Ever wonder how much of a ripoff X7R caps are? - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 19, 2014, 02:52:02 pm
content: They're not terrible, as it turns out. But do be mindful:- Typically, at rated voltage, capacitance is 20-30% of the nominal rating- Energy density is less than half that of a C0G part at full ratings (C doesn't drop with V)I hand tuned a rough best-fit curve to a typical graph, then integrated it to find total energy at ratings. See attached spreadsheet.The curve should be roughly typical of X7R from any manufacturer, in any rating. It should be different for very low voltages (under, say, 25V, the capacitance may remain high, I haven't checked), and will be different for other formulations (like, X5R or Y5P should be similar enough, but Z5U? bleh!). It also varies strongly with temperature.The fields at the bottom are for a typical part. The curve is normalized, so C(V) and E(V) should be reasonably proportional to C and V ratings, given the above assumptions.And at the very bottom, a comparison toa part I've mentioned before.Tim

author: con-f-use
author_rank: Unknown
timestamp: October 19, 2014, 04:15:24 pm
content: Dave did two videos on this...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 19, 2014, 07:15:25 pm
content: No, not just voltage, that's obvious from the curve. I wanted to see what that does to energy. And, as it turns out, E(V) is about linear, which is pretty interesting.Tim

author: nukie
author_rank: Frequent Contributor
timestamp: October 26, 2014, 10:57:45 am
content: It's a nature of ceramic capacitors just keep it in mind. How about dielectric aging? The low ESR nature of ceramic also causes in stability in some old tech regulators, is that a scam too? If you are not comfortable with ceramic you can avoid them and go for electrolytics. However they also need to be volt derated.I love them I cannot live without them. If possible, choose the largest capacitance and largest package possible. The bigger the package the less change to the capacitance.As a circuit designer you should take all these into account. There are plenty of electronics devices that works fine with X5R. Without them your phone will be a huge brick.

author: jeremy
author_rank: Super Contributor
timestamp: October 26, 2014, 11:03:03 am
content: Thanks for sharing, nice!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2014, 07:31:11 pm
content: One small datapoint on aging:Annealing as in, heated each lead with the soldering iron for 10 seconds. (Last time the part saw heat was probably 10-20 years ago.)I think the expectation is the value can drop by about half over age; probably much worse for Z5U.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 08:00:29 pm
content: I guess that was a 10n cap then, and big enough that it was not too bad with voltage change.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2014, 08:06:22 pm
content: "0.02Z", no other markings. Ceramic disk about 8mm wide. Probably 50V.Even after annealing, it didn't restore to specified value (presumably +80/-20%), go figure.It's reasonable to expect they made them thicker than necessary back then; this would be why MLCCs, with carefully refined modern process control, have such extreme coefficients in comparison. Or put another way, assuming it doesn't break down, this "50V" cap might've been rated for 200V by modern standards.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 08:29:16 pm
content: Disc cap, that would be about right, and it was a 20n nominal unit ( probably only if you could apply 0V as a capacitance check without having voltage derating) and it probably would work at 200V ( and I have seen some run at that but only in the cheapest of uncle Lo's stuff) as a 100pF capacitor.Then again I have seen some ceramic discs around 10mm in diameter rated at 100n 6V.

author: tszaboo
author_rank: Super Contributor
timestamp: October 27, 2014, 12:42:35 pm
content: I dont think so. The biggest NP0 in 0805 package I could find is 47nF, while the X7R was 4.7u (25V both). Even if it would lose 80%, it would be still 20 times the NP0. Maybe your conclusion is different because the high voltage?

author: TerraHertz
author_rank: Super Contributor
timestamp: October 27, 2014, 12:58:04 pm
content: Interesting. I was aware ceramic disk caps were unsuitable to use anywhere that needed an accurate capacitance value, but I didn't know their value was so voltage dependent.So, they'd actually be useful in VCO circuits. If you didn't care about absolute accuracy, or there was some other freq regulation in the loop.Are these type of Philips precision silver-ceramic caps still in use much? (See pic)Hopefully there's a SMD version these days.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2014, 01:20:22 pm
content: Don't think I've seen those before (or in quite a while??), but they look fancy...The heart of what I wanted to know in the OP was, how does dielectric affect energy storage and energy density? And the conclusion is: for small voltages, it behaves ideally, but at higher voltages, I discovered there is a different power law, and energy goes up linearly rather than quadratically with voltage. And, at rated voltage, the energy density is about 1/3 of C0G/NP0.Which verifies my hypothesis, because the same is true of magnetic cores: energy density is inversely proportional to permeability, which is why transformers (that don't store energy; as opposed to coupled inductors) use high-mu cores, and inductors use low-mu cores or gapped cores (which gives a low average mu -- same thing).Quote from: NANDBlog on October 27, 2014, 12:42:35 pmLike electrolytics, ceramics either go as C*V, or there's a weak low-voltage end before going as C*V^2. Both examples in the study are high voltage parts, giving the best chance for each of achieving the highest energy density.At low voltages, you're even better off with tantalum or an aluminum polymer, where sheer energy density is the requirement. (Offhand, "0805" package tantalums are showing up to 4.7uF in 25V which won't drop off with voltage, or including voltage derating, a comparable 1uF at 50V. They're a lot more expensive. Aluminum polymer aren't available that small, though the values they put into 2917 chips are pretty cool.)Tim

author: free_electron
author_rank: Super Contributor
timestamp: October 27, 2014, 03:03:25 pm
content: Quote from: T3sl4co1l on October 26, 2014, 07:31:11 pmthat is a well known effect. if you heat the cap dielectric over its curie point , which for BaTI [barium-titanate] based dielectrics like X7R and X5R , (basically anything above 1nf ), is 147 degrees c . 'resets' the time induced aging. TDK has a paper on that. 1% drift the first minute , another 1% the next 10 minutes , another 1% the next 100 minutes, another 1% the next 1000 minutes. and so on. it takes 5 YEARS to stopso : in any given system that uses a capacitor for timing or filter purposes and needs adjusting : can't use BaTi based dielectrics. they come out of the reflow or wave and you know they will drift for the next 5 years ...