 'fleabay'.

[John10b]

Yes Paul I was referring to ebay. I only wondered why buy from there and not other companies such as RS, Farnell, Cricklewood, just to name a few.
Cheers
John

[kalee20]

These 472 capacitors probably ARE rated at 3kV.

However they may not be any good for your application. There are ceramic capacitors and there are ceramic capacitors. These are small, so may be hi-K dielectric, which has high losses, a large temperature coefficient, and wide tolerance (could be -25% / +50%). Would be OK for decoupling, but not much use if you require a specific value or need a large AC current capability.

[Al (astral highway)]

Hi Wavey,

I agree with Kalee that these look on the small side- the genuine EHT caps I have that are in the nF range are quite a distinctive shape and characteristic blue colour. What’s the application? Is the 4.7nF value critical ? I may have a couple of spares if not.

In passing, I don’t think there’s anything generically wrong with EBay - it acts as a giant distribution portal, so no one is actually buying from EBay (John- relates to your comment ) I’m a repeat customer of several electronic component companies who use it ;often they’ll send small quantities first class (no min order)with free postage and are very personable.

[buggies]

Can't really judge the size from the pics - but this datasheet from Rapid has some representative dimensions from 1kV to 50kV.
https://www.rapidonline.com/pdf/08-1531.pdf

[paulsherwin]

I buy components from eBay sellers because they are cheap and it's normally a hassle free experience. I check out the sellers and use common sense. That said, I use them for simple homebrew projects and mending old radios, not to build heart pacemakers. It doesn't really matter if they go phut (though I haven't had any disasters yet).

I did buy a lot of stuff on eBay a while ago when the RoHS rules were coming in and workshops were disposing of non compliant stock, but that was a one off.

[turretslug]

Sometimes, I've been surprised at just how small HV ceramics are now- but then I think of just how small SMD ceramics are and the drive to make everything ever smaller, so I'm prepared to give leaded components the benefit of the doubt as benefiting from this trend. Equally, I find myself thinking "Nah!...." when comparing the size of modern electrolytics to those of half-a-century or more ago- but it's all backed up by spec.-sheets and manufacturers' reputation.

Still, I can see why there's a feeling of caution with particularly small high-voltage components.

[bikerhifinut]

I equally puzzle over why a standard 35mm diameter case can hold a 50 +50UF 450V cap and yet the same brand in 16 + 16 x 450V is in an identical size can. I suppose its just cheaper to have a limited size range of cans and have varying amounts of padding or fresh air inside?

A.

[WaveyDipole]

Al, thanks for the kind offer, but I had already placed an order with RS. I received some genuine Murrata 2kV caps today so here a a couple more pics alongside the known genuine one. Caps are in the same order in both pics: Murrata 4700pF 2kV, unknown 4700pF 3kV, unknown 470pF 3kV.

The intended application is a HV multiplier. 15kv output.

BTW, since I will have some of these left over, could these be used in place of RIFA mains filtering caps? The RIFA originals are 250v Y rated, same capacitance. I was hoping not to have to order another set of 10x Y caps, but will do so if these are not safe or appropriate to use.

[Al (astral highway)]

Hi Wavey,

I’m glad you found these great Murata ceramic caps. I’ve used this family a lot without problems.

I can see why you’d consider them for use as mains suppression to replace your Rifa caps between line and neutral, but I’m not sure how high the repetitive transient max voltage of the spec you have bought might be, compared to X-class rated. Class X need a couple of extra kV resilience or more. Others with experience of regulations could better make the call whether to use caps designed for that purpose or assume that these will be fine for a good while.

Def not suitable to replace Y-class, as these (the Murata ones) typically fail short, not open, as Y-class need to.

[paulsherwin]

Agreed, class X and Y caps have different design parameters and specifications. Class Ys in particular are considered to be safety critical components.

[WaveyDipole]

Al/Paul, thank you for your kind advice. When it comes to safety critical applications, I would rather err on the side of caution. Although I had presumed these might have a different dielectric, I wasn't entirely sure what the implication might be and hadn't appreciated that these will fail short, which of course, would be a major safety issue especially, if, for some reason, the chassis wasn't properly earthed. I would rather not cut corners in such matters so I have ordered additional suitable Vishay Y-class caps to replace the RIFA's.

[MrBungle]

Wise move.

The foil capacitors are self healing so they will take more than one transient. The ceramic ones are compromised on the first hit. If they survive that, on the second they tend to explode violently. I have seen a Heathkit power supply with quite a bit of damage where one went off. It was enough to blow a tag strip to bits.

RIFAs are pretty good capacitors to be honest but they have a lifetime which should not be exceeded. I usually put brand new RIFA capacitors (not NOS ones!) where there were RIFAs before and stick a sticker on the unit informing the next owner or me to replace the mains filter components 10 years after the date on the sticker.

[Maarten]

10 years? You're a brave man! But you have me puzzled a bit. Why use capacitors that are more expensive and even when new measure worse for every parameter than a modern polypropylene equivalent? They were still 'pretty good' in the 1970's, but after that? Developments in foil capacitors went pretty fast. Most manufacturers were happy to abandon their last paper product lines (some professional applications in sealed cans excepted).

[Argus25]

Originally Posted by MrBungle

Wise move.

The foil capacitors are self healing so they will take more than one transient.

Self Healing is a manufacturer's euphemism for progressive death. With each hit the paper and foil disintegrates, lowering the value of the capacitor.

Recently I checked all the X2 capacitors in my three air con unit control PCB's, the 1uF ones are already below 0.5uF and still falling.

I also set up an X2 capacitor in series with a 110V 2 watt mains clock motor with a pilot light (to run it from 230V), after about a year, the light dimmed off and the clock stopped. The capacitor was down to about 1/2 its original value. I replaced it with a 2kv rated poly cap, no problems since, it doesn't need to "heal" because its not unwell and not damaged by voltage transients, because its voltage rating is higher than any transient in the application (except perhaps a lightening strike, but they are only well defended by a GDT).

I wonder who thought up the idea of using a capacitor in an application where it would regularly be exposed to a voltage transient which was higher than its dielectric and film/foil could tolerate without damage and then have the nerve to call it self healing. The whole idea seems monumentally stupid.