[Fwd: Re: [Boatanchors] capacitor stocks]

[rrkrr]

"Mylar" is a DuPont trademark for a particular type of polyester film.  
It's both mechanically strong and a great electrical insulator ("high 
dielectric strength").  It's used as a dielectric in capacitors, and 
"back in the day" before Computer Assisted Drawing software, as a base 
for drafting paper with a roughened surface coating to take a special 
plastic pencil "lead".  If you look up "polyester capacitor" you'll find 
a lot of manufacturer's labeling their products as "Mylar" capacitors.

Bob K4ERR

shoppa_boatanchors at trailing-edge.com wrote:
> <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.
> _______________________________________________
> Boatanchors mailing list
> Boatanchors at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/boatanchors
> ** List Administrator - Duane Fischer, W8DBF/W9WZE **   ** For 
> Assistance: dfischer at usol.com **         $$ For vintage radio info, 
> see the HCI web site $$      http://www.w9wze.org   
>
>