Polystyrene Cap replacement, best option? - Page 1
Forum section: electronics / beginners

author: Midphase
author_rank: Regular Contributor
timestamp: March 13, 2025, 06:36:53 am
content: Howdy, I did a search on this forum and I haven't found a particularly conclusive answer.I need to replace a couple of 48pF Polystyrene capacitors on a synthesizer operating at 20MHz. My options at the moment are either a Multilayer Monolithic Ceramic Capacitor of the same value, a 5% Polyester Film/Foil Box Type Capacitor, or a 5% Polypropylene Film/Foil Box Type Capacitor which unfortunately I only have in 33pF value (although it might be close enough for my needs).What would you say is my best replacement option? I'm still not 100% sure what the specific advantage is of Polystyrene vs other types of capacitors.Many thanks as always for any helpful insight.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2025, 07:17:40 am
content: Polystyrene capacitors have very low leakage and low loss (dielectric absorbtion). For the synthesizer the exact capacitor type likely does not matter. So all 3 shpuld be OK for this application. From the dielectric absorbtion PP and C0G are similar to PS, polyester is about 10 x higher loss and thus possibly an issue in some applications. A few special applications (electrometers) that need super low leakage (<< 1 pA) may still want PS capacitors - but these would also not use a normal PCB.I would go with ceramics, as 47 pF should be easy to get, both as SMD and THT form and small size. 20 MHz may want SMD to make things compact.

author: benj38
author_rank: Frequent Contributor
timestamp: March 13, 2025, 08:53:02 pm
content: Quote from: Midphase on March 13, 2025, 06:36:53 amThat is not telling us much about the exact role the capacitors play, and what are the critical parameters to try to replicate.I would add to @Kleinstein's answer that polystyrene capacitors are also known for their excellent stability (i.e., very little capacitance and DF change) over their operating temperature range, as well as vs. frequency (within limits of course).Given no further information, among the options you gave, I would say the ceramic capacitor is the best ---provided it is C0G!

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2025, 09:20:51 pm
content: Polypropylene is probably the best replacement for polystyrene, although it can be harder to find polypropylene below, say, 100 pF.Note that "Multilayer Monolithic Ceramic Capacitor" is not an adequate description for that replacement: MLCC is a construction, not a dielectric.47 pF is likely to be C0G/NP0, which is a good dielectric. X7R, Z5U, etc. are inferior to C0G for signal purposes.A C0G ceramic capacitor is a suitable replacement.One difference between these replacement types that may not be important is the temperature co-efficient.Polystyrene had a tempco of roughly -125 ppm/K, polypropylene is similar at about -100 ppm/K (depends somewhat on manufacturer and construction).C0G is close to zero tempco.Polyester (PET) has mediocre loss (dissipation) and dielectric absorption: PS, PP, and C0G are all excellent.Polystyrene is still available, but very hard to find new. Its main drawback is low service temperature (only 85oC, requires careful soldering).

author: floobydust
author_rank: Super Contributor
timestamp: March 13, 2025, 09:33:27 pm
content: Mouser carriesXicon 23PSpolystyrene caps but 100pF is their lowest value I think.I'm suspicious of a 47pF polystyrene part, if it's been correctly identified?It certainly wouldn't be doing anything digital and in the audio chain, ceramic disc NP0/C0G parts would be used.Unless it's some audio filter where the cap dielectric can make a sonic difference.

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2025, 09:37:11 pm
content: I'm surprised at "48 pF", since 47 pF is a popular standard value.I do not concern my self with "sonic signatures", but the dielectric loss of polyester foil and bad ceramic dielectrics is a measurable problem.

author: floobydust
author_rank: Super Contributor
timestamp: March 13, 2025, 09:43:30 pm
content: In digital audio, we are down to the ppm level for THD so capacitor dielectric type does show up.PS, mica, C0G are among the best. But through-hole parts have significant ESL so that is part of it too.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: March 13, 2025, 10:33:49 pm
content: Polystyrene caps are very reliable. Are you sure they are faulty?SJ

author: Midphase
author_rank: Regular Contributor
timestamp: March 14, 2025, 12:07:45 am
content: Quote from: TimFox on March 13, 2025, 09:37:11 pmSorry...typo. It's a 47pF cap.

author: Midphase
author_rank: Regular Contributor
timestamp: March 14, 2025, 12:17:43 am
content: Quote from: Solder_Junkie on March 13, 2025, 10:33:49 pmThis type of thinking is why I ended up spending an inordinate amount of time diagnosing the issue...I simply didn't want to believe that the fault was in the polystyrene caps just because they're so known to be reliable. Unfortunately, after several days at poking around everywhere but there, I finally decided to pull them and sure enough at least one of them had failed.The purpose of those two capacitors is, from what I can best tell, to provide a reliable sync timing to the synthesizer circuit in order to keep the tuning stable.At the moment I plugged two ceramic capacitors of the correct value and they seem to be working fine. I'm going to keep the synth on for a few hours to see if there is some tuning drift, but I get the sense that what I have in there is probably ok for what they need to do. I also had a couple of 5% Polyester Film/Foil Box caps handy which is why I was asking in case they might be better, but from my tests the ceramic ones seem to work best.I was considering ordering some C0G's, but I hate paying $15 worth of shipping for $.50 worth of parts and at the moment I don't really need anything else so if what I have in there work well I'm leaning toward just sticking with those.For those curious, here is the schematic of the circuit (C18 and C19 are the two troublemakers):

author: floobydust
author_rank: Super Contributor
timestamp: March 14, 2025, 01:02:31 am
content: OK it's a Korg Poly 61? maybe thisKLM-477B?I see a two transistor multivibrator making CLK2. I would say C18, C19 the PS is for stability over temperature and precision. 1% resistors yet no trimpot here, and pitch bend joystick modulates power to it, like a VCO.So a C0G cap would be best here, but if you don't mind pitch drift I guess use less. Put it on the list or parts to get from Digi-Key lol.Polystyrene caps can't take soldering (process) heat and it can cause them to melt or years later, crack.

author: Midphase
author_rank: Regular Contributor
timestamp: March 14, 2025, 02:36:27 am
content: Yeah, I'm testing with the ceramic ones and so far the pitch seems pretty stable. Next time I order parts I'll add some higher grade caps so that I have them. There's always that one component that I wish I had, and tons of other components which I do have and that I never seem to actually need! ;-)

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2025, 02:46:28 am
content: Which ceramic type did you use?At 47 pF, they are likely C0G.("MLCC" is not an appropriate answer.)

author: floobydust
author_rank: Super Contributor
timestamp: March 14, 2025, 02:57:55 am
content: Quote from: Midphase on March 14, 2025, 02:36:27 amThe nuisance is possibly the instrument going out of tune during a live performance. This can happen from the temperature going up as the instrument warms up on stage. You can check tuning with a bandmate in the middle of the gig.I think PS was just the most stable cap (dielectric) they had at the timeNPO/C0Gwas rare in Japan back then I never saw anything better than Z5U or Y5V which are junk for oscillator circuits.I would use a frequency counter to calibrate the oscillator - the bend up/down and coarse tune trimpots, after the new caps are in.

author: wraper
author_rank: Unknown
timestamp: March 14, 2025, 03:21:28 am
content: Quote from: floobydust on March 14, 2025, 02:57:55 amNobody ever produced low voltage 47p capacitors with those shitty dielectrics. Low capacitance ceramics usually were worse than NP0 but not even close as bad as X7R, not to say Z5U.

author: floobydust
author_rank: Super Contributor
timestamp: March 14, 2025, 04:43:56 am
content: I said that based on board pics, the low cost disc caps used. Mystery meat?The "104Z" I get but no mention anywhere about the small ones, what their dielectric is. 47pF disc is only used for comp caps in the synth. I guess you'd have to look at the writing what's on them. They are low cost.

author: Midphase
author_rank: Regular Contributor
timestamp: March 14, 2025, 06:02:09 am
content: Quote from: TimFox on March 14, 2025, 02:46:28 amThis is what I have in there right now:https://www.taydaelectronics.com/capacitors/monolithic-ceramic-capacitor/47pf-50v-multilayer-monolithic-ceramic-capacitor.htmlThe other alternative that I could use are these guys:https://www.taydaelectronics.com/47pf-1000v-5-polyester-film-foil-box-type-capacitor.html

author: wraper
author_rank: Unknown
timestamp: March 14, 2025, 06:27:19 am
content: Part number starting with CT4 means class II dielectric = shit.

author: squadchannel
author_rank: Frequent Contributor
timestamp: March 14, 2025, 06:36:36 am
content: Polystyrene caps were often used in older japanese amplifiers(LUXKIT, SANSUI etc...). low leakage current and good sounds(for audiophile).I would replace them with polypropylene; WIMA's PP caps will work just fine.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2025, 07:50:11 am
content: Quote from: Midphase on March 14, 2025, 06:02:09 amI am surprized to see that they make 47 pF with X7R class dielectric. That would not be good, could still work in this circuit, as the transistors own capacitance is also nonlinear and chances are that for such a small capacitor the dielectric is not super thin, which makes it more nonlinear. C0G would be the right choice and is also available.

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2025, 02:41:49 pm
content: Quote from: Midphase on March 14, 2025, 06:02:09 amI was afraid of that: X7R is not suitable for a frequency control.Mouser has 47 pF C0G MLCC for $0.024 each in quantity 10, no need to pinch pennies further down to $0.02 each for inferior components.

author: Dave Wise
author_rank: Regular Contributor
timestamp: March 23, 2025, 12:59:36 am
content: Quote from: Kleinstein on March 14, 2025, 07:50:11 amI am head-explode astonished.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 03:29:32 am
content: $0.004 saved is $1/250 earned.

author: wraper
author_rank: Unknown
timestamp: March 23, 2025, 03:44:48 am
content: I suspect the might be not actual X7R but something a lot better. In the past quite a bit of types existed in between of C0G and X7R (such as N33...N150...N1500), but in the last few decades nobody bothers with those. So it might be they're calling whatever that is worse than NP0 as X7R.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2025, 09:00:02 am
content: Polypropylene and C0G/NP0 ceramic are a good replacement for polystyrene in critical applications. A pair of 100 picofarad polypropylene capacitors in series will be 50 picofarads.Quote from: benj38 on March 13, 2025, 08:53:02 pmIt is not that they have good stability, but that their temperature coefficient is reliably about -120 ppm/C which can be compensated for.

author: MarkT
author_rank: Frequent Contributor
timestamp: March 23, 2025, 09:40:05 am
content: Polystyrene are stable compared to many types of film caps because they don't absorb moisture and are below their glass-transition temperature (i.e. essentially crystalline, not random-chain), this stability was valuable for tuned circuits for instance. Polystyrene have basically vanished as they are not surface-mount compatible due to low melting point.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2025, 01:02:36 pm
content: Quote from: MarkT on March 23, 2025, 09:40:05 amThe last manufacturer of suitable polystyrene film shut down around 1996.Recently I was shown a memo ex­plaining that all manufacture of poly­styrene capacitors will cease in five years, because the only manufacturer of polystyrene film stock has quit the business. If your circuits really need polystyrene, you should contact your favorite capacitor supplier to make your one-time buy now. Then start redesigning your circuits. - Jim Williams (1996)Teflon and polystyrene dielectrics are even better but the Real World intrudes. Teflon is expensive and excessively large at 1μF. Analog types mourn the imminent passing of the polystyrene era as the sole manufacturer of polystyrene film has ceased production. - Jim Williams (2009)

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 02:42:51 pm
content: Quote from: wraper on March 23, 2025, 03:44:48 amN150, N220, etc. are still Class I dielectrics. They have substantial negative temperature co-efficient of capacitance, and somewhat higher loss than NP0 (nominal zero tempco), but don’t have bad voltage co-efficients like the Class II dielectrics (X7R, etc.).Ferroelectric dielectrics have much larger dielectric constants than the linear Class I materials, and can therefore be made smaller for a given capacitance.The N-series caps (in single-layer discs) were used to temperature compensate oscillators; I don’t think I’ve seen one in MLCC construction.In discs, they are available with higher capacitance than NP0 for a given case size.

author: wraper
author_rank: Unknown
timestamp: March 23, 2025, 03:57:24 pm
content: Quote from: TimFox on March 23, 2025, 02:42:51 pmYes, but nothing prevents you calling them as class II either because class does not specify a minimum boundary of how bad it must be.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 04:28:48 pm
content: No.Class I and Class II dielectrics are well-defined technical terms, related to the non-linear behavior of the high-dielectric-constant materials used in Class II capacitors as a trade off between physical size and electrical performance.The “N” series of Class I dielectrics (not so common now as in the higher voltage disc capacitor heyday) do not exhibit the non-linear behavior of Class II ferroelectric materials, but have stronger temperature dependence than NP0/C0G Class I units.

author: wraper
author_rank: Unknown
timestamp: March 23, 2025, 05:38:33 pm
content: Quote from: TimFox on March 23, 2025, 04:28:48 pmClass II does not specify itmustexperience non-linear behavior.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 06:02:17 pm
content: Quote from: wraper on March 23, 2025, 05:38:33 pmClass II dielectrics exhibit non-linear behavior. Name one that doesn’t. They are a different class of materials from Class I, and are used in practice due to their much larger dielectric constant (if not strictly constant) to achieve useful smaller physical sizes. It is analogous to the difference between paramagnetic and ferromagnetic magnetic materials.

author: wraper
author_rank: Unknown
timestamp: March 23, 2025, 06:15:47 pm
content: Quote from: TimFox on March 23, 2025, 06:02:17 pmYou can call whatever non-linear or linear ceramic as class II as long as it falls within the limits of class II. Nothing prohibits you from producing C0G capacitor and selling it as X7R. The same as you can produce 100V capacitors and sell them as the whole range rated for 6.3V and upwards. My argument was that since nobody is selling anything in between of C0G and X7R for decades, it could easily be anything in between in practice. In fact it may be problematic to produce 47p while using high dielectric constant ceramic.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2025, 06:23:18 pm
content: Quote from: wraper on March 23, 2025, 06:15:47 pmOf the many manufacturer's app notes, this one shows the different Class I dielectrics (with different tempcos) and the usual Class II dielectrics, reminding that Class II dielectrics use ferroelectric materials.Ferroelectric materials are inherently nonlinear, including their tempco.https://www.ufcapacitors.com/3854.htmlFor example, "N750" US military type Class I dielectric (-750 ppm/K) is called U2J in the current typing that includes C0G (Class I) and X7R (Class II).To quote that appnote:"Class II ceramic capacitors, formerly known as low-frequency ceramic capacitors, use ferroelectric ceramics as their dielectric material, hence they are also called ferroelectric ceramic capacitors. These capacitors have a higher capacitance, nonlinear capacitance change with temperature, and higher losses. They are often used in electronic devices for bypassing, coupling, or in circuits with low requirements for losses and capacitance stability. Class II ceramic capacitors are further divided into stable and usable grades. X5R and X7R belong to the stable grade of Class II ceramics, while Y5V and Z5U belong to the usable grade."From a "chattier" appnote from Kemet/Yageo:"Ceramic Capacitor PhysicsTemperature coefficients and tolerance over a temperature range is all fine and dandy, but explaining the next effects fully requires a bit of a dive into the physics and even chemistry of the dielectric material itself. Strap in, this is going to get interesting. It is all about the dipoles.A lot of the magic of a capacitor comes from the dielectric material itself. Some people would describe a dielectric as an insulator that prevents the two electrodes from shorting. That is true, but there is a bit more to dielectrics than just that. In a word, dipoles. A quick Wikipedia search will show that a dielectric is that it is “an electrical insulator that can be polarized” with the application of an external electric field. A piece of rubber is a great insulator, but it is a terrible dielectric. You can’t polarize rubber (very effectively). It is the presence of those dipoles in the dielectric material that makes for an effective capacitor. KEMET uses two main types of materials for ceramic dielectrics. Are you ready for some phrases that are going to take you back to chemistry class? First off is barium titanate (BaTiO3), which is used for our Class II/III dielectrics. Those are our X5Rs and X7Rs, among others. Next is our calcium zirconate, that’s what we use in our class I dielectrics. That would be C0G and U2J. This is where things get really interesting, calcium zirconate is paraelectric and barium titanate is ferroelectric. Those properties have some similarities to the concepts of paramagnetism and ferromagnetism, which is introduced in early physics classes.In ferroelectric materials, the dipoles are permanently present and will align themselves with an electric field. In paraelectric materials, the dipoles appear spontaneously aligned with the application of an external electric field. The dipoles created by class II dielectrics are a result of the materials and structure of the barium titanate itself....The effects brought upon by the ferroelectric nature of class II dielectrics has impacts to engineering and circuits that rely on class II capacitors. The so-called DC-bias effect, microphonics, and aging are all due to the dipoles created by the displacement of the titanium atom in barium titanate."

author: floobydust
author_rank: Super Contributor
timestamp: March 23, 2025, 06:26:17 pm
content: Quote from: David Hess on March 23, 2025, 01:02:36 pmPolystyrene caps are still made by Xicon, see23PSseries sold at Mouser as I mentioned in Post #4.

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2025, 01:09:36 am
content: Quote from: floobydust on March 23, 2025, 06:26:17 pmI saw that the Digi-key marketplace also has them, but the range of values is very limited.I think I remember reading somewhere that polystyrene film was being produced again.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2025, 08:29:58 am
content: There are still a few PS caps available, but there is very limitd need for them. For most cases C0G MLCCs are a better alternative (smaller, lower TC and SMD solderable). It is only for a very few cases where super low leakage (<< 1 pA) is needed that I would still consider PS capacitors. They are sensitive to solvents (even IPA) and heat and are thus best avoided if not absolutely needed.

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2025, 10:34:26 pm
content: Quote from: Kleinstein on March 24, 2025, 08:29:58 amI used a 150 picofarad C0G MLCC in my picoammeter as a feedback capacitor seeing 1 millivolt per picoamp. I just measured another one and the leakage was below 10 femoamps at 3 volts, the limit that I can reliably measure, so I wonder if film capacitors are really better. A random 150 picofarad disc ceramic was as good.Looks like PPS (polyphenylene sulfide) are available only down to 1000 picofarads, but Kemet has PP (polypropylene) down to 100 picofarads, WIMA down to 33 picofarads, and Vishay down to 100 picofarads.I guess I should invest in a set of low value PP capacitors, if only for comparison purposes.