[Boatanchors] capacitor stocks

[shoppa_boatanchors at trailing-edge.com]

<eugene at hertzmail.com> wrote:
> I was directed to justradios.com as having some very good capacitor kits.  
> http://www.justradios.com/capkits.html 
> They sell 5 basic types of caps. 
>
> 1. Leaded Film (polystyrene, polyester)
> 2. Orange dips (drops)
> 3. lytics (aluminum)
> 4. Mica
> 5. Mylar
>
>
> Question is, other than electrolytics, which I understand the importance of polarization, wv and capcitance value...
> If a circuit calls for a value and voltage rating (and temp?) does it matter which kind, film, orange, mica, or mylar I use? 

Orange drops are essentially a particular brand and style of film cap.
Internally "orange drops" might be polyester film/foil, metalized
polypropylene, metalized polyester, etc. depending on the exact
Sprague part number. In HV pulse circuits the differences are
important but in typical boatanchor use (e.g. that audio bypass)
they're hardly important.

Mylars are also a particular type of film cap but mylars are hardly
in production anymore. If they are really mylars, they are probably
from a big bag of factory rejects somebody picked up a few decades
ago.

Most films are in the range 0.01uF to the very low uF. Yeah, there are
some larger and smaller.

Polystyrenes run from a little below 100pF to 0.01uF or so. Polystyrenes
are not really very common anymore but come in very handy where their unique
temperature coefficient comes in handy. They are NOT for hot environments
(e.g. your typical boatanchor) because they deform under heat and do not
come back!

Micas run from a pF to a little over 1000pF.

So there is really very little choice among the ones you give. If
it's under a few 1000pF you'd have to choose the mica from your list. (But
you'd more likely use a ceramic, a choice for some reason absent from your
list, unless there was a good reason to use a mica.) If
it's over 0.01uF but less than a few uF you'd choose from the multiple
film types you list. If it's over a fraction of a uF then an
electrolytic would be used.

You will notice that the types you list do not cover very well, at all,
the range from 1000pF to 0.01uF. Yes, there are some really small films
and some really big micas, but not much. And while polystyrenes
cover that range, polystyrenes hardly ever appear in boatanchors
except for a couple of audio filters.

Ceramic discs, though, span from a fraction of a pF to multiple uF.
Multiple temperature coefficients and dielectrics that are radically
different so NEVER just lump all ceramic discs together.

> One of the motivations for this question is I want to buy stuff
> from justradios, but I don't want to buy every kind of cap they have

"I want to buy something from a Ford dealership but can't decide if
I should get a gas cap for a 1986 Escort or a 2008
Ford Expedition". In other words, you don't really know
what you want yet.

Mouser, Digikey, Allied, Newark etc. have almost all the caps you'll
ever need. They're all major stocking distributors and NONE OF THEM SELL
FLOOR SWEEPINGS. Buy the ones you need.

Tim.