In article , "Henry Kolesnik"
writes:

I was at a hamfest looking for some mylar caps for a circuit that called for
them. I got several opinions on how to tell one and finall gave up. Are
there any sure fire ways to identiy a cap as mylar and why are they bettter
than a silver mica or others. Does anyone have a short chart comparing the
characteristics? How did we get by before mylar was used?
tnx
hank wd5jfr

I would suggest looking at the Cornell-Dubilier or Illinois Capacitor
catalogs (both have websites, too). There you will find out more interior
whys and wherefores of capacitor types and their insulation and
construction.

The "wound" (as in winding) type used to be aluminum foil in a paper
strip sandwich. Mylar film was substituted for paper for better
environmental performance. Other plastic film types are used such as
polypropylene and polystyrene, even Teflon. With development of
deposited metalization, the aluminum foil was replaced with direct
deposit of metal on the film...which cost less to produce.

There are some notable differences in dielectric material versus temp-
erature performance with both positive and negative temperature
coefficients possible depending on material _and_ construction. You
have to see manufacturer's literature to get full details on that.

There is no real identification of the interior construction based on the
exterior appearance, lead placement, or color of the outside. Those are
all manufacturer's choices and there is no EIA standard on esthetics.

Mylar capacitors, as all the other film types, generally have higher
insulation resistance than paper, thus they are more suitable for tube
circuits' or FETs' high impedances. High withstanding voltages (300
VDC and more) are fairly easy to get with both paper and film types,
but that also leads to large sizes.

Silver-mica (usually called "dipped mica" due to the exterior coating)
is generally better above a MHz due to higher Q...but many ceramic
dielectric capacitors (very high dielectric constant) can be just as good.
The "ceramics" can be made with "zero", positive, or negative
temperature coefficients...and the common lower-voltage bypass-use
types have a high negative tempco but are cheaper than most. All of
those have good Q at a MHz and higher compared to the wrapped
paper/film variety.

There are two basic types of wrapped (actually rolled) capacitors. One
has the conductring material within the width of the paper or film...the
other (almost always using aluminum foil) has each end extending out
from the dielectric so that the leads can have all of the foil crimped on
them. The latter results in a much lower internal inductance compared
to the former and is a very important consideration for "RF" circuits.

With the miniaturization of SMT, the "co-fired" construction is leading to
very small high-capacitance values where metalized ceramic plates are
fired together in sandwiches. See the "Blue Cell" trademark construction
method used by several, including Mini-Circuits. In some cases entire
stripline structures, including capacitors, can be made as one unit.

Capacitors are chosen for a circuit depending on their capacity, working
voltage, temperature coefficient, Q, insulation resistance, lead placement,
and general characteristics. You should KNOW the circuit and what it
needs to determine what you can use...or follow some article's description
precisely, putting your faith in the author and editors. :-) [I've been
both
author and editor...]

Len Anderson
retired (from regular hours) electronic engineer person