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CDROM metal stripper

  • Thread starter RST Engineering \(jw\)
  • Start date
J

James Arthur

Jan 1, 1970
0
That wasn't my battle plan. My idea was to have the polystyrene "platters"
simply there to have a place to glue the plates onto.

The plates themselves are on the INside of the disks with a thin sheet of
plastic (kitchen wrap, saran wrap, a sheet protector cut in half...) between
them. Something like this, and an ASCII artist I ain't:

@@ grommet through the center hole
******************* top cdrom disk
--------- brass shim stock glued to top disk
=================== thin plastic dielectric
--------- brass shim stock glued to bottom disk
******************* bottom cdrom disk
@@ grommet coming through from top










I calculate somewhere between 500 and 1000 pf for the capacitor with 3 mil
mylar and a 3 mil airgap allowance on both top and bottom plates.

I'm sort of curious why I was off nearly an order of magnitude from the
experimental results.

Jim


Your arrangement was clear, but my response to John wasn't. I was
fixated on the parasitics, and hence the capacitance of the
metallizations through two layers of CDROM material was what
interested me.

I measured this, and I get 133pF +/- 10pF. That's for two full-
circle aluminum-foil plates separated by two CDROMs.

So, you're not off. Neglecting airgaps, half-plates at 1/20th the
distance, through mylar, should give about 10x that value, or 1.3nF.

Best,
James Arthur
 
H

Homer J Simpson

Jan 1, 1970
0
Cost. Cost. Cost.

But the last two sound interesting if I can figure out a cheap method of
implementation.

Consider pieces of cardboard with Al foil glued on - or two cardboard tubes
with foil, sliding in and out.


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P

Phil Hobbs

Jan 1, 1970
0
RST said:
Neither trick works with the Verbatim junkers I just played with, but thanks
anyway. How "strong" does the tape have to be? All I used was plain old
clear packing tape for one try and a really stuck-on paper label for
another.

Jim

Why not just leave the metal on there and accept a very slightly lower
capacitance ratio? You aren't using the thick polycarbonate as the
dielectric anyway.

This will result in a parallel capacitance on the order of

Cpar = Cmax*thickness(mylar film)/(2*thickness(CD)). Since the mylar is
probably 1 mil or thereabouts, and the discs are about 50 mils thick,
this would degrade your Cmax/Cmin ratio to--well, about 100:1. Not too
bad. I get about 1250 pF Cmax and 13 pF min with 1 mil dielectric, with
epsilon=4. There's the self-capacitance of the plate to worry about
too, which (in pF) is on the order of its diameter in centimetres, but
that's about the same size as Cpar.

Cheers,

Phil
 
B

Barry Lennox

Jan 1, 1970
0
I plan on using two junk CDROMs in my basic science class as a variable
capacitor for a crystal set -- shim brass cut to cover a little less than
half of each cdrom, glued to the surface, with a vinyl grommet in the middle
to hold the disks together and a disk of mylar cut from a sheet protector
between the brass plates as the dielectric.

Each student is going to have to prepare two disks each, so it isn't a
onesie-twosie job. I need a method that I can use fifty times a semester
every semester.

The problem is removing the metal "plates" that are there now. First
thought was to soak the disks in drano or aqueous lye and dissolve the
aluminum, but further reading and experimentation shows this isn't quite
enough. Apparently there is a lacquer or epoxy coating over the aluminum
and a nickel flash under the aluminum. This is only hearsay, but I know
from experimentation that a caustic solution by itself isn't enough.

Anybody KNOW what the various layers are on the disk and any suggestions on
how to remove them without destroying the plastic of the disk itself?

I just grabbed an old AOL one and stripped it in about a minute using
a Bahco stripper, basically a TC blade in a handle. You attack the
painted side. The trick is to bend the bend the CD ever so slightly
over something large (I used a bench vice) so as to present a slightly
curved face to the scraper blade.

Barry
 
J

James Arthur

Jan 1, 1970
0
Why not just leave the metal on there and accept a very slightly lower
capacitance ratio? You aren't using the thick polycarbonate as the
dielectric anyway.

This will result in a parallel capacitance on the order of

Cpar = Cmax*thickness(mylar film)/(2*thickness(CD)). Since the mylar is
probably 1 mil or thereabouts, and the discs are about 50 mils thick,
this would degrade your Cmax/Cmin ratio to--well, about 100:1. Not too
bad. I get about 1250 pF Cmax and 13 pF min with 1 mil dielectric, with
epsilon=4. There's the self-capacitance of the plate to worry about
too, which (in pF) is on the order of its diameter in centimetres, but
that's about the same size as Cpar.

I concluded the same--written up and illustrated with beautiful
ASCII art last night--but I calculate double the parasitic due to
additional shunt paths. Namely, fully unmeshed, the two shunt paths
are via each plate to the metallization on its substrate CD, across
the metallization, then across the CD+center dielectric thicknesses,
to the other plate.

This results in four roughly equal-valued stray capacitors in series-
parallel, with a net value equal Cstray(unmeshed) = Cmeshed * (central
dielectric thickness / CDROM thickness) (assuming equal dielectric
constants, which appears nearly to be the case).

That is, the variable cap's capacitance ratio is approximately the
thickness ratio between the CDROM and the mylar dielectric. For a
larger ratio, use a thinner dielectric.

Homer's sliding tubes suggestion is attractive, but Barry Lennox's
stripping technique suggests another possibility:

Why not use Barry's technique to strip half the metal off each of
two CDROMs, then glue brass (or aluminum foil) plates on top of the
remaining metallized area (rendering it moot), and assemble the CDROMs
with metal-sides inward, separated by your mylar dielectric?

Cheers,
James Arthur
 
That wasn't my battle plan. My idea was to have the polystyrene "platters"
simply there to have a place to glue the plates onto.

The plates themselves are on the INside of the disks with a thin sheet of
plastic (kitchen wrap, saran wrap, a sheet protector cut in half...) between
them. Something like this, and an ASCII artist I ain't:

@@ grommet through the center hole
******************* top cdrom disk
--------- brass shim stock glued to top disk
=================== thin plastic dielectric
--------- brass shim stock glued to bottom disk
******************* bottom cdrom disk
@@ grommet coming through from top










I calculate somewhere between 500 and 1000 pf for the capacitor with 3 mil
mylar and a 3 mil airgap allowance on both top and bottom plates.

I'm sort of curious why I was off nearly an order of magnitude from the
experimental results.

Jim

I read a reasonable 780pF with the whole assembly jacked tight
together. Used a Polyethylene seperator, 3mil (er 2.3 ?). Plates made
of tinned steel sheet 8mil, 0.2mm. "Glue" was double sided tape at
8mil.
Final assembly could best be described as sloppy.
john
 
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