Crude coil sheilding paint

[Tinkerer]

Good graph Tinkerer,

500 pf including including the cable capacitance if a good yard stick for the Minelabs.

Have you opened up a ML coil to see that the winding is level with the bottom of the foam in order to place the windings as close as possible to the target.......but in doing so the ML winding is hard against the Shielding thus adding maximum c. Their testing must of told them that it is more important to gain 4mm of depth in relation to coil to target proximity than to reduce the coil capacitance and to optimize DV/DT.

Thanks for the feedback.

The matter of the coil capacitance is quite complex. For example, only about half of the winding to shield capacitance actually does "show up" when we calculate the capacitance by the self resonant frequency of the coil. What happens to the rest of the capacitance?

We have the inter-wire capacitance and the winding to shield capacitance and the cable capacitances, maybe some of these capacitances add up in series and so get reduced?

It seems that for every answer to a question we find, so many more questions turn up.

Ah, by the way, the graph above shows the di/dt, the current, not the voltage, since it is the coil current that generates the magnetic field.

Tinkerer

[multieagle]

Somewhere I recall reading that carbon paper, the kind used by artists to transfer drawings, images to art paper, could be used as shield material. It would have to be pasted onto thin cardboard. I do not wish to spend the $25 locally to test it out before finding out if it can indeed be used. Anyone have an opinion? It certainly looks similar, except for thickness (tracing paper), to carbon coated paper I've seen removed from a Minelab coil , in a YouTube video http://www.youtube.com/watch?v=BY7WFyCVO_A.

[WM6]

Take your AVO meter to your store and ask for measure conductivity (resistance) of those artist carbon paper. If it show conductivity, then only question how to connect ground wire to those paper remain.

[satdaveuk]

Can't we use any good quality paint (that will stick to the coil shell or coil covering) and add the same ratio of graphite powder?

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

[WM6]

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

You mean as electrostatic shielding - mixed with graphite? Which brand/product did you use?

[multieagle]

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

This may be a foolish question, but guess a newbie is allowed that latitude. How is this grounded? Do you run a naked ground wire through the varnish/graphite/carbon? Are the coil wire/cable connections within, below, or above the epoxy or varnish shell?

[satdaveuk]

You mean as electrostatic shielding - mixed with graphite? Which brand/product did you use?

All you got to do when shielding styrene project boxes is sand the gloss off the styrene then stick copper type tape to the 4 corners and anywhere else after thats convenient where you want to solder your earth points not forgetting the lid which I always use a permanent soldered wire between the two, then get your Synthetic Graphite which is normally 99.98% carbon and mix with yacht varnish about 1:1 although not critical just make it so it brushes on nicely leaving a little copper showing where you intend to do terminations.
If you want less or more resistant just another coat when dry or lightly sand its as simple as that and sticks like pooh to a blanket.
When in the workshop next will get the manufacture for you, it really works well .
If you look on this forum a while ago I did a whole tutorial on using the graphite which started when first doing experiments right through to total constant success, done and dusted in my opinion for DIY shielding.
When doing coils if directly painting which most of the time I do just put plenty of layers of rubber tape on first to reduce internal capacitance then hospital type paper tape on top which is very adhesive and absorbent then copper tape at each end leaving 5mm or less gap between the two then same procedure as above.
Once this stuff is dry you can hit it with a hammer and it wont fracture yet on the other hand you can still bend your coils lightly when nulling and they easily sand if increasing resistance is required.
The actual bond between the copper and the graphite is good just remember as with the styrene you need to sand the surfaces before applying the mix otherwise it will not bond correctly.
When testing the Graphite for continuity at various points its paramount you lightly sand it first because the varnish comes to the surface which acts as a insulation so you will get false readings.
Another plus side is it actually looks nice very close to gun metal Gray.
Give it a try but don't deviate from what I said because lost count of different chemicals I used based around Synthetic Graphite, also talked for a considerable time with the manufactures who are based here in the uk for advise before experiments began.
Regards

[WM6]

Thanks for your experience.

I am usually spraying graphite direct to finished coil(s) and not to coil housing, but to lower parasitic capacitance your approach would be better.

[satdaveuk]

Thanks for your experience.

I am usually spraying graphite direct to finished coil(s) and not to coil housing, but to lower parasitic capacitance your approach would be better.

Hi
Graphite in a spray can is great the only problem is the price and I presume the carbon content within that graphite will vary according manufactures so some brands of spray you maybe using half a can or so before you get the conductivity you require.
As promised I will pop in the workshop later and tell you the make of graphite I am using because that also may well vary from brand to brand and maybe thats part of why we have had good and not so good results on here.
What may interest you is the synthetic graphite I use comes from the off cuts when they are making brushes for motors etc. they collect them up and with precision grind it all up to a powder thats finer than talc powder, thats why its better to wear gloves because it gets easily into the paws of your skin, anyway maybe you can understand its high conductivity due to what it originally came from, all interesting stuff isn't it .
1kg cost around £25, when shielding two coils your using about 3 teaspoonful's and throwing nearly half away so that container of graphite will last years if you think about it, its costing penny's per coil if that.
Regards

[Davor]

Somewhere I recall reading that carbon paper, the kind used by artists to transfer drawings, images to art paper, could be used as shield material.

Carbon papers are wax based, so don't expect much.

[Henrikras]

Maybee some stupid Questions. But why not mix the powder in to the plastic before you cast the Shell. then the Shell got the Sheilding.?? If you can spray/paint inside a shell, you properly allso can spray/paint outside the Shell, so why not between ??
Will it work If I sprayed the Sheilding outside a search coil, which didn't got enough spray inside before I closed it ? If I can connect to the wire.
Henrik

[multieagle]

Hi,
I'm very sorry, this preparation is way too corrosive, it has over time turned my ground wire connection to oxide powder.

The carbon needs to be extracted from the manganese dioxide and other chemicals and washed carefully before adding to the paint....maybe better to use graphite powder instead, if you can get it, or just use the powdered carbon anodes from the battery.

{Washing egg from face}
Kev.

You can easily remove the manganese dioxide from the other powders: http://www.youtube.com/watch?v=BVXm1O3MtqY Perhaps that would render the battery chemicals useful?

[multieagle]

Carbon papers are wax based, so don't expect much.

Thank you. I'm going to move on to 6v heavy duty batteries. Will try using both the chemical compound and the carbon rods. See my previous post.

[satdaveuk]

Hi WM6
The Synthetic Graphite that I use is made by Graphite Trading company UK.
Interesting thing to check if using Graphite spray check contents before buying because best you going to get is natural graphite mixed with alcohol which is 85% carbon but quite often its abstracts from coal which is not true Graphite, so thats one to watch out for because although conductive its not designed for what we use it for due to low conductivity and build .
If your mixing into Epoxy resin they advise manufactures to only use Synthetic Graphite due to its molecule build and conductivity.
All the best

[WM6]

Good remark, thanks Dave.