gregor said:
Is there a source of information you can recommend that deals with
the more escoteric aspects of component selection?
I know what a transistor is and what it does but is there a recommended
purpose for metalic vs carbon film type?
The same with capacitors. When do you use ceramic, taladium, electrolytic?
The main reason I ask is that often components mentioned in construction
articles are very specific as to type.
Sometimes these components are no longer available or difficult to obtain.
I'd like to know when and how substitutions can be made.
The answer is usually "it comes from experience". At 61 I don't have a
great deal of time
to accumulate much experience and am hoping there is a reference available.
I suspect many of us did learn from experience, so other methods
are not in our realm.
One thing about construction articles, sometimes they are specific
because there is a real reason for a specific component, and other
times they specify what they used (in effect saying, "try something
else at your own risk"). They may specific because the specific
is readily available, at least to them, or because they don't want
all kinds of questions asking "but what kind of capacitor should I
use at C1".
Though hopefully, when a specific type is necessary, it will have
something like "use only polypropolene capacitors here" rather
than "the .1uF capacitors are polyproplene" (the former indicating
there are no options, the latter an indicator of what was used
by the builder).
Not really useful, but it is background.
One thing about specific capacitors, one necessity is that the larger
the value, the harder it is to get that sort of capacitance in a reasonably
sized package. Hence, the move to electrolytics and tantalum (which
causes and incidental move to polarized capacitors, since the makeup
of such capacitors cause them to be polarized).
So small value capacitors, 20pF, might be chosen for high tolerance,
because much variation from the marked value could affect circuit
operation. But, that too is variable, because a coupling capacitor
might not be a problem, while tuning an LC circuit it would. You
might also choose a type of capacitor that isn't affected by temperature,
since again a small variation might affect circuit operation.
Ad capacitance goes up, their tolerance is less important, because
the circuits needing larger capacitance generally aren't fussy. The
caveat being that if the capacitor is used for timing or some tuned
circuit, precision is likely a factor.
So some types of capacitors are more stable with temperature, and
others are available in smaller tolerances.
One way of looking at it is the more general something is, the project
or the use of that capacitor, the cheaper the capacitor can be, and
thus the cheaper the capacitor, the less it matters what type.
When I was a kid, you'd use electrolytic capacitors for any value
over 1uF (because that's about all you could get in such high
values, and if you found any non-electrolytic of that value or
higher, they tended to be really large). Polarized because
that's the only option electrolytic capacitors provided.
..1 and .01uF, so common as bypass capacitors, they'd be ceramic.
Smaller values tended to be ceramic or better silver mica.
You didn't fuss over brand, you bought what was available, what
was cheap, or better yet scrounged what you could.
IN recent years, I've seen people fuss over specific brands, and
fuss over what type capacitor to use for really general applications,
which is fallout from a) a wider range of methods to manufacture
capacitors and b) it's all out there in the catalogs for everyone
to see. It start becoming complicated because the variety is there,
rather than because that much has changed.
You look at schematics, and read the theory beside them. You start
to note how common it is to bypass an emitter, you start to see the
handful of basic uses of capacitors (bypass, coupling, timing or
tuning), and you discard the fuss over some things, and then
that leaves just a few capacitors that would ever need to be
fussed over (and even then, with more thought and study, that
number drops even further).
You used to be able to get books on every single type of component.
I suspect you still can, but they are no longer aimed at hobbyists
and thus at the very least carry a high price. But a book like
that on capacitors is bound to tell you far more than you'll
ever need to know on the subject.
Manufacturer's literature might be helpful, though I suspect little
is being published on paper nowadays. Dig out the names of the
big capacitor companies, and look for their websites, to see if
they have any info there.
Michael
