Graphite coil shielding for TGSL problems

[billr]

I made a new coil coated with graphite paint being sure to leave a 1/2 space between start and finish. The graphite of each coil was grounded by wire to the circuit. The coil itself works well for range but it is very sensitive to any contact with my hand when placed close or on the coils. Don't want to pot it up until it has been given the best chance of interference suppression.

Managed to reduce the interference by hand when a thin uncoated wire was wrapped around each coil and grounded but this also reduced the range. So not ideal.

Can someone walk me through or link the procedure of shielding the coils and shell for best performance using graphite?

Thanks.

[rickb]

First, the coil must be grounded. Second, the resistance of the shield must be low enough to eliminate electrostatic interference, but high enough to not reduce electromagnetic pickup. The resistance should be high enough to not need the gap.

Read this and the link.

https://www.geotech1.com/forums/show...187#post241187

[billr]

Thanks Rick.
I'm getting nearly 2k ohms on the shielding of each coil when checked from start to finish points of the paint. The coil housing was only in the low hundreds so I added another coat to that to see what changed.

The range is the same as before but I noticed after making the best nulling a slight audio signal is given off and no adjustment can cancel it out. However. When I touch the coil housing the noise goes away and the range stays the same.

Is this telling me the coil or the housing needs more paint?

When the coils are removed from the housing and placed on a board. Nulling can be achieved to silence. But there is some noise given off when my finger touches either coil.

Both coils are isolated from making contact with each other using pvc tape.

[rickb]

2k ohms is to low! The detector is detecting the eddy current in the shield. I have never heard of anyone panting the tx coil in an I/Bdetector. With resistance that low, the depth is reduced. Read the link I posted about ohms per square. I see no alternative but to strip off the paint and try again.

Commercial detector mfg. paint the inside of the housing.No need to paint the coil, and much easier to fix a mistake. Painting 2 sheets of cardboard or thin plastic has been done. Read the link I posted about ohms per square (sheet resistance).

Thin the paint and apply thin coats, and test for acceptable shielding between coats. Each coat will lower the resistance. Don't go any lower than necessary to achieve static elimination.

Has the detector and coil been tested for proper operation and detection distance before shielding?

Use the forum search for shield, shields, shielding, static.

[billr]

Hey Rick.
This might be a long one but lemme explain. Read your posts and links. Appreciate the info in them and it kinda helped up to a point. Then it all went south.
Took the coils out of the shell and placed them on a board. Used a plastic adjuster screw to regain null. Got the null pretty darn close to silent when the thing was running with the intermittent light chirp every few seconds on audio. The GB knob did not correct this. Coin detection was back to 13.5" as it was before.
Made an error in my last post re the 2K shielding. It is in fact 10 to 20 ohms per coil as measured above. So I left the paint as it was.

Used a cotton swab with thinned graphite paint to spread an even layer on some paper. Placed them on top of the coils and all noise stopped. A slight tweak with the GB pot and nothing came through when the coils were touched by hand. Great........I thought.

So I coated the inside of the shell with a thin layer of paint and placed the coils inside. After securing in a few places with hot glue I set about nulling again.

That's where it all went out the window. It chatters like a crazy. Cant null it as before. Even placed the coated paper on the coil shell to no avail.

I dont know if the coil shielding is reacting with the shell shielding when in contact and causing over compensation but it's untunable. Did consider wrapping the coils in tape to insulate but after many hours of trial and failure ......... I'm pooped.

Real shame as these coils had better range than the aluminum foil ones I first made.

I need a drink to drown my sorrows.

[rickb]

The low resistance of the coil shielding makes your coils unusable. Remove all of the coil shields or make new coils.

Did you paint graphite directly on the bare coil? Stop adding graphite.

[billr]

You're confusing me. The coils need to be shielded or else they wont work. Yes? So how can they be shielded if there isnt any shielding added to them? Are you saying the coil shell itself is enough shielding to work? A little elaboration in detail would really help as I am only new to electronics.

[surfdetector]

You're confusing me. The coils need to be shielded or else they wont work. Yes? So how can they be shielded if there isnt any shielding added to them? Are you saying the coil shell itself is enough shielding to work? A little elaboration in detail would really help as I am only new to electronics.

Bill,
You might find this article to be of some use. You are doing a DD but the section on shielding is relevant to your situation:
https://www.hobbielektronika.hu/foru....php?id=139970

[billr]

Ah now I see.
Didn't know you could do it that way. All the coils I've made have always had some form of shielding wrapped around them. I thought the graphite coating in the housing was for ...........actually I'm not really sure what I thought it was for until now. Some form of wet grass or water contact deterrent thing to stop false positives I guess.

LOL Rick must be tearing his hair out in frustration at my graphite fetish to cover everything including myself. Sorry Rick

Thanks Surf.
The file really explains a lot.
Armed with so much info I can really do some serious damage now.

[billr]

Out of curiosity.
How did this dude manage to get his painted coils to run so silently when nothing was placed in front of em?
Mine sounds like Morse code.
Asking cuz I like to learn.

https://www.youtube.com/watch?v=XnrStRklcUQ

[Koala]

2K sounds usable?

do you have you pcb in a metal case ?

I would try running a separate cable from the Rx coil shield direct to the pcb ground. Temporary leaving the Tx off. Just to test

[kt315]

2K sounds usable?
---
general understanding of electronics basics. you are testing in range of your batt setted in the tester.
this is 9 V batt. while we mean __the high_voltage_electrostatic field _charge-discharge process, in range 1...10+ kV (KILO-).
are you able to notice THE difference? your '2k' sounds nothing. and sure unusable.

[Koala]

how confusing

one person says its too high one too low

I still think it will be all right. Especially with a spiral copper tap wire

[kt315]

general misunderstanding of electronics basics***

sorry

[waltr]

Out of curiosity.
How did this dude manage to get his painted coils to run so silently when nothing was placed in front of em?
Mine sounds like Morse code.
Asking cuz I like to learn.

https://www.youtube.com/watch?v=XnrStRklcUQ

That is a good video.
That is also how I did a DD coil for my TGSL including the plastic null adjusting screw (this is really nice).
I did ensure a 'break' in the shielding on each coil and then insulated each coil with electrical tape before mounting in the coil shell.
DD coil works very well and has a little more distance than the Aluminum foil shielded DD I made.