Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "sbdwag" <sbdwag@...> wrote:
>
> Im curious how do big capacitors have internal inductance. If they
> have inductance wouldnt that cancel some of the capitance that they
> have? Are you talking about the leakage current? This would be the
> only thing that passed dc. 

It has to do with how big electrolytics are constructed.  One "plate"
of the capacitor is a conductive liquid (the electrolyte that gives
the type its name).  But the other plate is a roll of thin aluminum
foil with a connection to the capacitor pin.  The surface of the
aluminum is oxidized by an electrolytic process that coats every bit
with a uniform layer of insulation, after the metal is made very
porous so it has lots of surface.

Current can get to every part of the oxide through the electrolyte by
a fairly direct path (though there is an usually an unoxidized roll of
foil sitting in the liquid to lower the liquid's resistance, and that
adds a little inductance).  But current gets to the metal side of the
oxide by travelling around the roll of foil (many turns to get to the
far end).  And that generates a magnetic field and produces inductance
in series with the capacitor.  So if the current may have to travel
through feet of rolled up foil, inside the capacitor, it makes little
sense to worry about millimeters of trace, in series, outside the
capacitor.
http://www.bartleby.com/images/A4images/A4capaci.jpg

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