Author Topic: Ceramic capacitor selection hell  (Read 3467 times)

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Offline Lomax

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Ceramic capacitor selection hell
« on: October 27, 2015, 04:06:26 am »
I'm trying to assemble a complete(-ish) kit of good quality ceramic disc capacitors to replace the junk currently taking up space in my component trays (and pissing me off at every use). I'd like them to have good tolerance (<=10%), reasonably stable (class II or I) and be of a high-temp (>100degC), high-voltage (mains capable) type for maximum flexibility.

I've spent hours looking on both Mouser and DigiKey and at series from Murata, Vishay and TDK, trying to find something that is in stock with good specs and good coverage at a reasonable price, and I'm about to take the plunge on the X1/Y2 rated CS range from TDK from 10pF-470nF (±5% 10-68pF, ±10% 100-680pF, ±20% 1-470nF). The price is pretty reasonable (e.g. 10pcs 470pF @ £1.73, or £10.30 for 100pcs), but I just thought I'd double check here before pressing the button: am I right in assuming that these caps will be usable, if a little wasteful, anywhere a cheap 50V ceramic disc is used? Or is there some quirk in the design of AC rated ceramics which makes them unsuitable for some applications?

To further confuse things, there's a similarly priced CC45 "low dissipation factor" series, which eschews the X1/Y2 certification and instead offers better high frequency performance (e.g. for switched mode PSUs) - though that series only goes to 470pf. And there's a plain CK45 series as well, from 100pF to 4.7nF. If you were to pick only one series, which one do you think would be most versatile? Should I look at another series/manufacturer altogether?

 :-//

Any wisdom shared gratefully received!

P.S. In addition, I'm planning to get a set of class I MLC capacitors for critical (audio) stuff, but here a 50V type will suffice.
 

Offline wraper

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Re: Ceramic capacitor selection hell
« Reply #1 on: October 27, 2015, 04:15:35 am »
Why the sane person would like to use safety rated capacitors for low voltage general purpose? Not only they are big but also with inferior specs. If you check the datasheets, you'll notice that temperature coefficient is just awful.
 

Offline Lomax

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Re: Ceramic capacitor selection hell
« Reply #2 on: October 27, 2015, 04:35:54 am »
I never made any claim of sanity! The idea to stock HV rated caps came from a few things:

1) they are available, unlike 50V caps which it seems impossible to find a complete series of from any supplier
2) they aren't that much bigger diameter wise, but yes about 2x thickness
3) they can be used in more places than a 50V part (PSUs, line filter...)
4) they cost about the same

Electrolytics, disc ceramics, MLC, tantalums, polyesters, mica - having a range of capacitors on hand can take up a lot of space (and money) to cover all most bases; it was my hope to combine a pF range high-voltage type with other general pF range uses. Just to save space and money really. I didn't know about the poorer thermal performance though, that explains why this information is not shown in the datasheet(s) :) I guess I'll just skip stocking a HV type, but then maybe I should forget about disc ceramics altogether and just get a nice set of MLCCs. When doing repairs, would an equivalent MLCC always normally be a suitable replacement for a disc cap?
 

Offline wraper

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Re: Ceramic capacitor selection hell
« Reply #3 on: October 27, 2015, 05:16:41 am »
I didn't know about the poorer thermal performance though, that explains why this information is not shown in the datasheet(s) :)
Info is there, just read more carefully. The thing is that they cannot use decent (NP0) ceramic even at <100pf capacitance and be able to get a high voltage rating at the same time.
Quote
I guess I'll just skip stocking a HV type, but then maybe I should forget about disc ceramics altogether and just get a nice set of MLCCs. When doing repairs, would an equivalent MLCC always normally be a suitable replacement for a disc cap?
Won't be an issue as long as you don't use some inferior type instead, like replacing NP0 with X7R. Disc capacitors are mostly used only because being the cheapest stuff.
 

Offline Lomax

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Re: Ceramic capacitor selection hell
« Reply #4 on: October 27, 2015, 06:32:23 am »
Ok, many thanks :) I'm now looking at the Murata RDE series MLC caps @ 50V, they use C0G/NP0 material up to ~10nF, after that X7R, and cost about the same as the HV TDKs (£10/100). The confusion with the TDK caps came partly from their unusual material designations, what does "B" mean for example? The 5% tolerance implies something like a class I dielectric, which is clearly not right (considering size vs. voltage) - thank you for pointing out the issue!
 

Offline T3sl4co1l

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Re: Ceramic capacitor selection hell
« Reply #5 on: October 27, 2015, 07:15:49 am »
Question: Why are you looking for ceramic discs at all?

Times have changed!  If you're used to the olden days (I kind of get that impression, sorry if that's incorrect), ceramics were used all over, but they're very expensive now -- it takes so much more material to make them than other kinds.  And the materials have improved to the point where the same values can be had in microscopic sizes, 1000s of times smaller than vintage discs.

If you still need through-hole, moderately to high voltage rated parts, film capacitors are the way to go.  Polyester is good for general purpose (timing, signals, and low ripple filtering), while polypropylene is excellent for higher stability, and high ripple filtering applications.

This includes line filtering, which requires an X1 rating for line-to-line applications.  Y1/X2 ratings are for line-to-ground, which can be had in film types, but ceramics are usually cheaper and preferred there.

For very high frequencies (such as you'd see in RF amplifiers), MLCCs still offer good performance, as long as you get C0G type dielectric, and a suitable size (e.g., a 1nF 630V part will be in a 1206 SMT package, or similarly sized dipped and leaded THT part).  Discs are still reasonable for small values (maybe 100-200pF and below), but larger values (aside from Y1 rated types) just aren't economical anymore.

By the way, for RF purposes, avoid type 2 dielectrics.  Lossy, even when rated for much higher voltages than your need.  Get C0G (sometimes labeled NPO), or something nearby (N750 or other tempco -- if you don't mind, or actually need, a tempco of course!), or SL at worst, I think.

Tim
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Need engineering assistance? Drop me a message!
 

Offline wraper

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Re: Ceramic capacitor selection hell
« Reply #6 on: October 27, 2015, 08:01:33 am »
5% tolerance implies something like a class I dielectric, which is clearly not right (considering size vs. voltage) - thank you for pointing out the issue!
5% tolerance is not so simple. It might be within 5% when new, at room temperature and zero bias voltage. For example, you can buy X7R cap with 5% tolerance. Then capacitance start to drop with ageing and after a few years can drop by say 15%. When heating above certain temperature (soldering process), it will regain it's capacitance and will start ageing again. When bias voltage applied, at full rated voltage it can loose as much as 80% of it's capacitance, especially those with very high capacitance for it's size. So your 5% cap actually isn't 5% at all.
 

Offline Lomax

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Re: Ceramic capacitor selection hell
« Reply #7 on: October 27, 2015, 08:53:24 am »
5% tolerance is not so simple.
Yes, I've realised that now :) I did read about the gradual loss of capacitance with some ceramic dielectrics, and that capacity can be restored by applying heat. There's also a voltage coefficient which is problematic with some materials. I don't need the best of the best though, just something I can confidently use to replace a failed part in old HiFis etc, without worrying about it affecting the specs negatively, or itself failing at some point.

Why are you looking for ceramic discs at all?
Mainly because of the false impression I had that they were ubiquitous and cheap :) Also since I have some concerns about using a completely different type when repairing old equipment. And yes, I am a bit old-school too! I couldn't understand why I was having trouble finding (50V) stock at reasonable prices - now I know. It seems TTH monolithic ceramics are mainly used for high-voltage caps these days, though I still see lots of SMDs.

Polyester is good for general purpose
Again my (false) impression would be that these are (much) more expensive than ceramics, and perhaps not always possible to fit when making a replacement. I was planning to get a range of polyesters as well though (and maybe polypropylene), of higher values to use in loudspeaker cross-overs for example, but considering how long it's taken me to select the small caps I'm not looking forward to it.

MLCCs still offer good performance, as long as you get C0G type dielectric
Yes, and I've now decided on getting the Murata RDE MLC capacitors @ 50V in E6 values. 50-100pcs 22pF, 47pF, 220pF, 470pF, 1nF, 4.7nF, 10nF, 47nF and 100nF and 20pcs of the other values. Should make it worth my while labelling things up properly! Unless there's something else I've missed and these are also unsuitable as "use anywhere" / everyday caps?

Thanks everyone for your kind help and insights!

Edit: Murata RDE only uses C0G material up to around 10nF, above which it's X7R. You can get (some) higher value ones with C0G but the cost is prohibitive.
« Last Edit: October 27, 2015, 08:58:34 am by Lomax »
 

Offline Lomax

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Re: Ceramic capacitor selection hell
« Reply #8 on: October 27, 2015, 09:18:17 am »
And here's an example of what I mean by "labelling up properly"... :)
 

Offline T3sl4co1l

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Re: Ceramic capacitor selection hell
« Reply #9 on: October 27, 2015, 12:32:14 pm »
I don't buy MLCC C0G above 1nF or so, for noncritical applications.  Everything else, X7R.  Yeah, large C0Gs are actually available, but they're ridiculously expensive, definitely not worth it when a polypropylene is just as good.

Tim
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Offline bingo600

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Re: Ceramic capacitor selection hell
« Reply #10 on: November 01, 2015, 08:19:54 pm »
I just bought a set of these on aliexpress (search for) : 1260635783

Price was low enough , that i'd give it a shot.
IF the description fits (Murata COG & X7R) , it's not a bad deal.
If not well , i just don't know how to test if it's X7R vs. "the bad x5x"

The 1206 kit was containing a lot of Y5V , so i dropped that.


Ohh
Quote
Why Your 4.7µF Capacitor Becomes a 0.33µF Capacitor
is a nice read.
https://www.maximintegrated.com/en/app-notes/index.mvp/id/5527

Or attached

/Bingo
 

Offline Lomax

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Re: Ceramic capacitor selection hell
« Reply #11 on: November 02, 2015, 03:48:48 am »
i just don't know how to test if it's X7R vs. "the bad x5x"

Try heating one up and seeing how much the capacitance changes. X7R should only give you a ~15% change, IIRC.
 


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