"Leon Heller" wrote in message ...
"Henry Kolesnik" wrote in message
...
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?

Mylar caps aren't used very much. Metallised polyester are easier to find.

Leon

Erk??!! Mylar is a DuPont brand name for polyester, actually.
"Mylar® is an extraordinarily strong polyester film that grew out of
the development
of Dacron® in the early 1950s."

Mylars (polyesters) are generally good for audio work. They can
contribute a small amount of distortion, but that's unlikely to be an
issue in ham work. They have higher loss (dissipation factor) than
polypropylenes, and the latter should be used in applications where
there is appreciable AC current such as in switching power supplies.
It's very hard to ID them from just the appearance. You may be able
to look up a manufacturer's part number, and if you wanted, you could
measure some parameter such as temperature coefficient, which
generally is a good indicator of the dielectric. But I submit that
it's not worth it! If you can identify it as a film capacitor (as
opposed to an electrolytic, ceramic, or mica), and the application
says that Mylar is good, then probably what you have will be fine.
Polyester and polypropylene are, I believe, far and away the most
common film caps, and polyprop should serve very nicely anywhere a
polyester is called for.

Sometimes internal construction is important; a design may call for a
"non-inductive" capacitor. Then you probably should buy the right
thing, to be sure, or at least measure the cap at high enough
frequency to see if it has significant inductance.

Cheers,
Tom