Homebrew PCBs

John,
Thanks for the info in your post.  It's now in my "Design" folder on
my hard drive.

Your explanation provides a good example of why we don't use film
capacitors at RF, only at audio frequencies.

Cheers,
Ted

--- In Homebrew_PCBs@yahoogroups.com, "John Popelish" <jpopelish@...>
wrote:
---snip---

> Good high quality film capacitors also have rolls of foil in them (or
> rolls of plastic film with metalization coated on them).  But the edge
> of the foil roll is coated with metal to short the coil of the roll,
> all together, so no current has to go round in circles to get in and
> out.  Here are pictures of these well designed extended foil rolled
> capacitors:
> http://www.parallax.com/images/prod_gif/201-01030.gif
> http://www.prestige.com.tw/pics/16b.jpg
> 
> Some of the lowest inductance designs have rectangular stacks of metal
> coated film, with the opposite ends of the stack (where the metal
> protrudes) coated with more metal and wires bonded on.  
>
http://akamai.globalsources.com.edgesuite.net/f/593/3445/5d/pdt.static.globalsources.com/IMAGES/PDT/THUMB/669/T1001155669.jpg
>
http://img.alibaba.com/photo/50332846/Sub_Miniature_Metalized_Polyester_Film_Capacitors.summ.jpg
> 
> Small extended foil rolled capacitors are often indistinguishable from
> stacked film capacitors, without a dissection.
> 
> The cheap and crappy film capacitors have a roll made of two layers of
> foil and two layers of film with the wires connected to the inner ends
> of the two metal layers (because production is cheap).  These are
> usually dark green, for some reason (called "Greenies" with a curled
> lip), though dark red is also a common color.  They also have lots of
> series inductance, even though the two currents travel round the roll
> in oppposite directions.  They are, in effect a rolled up transmission
> line.  You can recognize these (and avoid them) by the shape that has
> the roll with a vertical axis (before it is squashed flat) with the
> two wires coming out of the center of the roll.  They look a lot like
> extended foil rolled capacitors with radial leads, except that the
> leads are closer to the center(inside the roll).  Look at these
> picture and burn them into your brain like you do poison ivy leaves:
> http://www.hosfelt.com/media/15417.jpg
> http://www.interq.or.jp/japan/se-inoue/picture/condb.jpg
> 
> If you want more details, here is a link to a very good capacitor site:
> http://my.execpc.com/~endlr/index.html
>