Film capacitors

[jheatac]

Are the mylar film capacitors the same as the plastic box capacitors just in a different package?

Are the mylar film capacitor colors indicative of anything universal? Or is just a manufacturer's choice?

Thank you!

[TimFox]

There are different plastic films used to make film capacitors.
The popular ones are polyester (Mylar is a trademark for polyethylene terephthalate) and polypropylene.
Others include polycarbonate, polystyrene (now almost obsolete), and polysulfone.
Polyester is what I call a mediocre dielectric, and polypropylene is one of the best dielectrics (low dielectric loss and dielectric absorption).
However, since polyester has a higher dielectric constant, polyester film capacitors are usually physically smaller than polypropylene (for same voltage rating and capacitance).

[golden_labels]

And to answer the other end of the question: packaging (coated or a plastic box) is determined by how well the capacitor should be protected from moisture. Boxed types provide greater protection at the expense of compactness. Coated ones are only wound capacitors, while a box can accomodate both wound and stacked layouts.

To add to the mix, there are also axial capacitors. And in this case you have a good example of identical technologies just packaged in a different way: CBB60 and CBB61 motor start/run caps.

[David Hess]

And to answer the other end of the question: packaging (coated or a plastic box) is determined by how well the capacitor should be protected from moisture. Boxed types provide greater protection at the expense of compactness. Coated ones are only wound capacitors, while a box can accomodate both wound and stacked layouts.

Epoxy coated capacitors may be wound or stacked.

[T3sl4co1l]

I'm not aware of any consistency with color.

If you have random (salvage/surplus) parts, you may have to test them to identify.  PET has modest Q (30s) at most frequencies, and a strong negative tempco (Q drops as temp rises).  PP has high Q (100+) and flat tempco.

C, Q and tempco (of C and Q) have specific measurement methods; you usually have a good-enough C test on DMMs, but Q usually requires an impedance bridge or RLC meter.  An impedance bridge is easy enough to construct with standard lab equipment (i.e. signal generator and scope).  Tempco can be measured as crudely as pointing a heat gun at the part, or more precisely by immersing in a liquid of measured temperature and letting it soak.

All film caps are acceptable for signal/timing applications, assuming you don't need anything special like precision timing, or low absorption (S&H etc.).  Precision and low-loss applications, you'll have to do more checking to tell, or, just buy good new ones.

Boxed types are usually PP (MKP), but MKT (PET) also exists, check the markings.  You can often find datasheets for parts these days, even some seemingly no-name Chinese types.  Most of the markings will be regulatory (safety) approvals and specifiers, recognize them and

There's also metallized paper, usually in a clear/gold molded or white box; Q is modest.

Other dielectrics are uncommon in leaded parts I think, but the other ones are good quality (e.g. PPS) so it's not a bad thing.

There are also dipped leaded ceramic chips, in various small and medium outlines that may look like small film caps.  Ceramic of small values may have low tempco and good performance in general (C0G type), while larger values tend to be much more dubious (Q ~ 20, value declines sharply with temp >85°C, and with DC bias(!)).  How much so, depends on the relative size, rating and type (Y5P, Z5U, etc. worse than X7R, etc.).

Round ceramic disc types, and occasionally square, are distinctive by shape, finish, lead position, markings, etc.

Tim

[Gyro]

In addition to MKP (Metalized Polypropylene), you can also get KP (Polypropylene Film and Foil) in 'plastic box' format. These have better RF performance and pulse current capability than MKP. Because of the foil thickness, they have lower capacitance values for a given package size than MKP. I don't think anyone ever bothered to do a foil version of the lower quality PET dielectric, so I think only MKT exist there.

If looking through collections of old capacitors, you may also come across MKB, which are Metalized Polycarbonate, usually by Wima. In dielectric quality terms they come between PET and Polypropylene. They are no longer manufactured, mostly overtaken by MKP. They do have a useful temperature coefficient curve though that could flatten the overall TC when used in parallel with other film capacitors, useful in some RC oscillator applications.

[Haenk]

Since we are at this topic - one circuit I want to recreate asks for a (tiny) PPS cap - not only are these rare, but I struggle to find the (low) capacity.
Any opinion of exchanging it with a mica or PC type?
(Only audio application, so actually no high temp range or frequency range and lowish voltage...)

[TimFox]

If you are looking for good audio capacitors in the, say, 100 pF range, NP0 disc capacitors are a good choice for high voltage (say, 500 to 1000 V working).
C0G MLCC capacitors (same material) are also suitable in their voltage range (100 V or below).
Do not use X7R or other non-linear ceramic dielectrics (e.g., Z5U).
Otherwise, you may still be able to find small polystyrene THT capacitors (probably new-old-stock) or silver mica.