ID cap type by opening it up?

[Freq Band]

Is it possible to determine a film cap's dielectric by cracking one open?

Here is a cap, and cannot find any info about. I am hoping it may be a polystyrene.
I did crack one open the other day, and it has the typical silver-coloured wrapping...so, are there any sure-fire clues ?

=FB=

 

[JohnRoberts]

Why not test it for DA, DF, and if it tests good it is good,,,

The .1uF PS caps I have seem a little fatter and shorter but it hard to tell from pix.

Compare them to mylar and if they look better in typical S/H type application maybe they are.

I vaguely recall some old WJ articles with bench tests for DA in electrolytics. It may be tougher to parse out differences between better film types.

JR

PS I typically used polypropylene for .1uF and above due to size and difficulty getting big PS.

 

[Freq Band]

Why not test it for DA, DF, and if it tests good it is good,,,

The .1uF PS caps I have seem a little fatter and shorter but it hard to tell from pix.

Thanks John.
Actually the wrap inside is "shorter/fatter"....a large portion of the ends are epoxy filled.

=FB=

 

[PRR]

> the typical silver-coloured wrapping

Make a sandwich of kitchen aluminum-foil and plastic-wrap.

The "silver" is meaningless. Except: if you can peel it off, it is foil; if it is permanently stuck to the plastic, you have "metalized" (they spray molten metal on the plastic). In a few uses this makes a difference.

Look at the -plastic-. What kind is it?

Well, this is a plastic-ridden world yet few of us know one from another.

Most of the bulk-plastic tests, like scratch-color, are hard to apply to super-thin metalized films.

Melting point is one clue. Need a good stable skillet and very good temperature measurement, since all the likely suspects have melting-range at similar temperature.

Hot polyethylene smells like candle wax. Cellulose Acetate smells like vinegar. But don't get your nose too close: many of the plastics make toxic fumes when hot.

To your point: polystyrene makes a very distinctive sooty flame when burned. Hard to describe, but once you seen it on known-polystyrene (plastic model cars, old Bell telephones) you know it anywhere.

http://www.modernplastics.com/how_to_identify_plastics.htm

 

[Freq Band]

Now we're talk'n.
Opening this capacitor up, it is a rolled sandwich of a clear plastic film, and seperate layer of metal foil.

Here's a chart for testing plastics.
http://www.consultekusa.com/pdf/Tech%20Resources/New%20ID%20chart%20.pdf

A sample from my capacitor....the plastic layer sinks in water, eliminating polypropylene, and leaving polystyrene or polyester as suspects. (I put a little dish-soap in the water,  because surface tension will make anything this thin, float.)
It burns sooty, yellow, with no blue flame, and will not extinguish itself once up to temperature.

But when burning it....I said to myself..."I know that smell."....then it came to me....that is the exact smell when cutting white expanded polystyrene foam with a hot-wire (typical white bead foam). We do that at work sometimes, and it smells nasty.
My sample melts readily with acetone, as does the white foam.

Can we consider this polystyrene ?

=FB=

 

[Freq Band]

BTW,  polystyrene's usually have one side marked. (these do not)
There was a thread in the old forum, discussing the  differences in the way various film caps are oriented, if they include a marked lead, and the preferred orientation in a circuit....

...anybody have that info?

(I will be using these caps in an active filter)

=FB=

 

[Svart]

I have polystyrenes and they aren't marked for direction..  ???

I don't think I've ever seen a film cap of any kind be polar..

 

[dabo]

The caps were made by Southern Electronics Corp. (Burbank, CA) The "PS" designation is for polystyrene. The molded shell was an option they offered for film/foil caps.
Southern / F-Dyne was purchased by Electrocube in 2007.

 

[Freq Band]

There's a confirmation I'll raise my mug to...

cheers... "clink"

Thanks.

=FB=

 

[mcs]

I have polystyrenes and they aren't marked for direction..  ???

A lot of the ones I have are marked. The right side is usually the outside foil side on the Europeans at least.

I don't think I've ever seen a film cap of any kind be polar..

They aren't

Best regards,

Mikkel C. Simonsen

 

[JohnRoberts]

BTW,  polystyrene's usually have one side marked. (these do not)
There was a thread in the old forum, discussing the  differences in the way various film caps are oriented, if they include a marked lead, and the preferred orientation in a circuit....

...anybody have that info?

(I will be using these caps in an active filter)

=FB=

Most caps in filter circuits have one end tied to ground or a low impedance (like opamp output). Connect the outer wrap to the low impedance circuit node and you will get some self-shielding.

JR

 

[Svart]

Connect the outer wrap to the low impedance circuit node and you will get some self-shielding.

Yeah now that you mention that, I remember reading about that somewhere.  I'm not sure how much it could buy you though.