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Polystyrene capacitors in parallel - advantages?

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Hi!
I need some expert opinion about connecting two or more polystyrene capacitors in parallel. I must use one 33nF polystyrene (or styroflex) capacitor in an oscillator (for ESR meter) in combination with 100μH inductor. Since I don't have 33nF styro, I used WIMA-FKP1 (poliypropylene) 33nF/2000V, and I got stable oscillations, with some output (i.e. "test") voltage generated about 300mV - that is just for comparison - it is good value but in my case "the higher is better" - so I strive to achieve some optimal 400mV. I tried mylar (polyester), but I got (again, don't bother with this value), for comparison, barely 150 mV. Than I collect three of my largest polystyrenes of different values from some 7nF to 16nF IN PARALLEL (to match 33nf) and got surprisingly high voltage of 400mV - exactly what I want! I don't have enough place for that number of capacitors, so I wonder, should I use one 33nF styroflex capacitor OR, to deliberately use 2 or more styrofex in parallel in order to achieve better overall results? I know that total ESR will be lower, but what about other features (like total inductance)? It is hard to find 33nF styroflex capacitor, but I can (pretty cheap) find 10nF, 15nF or 22nF, so, maybe, I could achieve even better results with e.g. 3x10nF than with one 33nF cap?

TIA!
 
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Hi,

The disadvantages to connecting caps in parallel is the extra leakage due to two or more leakage currents instead of one. However, this might not matter in most oscillators that dont have to be super efficient. It would matter more in an application like a sample and hold circuit where we'd want minimum droop over some maximum time period.

In your case the best answer is probably to actually try this yourself and make some more measurements. As you noted there are a number of factors involved which are hard to predict exactly for what you are looking for without actually trying different things. It should not cost you too much to try, and if the application is important enough to you then you wont mind spending a couple extra bucks anyway.
 
Thank you MrAl!

Sorry, I'm not some expert in this area:eek:, but what "super efficiency" in my oscillator means in "real life", i.e. how can I evaluate my circuit with 2 or more capacitors in parallel? So far I didn't notice any "fluctuations on display" (LCD shows rock-solid 0,00pF for a long period, for example). It is not about money, but I can't find easy (even from ebay) 33nF styroflex where I live, so I'm "forced" to use combination.
I will attach schematic of my LC/ESR meter if that helps.
View attachment 63828
 
The disadvantages to connecting caps in parallel is the extra leakage due to two or more leakage currents instead of one.

That is not true.

The same leakage current applies, not matter if the value is a single unit or two parallel units.

The reason why styro is suggested in this design is purely because they have a better tolerance. For no other reason in this design.
Don't be misguided by incorrect suggestions.
 
colin55 said:
The same leakage current applies, not matter if the value is a single unit or two parallel units.
I hope you're right! I repeat: I'm not some expert, and your claim that leakage current is the same sounds strange to me - if we imagine two "leakage resistors" in parallel it should be like two resistors in parallel i.e. smaller "leakage resistance" and thus higher total leakage. BUT, I can confirm - I measured a few capacitors with known leakage, separate and than in parallel (even 3 or more) and leakage DIDN'T became any higher! Maybe it is strange, but it is so! (IMO)
colin55 said:
The reason why styro is suggested in this design is purely because they have a better tolerance. For no other reason in this design.
What do you mean under "better tolerance"? You mean tolerance of the nominal value, e.g. 10nF/2% or better stability (tolerance to temperature change etc)?
I repeated experiment with 3xstyro-configuration, just in case, and I got (again) unbelieveable good results compared to polypropylene for example! With same inductor I got much better oscillating circuit, very stable! Only to mention, mylar (polyester) capacitor gave very, very poor results!
 
That is not true.

The same leakage current applies, not matter if the value is a single unit or two parallel units.

The reason why styro is suggested in this design is purely because they have a better tolerance. For no other reason in this design.
Don't be misguided by incorrect suggestions.

*That* is not true. Two parallel units will each have their own leakage current, which adds up to twice as much as one unit when they are in parallel. Not too hard to understand this concept.

The bottom line however is to try it in the application and see what happens. That's the best way.
 
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Two parallel units will each have their own leakage current, which adds up to twice as much as one unit when they are in parallel. Not too hard to understand this concept.

You are obviously not understanding the problem.

Suppose 15n has 1uA leakage.
Two in parallel will be 2uA.
A 30n will have 2uA leakage.
The result is the same.
 
Hi Colin,


Well that's quite amazing how you can tell what the leakage is for each capacitor just knowing the capacitance alone and not knowing the voltage rating or the manufacturer.
Case in point, circa 1980: I was informed by the Chief Engineer not to use more than one capacitor for any of the sample and hold circuits because the droop rate goes up due to increased leakage with more than one cap. That's one application where leakage is often critical. So perhaps you should argue with him not me :)
There is also the absorption factor which would also effect droop rate and efficiency, as well as hysteresis.

Luckily, this actual application probably wont suffer too much but i think a test is a good idea anyway.
 
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Polystyrene caps primary claim to fame is low ESR / low dissipation factor, and very low leakage current. We used them for PLL loop filters.

As with all caps, not all polystyrene caps are created equal. The low leakage is dependent on good sealing from humidity intrusion. Good poly caps use aluminum foil for plates.

They are also succeptable to damage during soldering.

Generally, the higher the voltage rating the thicker the polystyrene film and thus the lower the leakage. If you want lowest leakage use a cap with a voltage rating several times the actual circuit voltage.
 
They are also susceptible to damage during soldering.

Generally, the higher the voltage rating the thicker the polystyrene film and thus the lower the leakage. If you want lowest leakage use a cap with a voltage rating several times the actual circuit voltage.

This is the method I also use when selecting caps for low leakage applications.
The small increase in physical size can easily be allowed for.
 
Hi Colin,


Well that's quite amazing how you can tell what the leakage is for each capacitor just knowing the capacitance alone and not knowing the voltage rating or the manufacturer.
Case in point, circa 1980: I was informed by the Chief Engineer not to use more than one capacitor for any of the sample and hold circuits because the droop rate goes up due to increased leakage with more than one cap. That's one application where leakage is often critical. So perhaps you should argue with him not me :)
There is also the absorption factor which would also effect droop rate and efficiency, as well as hysteresis.

Luckily, this actual application probably wont suffer too much but i think a test is a good idea anyway.


Leakage for the most part is per capacitance

The leakage through the packaging and outer shell and through the contaminates on the surface of the capacitor is an unknown.
Also, since the volume scales faster than the surface area, bigger caps will have theoretically at least lower leakage.

Film capacitors these days have such low leakage rates they do not matter for oscillator duty, because the dissipation factor exceeds the leakage by a few orders of magnitude.
 
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Hi,

There are other factors too that enter into the picture which are not going to be very predictable, and the main point i was trying to get at was that a test would be best. This should cover the field even if the cap manufacturers, voltage ratings, dielectric thickness, etc., are different. No doubt that theoretically leakage follows surface area, but that's also with a constant dielectric thickness and even that's in a perfect world.
 
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