Distinguishing ceramic from film capacitors electrically? - Page 1
Forum section: electronics / projects

author: mcovington
author_rank: Regular Contributor
timestamp: December 10, 2019, 05:25:56 pm
content: I see that several microcontroller data sheets recommend ceramic bypass capacitors, presumably because of their lower ESR.I have a few dozen 0.1-uF capacitors and don't know which kind they are. They are blue and plastic-encased. Obviously the usual green or red film capacitors are easy to distinguish from tan ceramic ones, but these aren't either one.Is there a practical electrical test to tell me? (Do ESR meters read low enough?) Or should I cut one open and look at it under the microscope?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2019, 05:45:46 pm
content: There are quite different types of ceramic and film caps. The good thing here is that the main ceramic type used for decoupling is class 2 ceramics like X7R and similar. These capacitors are relatively easy to identify: there capacitance is quite a bit voltage dependent. With film caps the capacitance is essentially not effected by a DC voltage.So one could build a simple oscillator (e.g. 74HC14 + resistors + cap) and see if the frequency changes with voltage at the cap:It should be something like <0.01% change for a film cap and some >1% change for X7R if a reasonable voltage is applied. 2 Caps in series and a high resistor to apply the voltage (e.g. 0 V or 1/2 the supply) should do the trick.The main reason they suggest ceramic is that these are small and cheap - if small enough it would be OK to use a film cap too.

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2019, 05:48:30 pm
content: You are not probably going to distinguish C0G/NP0 ceramic from a decent foil one - easily.But what might give, is if you'd stick the cap to a VNA, to measure the impedance. Foil type shall have higher parasitic inductance. But still - in some cases such difference may not be that high.//EDIT: Also, comparing the leakage current may lead somwhere (ceramic should have higher) and dielectric absorption. Polystyrene shall be superior.

author: nfmax
author_rank: Super Contributor
timestamp: December 10, 2019, 05:52:48 pm
content: Even simpler - most ceramic capacitors around the 100nF mark will use a dielectric like X7R that is strongly temperature sensitive, i.e. not C0G Pop one in the capacitance meter and monitor the value (not the ESR) as you warm it up. If it decreases significantly as the temperature rises, it's ceramic!

author: bob91343
author_rank: Super Contributor
timestamp: December 10, 2019, 06:06:53 pm
content: One of the distinguishing features of ceramic disc capacitors is the low ESL. This results in a high self resonance frequency, important when working with high speed components.A dis ceramic capacitor is basically a slice of ceramic with leads bonded on the faces. If done well, there is very little opportunity to create inductance.One should also realize that ceramic can be piezoelectric, causing circuit noise when subjected to movement.Multilayer ceramic capacitors are a different animal.

author: Gyro
author_rank: Super Contributor
timestamp: December 10, 2019, 06:22:16 pm
content: Regardless of the electrical methods, your OP blue plastic cased ones will be film type (assuming that they're rectangular). Hopefully there should be some markings, eg MKP, MKS etc to narrow down the film type.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2019, 06:26:40 pm
content: Easy: since we're talking small values for bypass, film caps are bigger!The ESL for either kind is essentially lead length. (Foil connection is not done at a point, but by spraying metal onto the entire exposed edge of the roll.)Also, unless the ceramics are C0G or similar, they will be nonlinear so the capacitance will drop noticeably under modest bias -- maybe 20-100V. C0G are usually distinguished by marking as such, or maybe a black dot on top, and are also very uncommon (and expensive!) in large values (>10nF?) and low voltages.Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2019, 06:44:08 pm
content: Hit it with a hammer, and look at the shattered remnants. if it is all brittle it was ceramic in a case, but a foil type will have some plastic film with metallisation on it, and that is easy to see under low magnification of the remains.

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2019, 06:46:40 pm
content: Quote from: T3sl4co1l on December 10, 2019, 06:26:40 pmAlmost all foil caps these days yes, but there are heaps of older foil types without the side metallization on the side of the roll, so beware.I have never seen a COG cap marked with a dot. Certainly not a surface mount one. Do you have any photos of those?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 10, 2019, 07:06:29 pm
content: Most foil caps are more rectangular shape. The tricky shape is a capacitor dipped in some kind of epoxy: these may be tantalum (usually marked for polarity), ceramic (especially X7R or similar) but also foil type. Still the shape my give away the type.NP0 ceramic is rather difficult to distinguish from a good foil type by the electrical performance. 100 nF NP0 is high value and would be rare and large, unless rather new as a MLCC type. Leakage, loss and DA can be well comparable to PP film caps. The temperature dependence of NP0 ceramic should be less than most foil types.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2019, 07:21:10 pm
content: I'm assuming the context is THT, since OP mentioned "plastic cased", which I'm guessing are dipped since the boxed are almost always film (not sure right now, if I've seen boxed ceramic?)...Chips are easy, films are shiny on top, while ceramics are ceramic of whatever white to drab color; neither is almost ever marked. (Have rarely seen ceramic chips with markings; I've seen film chipsat allabout as often as marked ceramics...)Anyway, here's a variety of markings from my bin --Top: Vishay/BC 1nF (50V?) C0G, no identifier, good luck figuring it out if you salvaged it.Bottom, left to right: EPCOS/TDK 22p, generic 10p C0G, someone (possibly Xicon, I forget) 33p 50V C0G ("NP0" being the old designation, still seen frequently today), and 180p 63V TDK.I don't know if the colored top is actually standard or listed anywhere, but I've seen it often enough to suspect it's what they mean.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 10, 2019, 07:30:28 pm
content: Quote from: T3sl4co1l on December 10, 2019, 07:21:10 pmThere are, indeed, boxed MLCC. The infamous CK05 series comes to mind. E.g. - like this little doodad:https://www.newark.com/kemet/ck05bx104k/ceramic-capacitor-0-1uf-50v-x7r/dp/87F4662***To the OP: polyester film and the X7R ceramic dielectric are roughly comparable, while polypropylene film is quite similar to the NP0 ceramic dielectric (as far as AC losses and ESR go). NP0 has a much higher usable temperature, however (125C vs. 85C).

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 10, 2019, 07:34:28 pm
content: It would help if you posted some photos.But by your very brief description, they likely are stacked film types.One thing that ceramic capacitors have, specially the MLCC types, is that they have very high volumetric efficiencies. For instance, one can get a 1uF, 10V device in a 0201 SMT package!

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2019, 08:21:37 pm
content: In a decoupling application I would not worry too much about the difference. Film capacitors work just as well in decoupling applications. Ceramic capacitors are less expensive now but in the past that was not always the case.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2019, 08:27:41 pm
content: Oh yeah, of course, I've got a bunch of M39014 style ceramics.Tim

author: mcovington
author_rank: Regular Contributor
timestamp: December 10, 2019, 10:13:27 pm
content: OK, the ones I'm curious about are the blue ones here. (These are two from the same batch, showing both sides.) The red one is obviously film.The blue one looks a lot like a Murata ceramic capacitor, but I can't find "A5K" in the catalogue.

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2019, 10:15:30 pm
content: MLCC ceramic.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 10, 2019, 10:19:31 pm
content: Agree, MLCC ceramics with leads.

author: mcovington
author_rank: Regular Contributor
timestamp: December 10, 2019, 10:26:43 pm
content: How do you know?BTW, I just did some testing. Temperature stability is good (capacitance is down 1% when heated too hot to touch with a hot air gun) and there is no appreciable leakage even at 600 volts.Can anyone identify the manufacturer?

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 11, 2019, 12:49:11 am
content: As mentioned previously, a hunch based on volumetric efficiency.But could be wrong.But if you are willing to sacrifice a device in the name of knowledge, cut one in half.

author: David Hess
author_rank: Super Contributor
timestamp: December 11, 2019, 12:54:31 am
content: Quote from: mcovington on December 10, 2019, 10:26:43 pmI can tell from the form factor and size for a 0.1 microfarad part. A disc ceramic or film would be larger at that value. The film example shown is a polyester stacked metal film capacitor; they work fine in place of ceramic capacitors in decoupling and filtering applications.Both of those are constructed by welding leads to a common rectangular surface mount part and then dipping it in epoxy. If you heat the part and twist the leads, they will separate exposing the inside construction.

author: MT
author_rank: Super Contributor
timestamp: December 11, 2019, 01:15:32 am
content: I wonder how much of fan, harddisk etc vibrations creeps into pow supply via SMT decoupling cap's? Any studies around?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2019, 01:29:38 am
content: "A5K" sounds familiar and ceramicy, though I don't know what it means (it's not a type code).The shape suggests some odd ceramic, but they could be stacked film as well.The tempco is definitely(?) too low for X7R or worse. If they're PET, they'll have somewhat higher losses than PP or C0G, which you might perform an experiment on.You'd do that by setting up a few watts from a signal generator or oscillator, and tune a resonant tank to it. Most of the losses will probably be in the inductor, but there should be enough that you can feel some in the cap, and perhaps compare to known types.Quote from: MT on December 11, 2019, 01:15:32 amA few mV, give or take. Yes, search for "ceramic capacitor microphonic".Tim

author: MT
author_rank: Super Contributor
timestamp: December 11, 2019, 02:33:18 am
content: Quote from: T3sl4co1l on December 11, 2019, 01:29:38 amI im aware of the standard piezo microphonics what im wondered about was "studies" made on fans and hardisk vibrations. Surely this depends om cap material, mounting material PCB, distance of vib source, microphonic bandwidth, resonant frequencies of/on the board, etc, etc, etc.Even in Daves EVVBLOG some time ago in which he taps the front of various scopes made aHUGHEdifference and not some mV!

author: bob91343
author_rank: Super Contributor
timestamp: December 11, 2019, 02:53:21 am
content: I would guess A5K means 5 pF and 10% tolerance. Dunno about the A. But that disagrees with the other side. Still, the K tells me 10%.I recall the days with the six or three painted dots on mica capacitors. I have some and not only can the coding be confusing but the old paint changes color.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2019, 03:51:05 am
content: Quote from: MT on December 11, 2019, 02:33:18 amWell, yeah, it depends on all of those things. What good would a study be?Alternately -- are you simply looking for a claim that "measurements in [typical server rack] showed XmV of noise attributable to microphonics"? What significance would you draw from it? Is the measurement bigger or smaller than you were expecting, and if so, why? If they are able to attribute the signal to some source or another -- if indeed the signal is separable at all -- since, that's the point of such an environment, there's just a lot of stuff going on?From a systems point of view, the approach is simple -- there is only one mechanism by which voltage gets induced: strain (flexural or otherwise) of the component of interest. Knowing the dimensions and elastic modulus of the component, you can attach it to a mechanical model of the PCB, and other components and enclosure and vibration sources, and it's all just a mechanical filter network from source to target. Whatever the gain is, that is your answer.Tim

author: MT
author_rank: Super Contributor
timestamp: December 11, 2019, 05:55:32 am
content: Quote from: T3sl4co1l on December 11, 2019, 03:51:05 amI dunno, just interested.QuoteAs said not looking for claims just interested to see outcome if any, clearly there are in Daves tap oscilloscope tests,Servers i dont think it matter much so much due to clock/date noise anyway.QuoteEarth is flat?QuoteTap the front of the oscilloscope.QuoteYou excluded unknown resonances that might appear here and there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2019, 01:15:06 pm
content: What unknown resonances? They are only unknown if you didn't include them in your model!You could even take a finished product to a vibe lab, give it a shake (at lowish amplitudes so as to maintain linearity) and measure acceleration at a component (or its voltage directly). The empirical method solves any issues with modeling. (Or even, do both and adjust the model until you've matched reality, and bring those insights into the next model and so on.)Outcomes are simple for the most part: most digital logic isn't going to care, and most analog circuitry is going to have either some coupling factor (via PSRR, or such caps directly in the signal path), or some parametric effect (e.g., microphonics acting upon a compensation capacitor in a PLL, or the digital equivalent being supply variation causing jitter).The oscilloscope test is simply the former: caps in the signal path, the amplitude of which may or may not depend on range, hence you might get 10mV of noise on one range or 1V on another.Hard drive racks are sensitive to noise simply because they contain mechanical elements, extremely precisely positioned; I would guess the actuators are entirely digital control, so that there's no capacitor in their signal path. Supply noise may be a small contributor, but again it may manifest more as jitter, and the data streams are self-clocking and heavily error-corrected, so it should make little difference to that end.Tim