If the CDROMs are just there to support the brass plates--with the
mylar dielectric in the middle--why not just face the intact
metallized sides outward, whereby the large gap will minimize any
affect on total capacitance?

Namely, in cross-section: (view using Courier font)

. ||xxxxxxxxx #### mmm #### xxxxxxxxxx|| <--aluminized layer
. \_________/ \__________/ facing outward
. CDROM CDROM
. B M B
. r y r
. a l a
. s a s
. s r s

If the resulting minimum capacitance is acceptable it'll save a lot
of effort stripping.

Best,
James Arthur

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?

Jim

"James Arthur" <dagmargoodb...@yahoo.com> wrote in message
news:1172099870.720179.40580@p10g2000cwp.googlegroups.com...

If the CDROMs are just there to support the brass plates--with the
mylar dielectric in the middle--why not just face the intact
metallized sides outward, whereby the large gap will minimize any
affect on total capacitance?

Namely, in cross-section: (view using Courier font)

. ||xxxxxxxxx #### mmm #### xxxxxxxxxx|| <--aluminized layer
. \_________/ \__________/ facing outward
. CDROM CDROM
. B M B
. r y r
. a l a
. s a s
. s r s

If the resulting minimum capacitance is acceptable it'll save a lot
of effort stripping.



Was just thinking that myself but am calcing about 50pF max across the
variable capacitor but a constant 30pF in the parasitic shunt paths
(100-50-100).
Suggests something like a 40 to 80pF variable cap.
Better still and in the interests of science (i.e. the bits were at hand),
spent 5 minutes making one. (the grommet idea works well).
Min C=64pF, Max C 102pF.
But with finger pressure making the adjusment results in 88pF to 330pF
john

On Feb 21, 5:48 pm, "john jardine" <j...@jjdesigns.fsnet.co.uk> wrote:

Was just thinking that myself but am calcing about 50pF max across the
variable capacitor but a constant 30pF in the parasitic shunt paths
(100-50-100).
Suggests something like a 40 to 80pF variable cap.
Better still and in the interests of science (i.e. the bits were at
hand),
spent 5 minutes making one. (the grommet idea works well).
Min C=64pF, Max C 102pF.
But with finger pressure making the adjusment results in 88pF to 330pF
john

Dang, actual data! Good show. But, I calculate/estimate the
capacitance of two full-disc plates through two CDROMs (two
thicknesses of 1.3mm polycarbonate, dielectric coefficient=2.9) as:

k * e0 * A 2.9 * 8.85e-12 * pi * (0.060m)^2
C = ---------- = ------------------------------ = 112pF
d 2 * 1.3e-3 m

so this real-world result bugs me.

Hmmm. Why wonder when you can measure? I must try it.

"RST Engineering (jw)" wrote:

Why not use thin fiberglass PC boards? Etch the plate areas you
want. Another approach is the old "Book Capacitor", or a homemade
piston trimmer capacitor.

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

"James Arthur" <dagmargoodb...@yahoo.com> wrote in message

news:1172118611.124181.155780@a75g2000cwd.googlegroups.com...



On Feb 21, 5:48 pm, "john jardine" <j...@jjdesigns.fsnet.co.uk> wrote:
Was just thinking that myself but am calcing about 50pF max across the
variable capacitor but a constant 30pF in the parasitic shunt paths
(100-50-100).
Suggests something like a 40 to 80pF variable cap.
Better still and in the interests of science (i.e. the bits were at
hand),
spent 5 minutes making one. (the grommet idea works well).
Min C=64pF, Max C 102pF.
But with finger pressure making the adjusment results in 88pF to 330pF
john

Dang, actual data! Good show. But, I calculate/estimate the
capacitance of two full-disc plates through two CDROMs (two
thicknesses of 1.3mm polycarbonate, dielectric coefficient=2.9) as:

k * e0 * A 2.9 * 8.85e-12 * pi * (0.060m)^2
C = ---------- = ------------------------------ = 112pF
d 2 * 1.3e-3 m

so this real-world result bugs me.

Hmmm. Why wonder when you can measure? I must try it.

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

Cost. Cost. Cost.

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

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



"Phil Hobbs" <pcdh@SpamMeSenseless.pergamos.net> wrote in message
news:45DC9266.7010206@SpamMeSenseless.pergamos.net...

If you use CDRs, you can usually delaminate the coatings by flexing the
disc a few times. Peeling with strong tape is another possibility.

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?

RST Engineering (jw) wrote:
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

"Phil Hobbs" <p...@SpamMeSenseless.pergamos.net> wrote in message
news:45DC9266.7010206@SpamMeSenseless.pergamos.net...

If you use CDRs, you can usually delaminate the coatings by flexing the
disc a few times. Peeling with strong tape is another possibility.

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.

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

"James Arthur" <dagmargoodb...@yahoo.com> wrote in message

news:1172118611.124181.155780@a75g2000cwd.googlegroups.com...



On Feb 21, 5:48 pm, "johnjardine" <j...@jjdesigns.fsnet.co.uk> wrote:
Was just thinking that myself but am calcing about 50pF max across the
variable capacitor but a constant 30pF in the parasitic shunt paths
(100-50-100).
Suggests something like a 40 to 80pF variable cap.
Better still and in the interests of science (i.e. the bits were at
hand),
spent 5 minutes making one. (the grommet idea works well).
Min C=64pF, Max C 102pF.
But with finger pressure making the adjusment results in 88pF to 330pF
john

Dang, actual data! Good show. But, I calculate/estimate the
capacitance of two full-disc plates through two CDROMs (two
thicknesses of 1.3mm polycarbonate, dielectric coefficient=2.9) as:

k * e0 * A 2.9 * 8.85e-12 * pi * (0.060m)^2
C = ---------- = ------------------------------ = 112pF
d 2 * 1.3e-3 m

so this real-world result bugs me.

Hmmm. Why wonder when you can measure? I must try it.

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