[R-390] Source for axial caps

[Bob Camp]

Hi

Most of the polystyrene parts I have seen have been in clear cases. 
They tend to be low value parts. I don't think I have ever seen a 0.1 
uf polystyrene part. Polystyrene is what your good old model airplane 
kits were made out of. The stuff has a pretty low melting point.

The two common plastics in use for capacitor dielectrics these days are 
polyester and polypropylene. Polycarbonate used to be an option but 
then the last guy making the film went out of business. Teflon is 
another option that is out there but it's so expensive you should not 
consider it for the kind of stuff we do. Mylar is a trade name for 
polyester so it does not count as a separate material.

Polyester is usually a 105 to 125C rated material. Polypropylene is 
commonly rated from 85 to 105C. Packaging can make a bit of a 
difference as can the size they are trying to achieve on the part.  The 
guys that go super compact on the parts seem to rate them a bit lower 
in temperature . The temperature is often a "rated" temperature rather 
than a "failure" temperature. As long as you are not putting a lot of 
AC current through the parts (and we don't) *and* you de-rate the 
voltage by a bit you can use both polyester and polypropylene up to the 
125 or 105C temperatures.

A quick check on all this is at http://www.illcap.com/Film.asp . The 
page applies to Illinois parts specifically, but most of their 
competitors rate parts the same way.

Orange drop capacitors are available in both dielectric materials. The 
715 series that most of the audio guys like has the polypropylene 
dielectric.

The polypropylene material usually results in a capacitor that is 
larger than the equivalent polyester part. The polypropylene part will 
normally have a lot less loss for a given size part (0.05% versus 1%) . 
  The only advantage to the polyester part is that it's smaller size 
will normally give you a lower inductance. What ever gain you get from 
lower inductance normally is wiped out by the higher loss of the 
polyester material.

In a bypass application the fact that the polyester part has an 
impedance 1% higher than the polypropylene part hardly matters at all. 
There is no reason to pull out parts you already have installed. If you 
have a choice *and* they will fit then use the polypropylene parts.

How important is all of this? Well I for one don't pay any attention to 
it at all. I pretty much use the two types of capacitors 
interchangeably in R390 rebuilds. Epoxy coating is nice, but keeping 
the soldering iron away from capacitors isn't all that tough. I often 
can succeed at it three times out of four.

	Take Care!

		Bob Camp
		KB8TG




On Aug 1, 2004, at 11:44 AM, Barry Hauser wrote:

> Ooops?!
>
> Bob wrote:
>> Provided the voltage ratings are adequate and you don't get parts that
>> are only rated to 85C (like polystyrene) just about any modern plastic
>> part will do a lot better than the paper parts it's replacing.
>
> I've stocked up on some yellow poly's here and there -- don't remember
> whether they're polystyrene or polyethylene polypropylene or what.
>
> What are the typical temp ratings of the various yellow axials?
>
> Barry
>
>