Problem with Coil shielding (Not Again)

[Geo]

Hi All. As i wrote 10 days ago i had problem with the shielding at my DD coil (i had use foil). As Reg told me i made a new coil with semi-conductive tape for shielding. Now the coil do not response. I can't balance the coils so i have at least a signal of 200mv at the Rx coil. When i place a metal object near the coil (1...2 cm near) i see at oscilloscope a phase shift but not an increase of the amplitube(width) of signal (maybe shield stop it). I measure the resistance of tape and i find 14Kohm from one side to the other of Rx coil and 34Kohm at the Tx coil. Can anyone (where are you Reg??) tell me which must be the resistance of the tape ??, or what other is wrong ??

[Leto]

Whites oldie

Hi Geo, according to US4,862,316 Whites patent (Static Charge Dissipating Housing for Metal Detector Search Loop Assembly), column 5, lines 10-14;

Tests have shown that the preffered material for use as a housing according to the present invention has a volume resistivity of 3.00 Log cm2/cm, and a housing exhibits about 1,000 ohms between pionts on its surface separated by a distance of one inch,...

[Reg]

Hi Geo,

I am not sure what is wrong but I doubt the shielding is the problem. The shielding can range from 10,000 ohms per inch down to 1000 ohms per inch and not cause much of a problem. Actually, it can go lower than that, but the slightest movement can then cause big problems. The lower resistance does a little better job of reducing the E field.

Now, make sure that you have a gap in the shielding and doesn't go all the way around the circumference of the winding. This is true of both coils. Also, I would recommend you insulate the windings where they overlap so the two shields do not touch at that point.

How do you have the shields connected? They can connect together but should do that at a common point. On a VLF, this can be a common ground or shield wire.

If the coil is for a VLf, check the basics and make sure you haven't swapped the transmit for the receive winding. Also, just looking at your windings, it appears they are too far apart and may have to be much closer to get a null.

How are you looking at the signal to check for a null? Are you injecting a signal into the transmit winding and looking at the receive or are you looking at the output of an amp? If you are looking at the output of an amp, make sure the gain isn't messed up on the amp circuit.

Basically, the shield shouldn't do anything that would cause what you mention. I would expect an amp circuit to do it.

Finally, make sure there is a load on both windings. It could be one is simply oscillating making it impossible to find the null.

On a PI, each shield should generally connect to the common of the particular coil winding and that wire should connect to the shield of the separate coax. The two shields should not connect, nor should the shields connect inside the coil housing.

Now, on a PI, one can connect the two shields together but only connect the shields to one of the two coax shields.

On a PI the two coax shields should not connect except back at the pc board or the connector.

Check what I have mentioned and let us know what you find.

Reg

[rumensat]

never use foil for shild.
for this purpose have to use non metal but conductive material - graphit.
use Graphite spray 33
first put small wire to plastic and fix with solder, spray graphite , put coils,
balance leaving small part open, fix with epoxy and spray again,conect two wires to ground ,balans again (use open part of coils) and fulfil with epoxy rezin.

[Geo]

Hi all.
Thanks for your reply.
1. For Leto and Rumensat. As you can see at my attached photo i use a open coil housing so it is difficult to shield it with graphite. Reg said me a way with a piece of wire parallel with the coil and a graphite paint but i can't find it. BTW Reg can yoou ask your friend if he can send me only 2 sprays EZ slide ???
So i wrap the coil with semi-conductive tape. First time that i wrap the coil with foil it worked fine but latter i had unstable operational because i used pieces of Al and it had bad conductive.
2. For Reg. I told you that coils are not my "strong point". Now lets see what i made:
a: i have a gap in the shielding and doesn't go all the way around the circumference of the winding. I have insulated the windings where they overlap so the two shields do not touch at that point.
b: I connect every shield at a common point. It is a VLF, and the common point is going at the shield wire.
c: I checked the windings and are ok (fat to Tx, and thin to Rx)
d: How am i looking at the signal to check for a null?? I connect the transmit winding at the detector (turn it on) and looking at the receive winding with the Op.Amp out of the socket.
I used a semi-conductive tape of unknown firma (i had at my Lab), not the 3M Scotch 13. Do you think the tape to have any other constitutive that makes the coil useless?
What are u say if i use a large piece of foil and to use the semi-conductive tape to connect the foil with the ground wire ????
Regards

[detectoman]

hello geo, probably can ocurr one short circuit in middle the wires, or short electrical on static,whit aluminum tape, proff whitout al.tape, or you nedd other wire tickness, most rounds
regards
detectoman

[Leto]

Hi Geo,
how have you connected shield to gound (I mean tape to ground wire)?
If you used nonisolated wire to make contact with shield -> it should have also gap on same spot.
I guess Reg is right about resistance of shield - you are well within limits, should be no problems there. Check the resistance of inner side of tape also.
At last (if possible..) you can replace only Tx winding no urgent need for shield there and try to null the coil again.

[Reg]

Hi Geo,

I looked at your picture of your coil and it appears the windings have the semi conductive tape wrapped around them plus a wire wrapped around the tape. Is there a wire wrapped around the windings and is it something you put on or is it in the tape?

If the tape is a rubber semi-conductive tape then all you need to do is wrap it around the windings and take a small wire, strip the insulation off about 2 inches of the wire and tape this bare wire to the rubber shielding. The other end of this wire goes directly to the common shield connection. This should be sufficient for a connection.

Now, when checking the receive winding, make sure there is a load resistor across the receive winding to make sure it doesn't oscillate freely. You want to be looking for the transmit signal induced into the receive and not the receive just oscillating by itself.

There was a discussion that you don't have to shield the transmit coil. You should shield the transmit winding also, but it isn't as critical as the receive winding. If this winding isn't shielded, then there can be a change in the field because of the capacitance involved and this can change things.

Try connecting a 1K load resistor across the receive winding and then try to adjust for a null. Once you are done, you can remove the 1K resistor. If the 1K resistor loads things too much, try a higher value, maybe a 5K or even a 10K. However, I would try the lower value first to assure the secondary coil has a load.

Reg

[Max]

Hi all.
Thanks for your reply.
1. For Leto and Rumensat. As you can see at my attached photo i use a open coil housing so it is difficult to shield it with graphite. Reg said me a way with a piece of wire parallel with the coil and a graphite paint but i can't find it. BTW Reg can yoou ask your friend if he can send me only 2 sprays EZ slide ???
So i wrap the coil with semi-conductive tape. First time that i wrap the coil with foil it worked fine but latter i had unstable operational because i used pieces of Al and it had bad conductive.
2. For Reg. I told you that coils are not my "strong point". Now lets see what i made:
a: i have a gap in the shielding and doesn't go all the way around the circumference of the winding. I have insulated the windings where they overlap so the two shields do not touch at that point.
b: I connect every shield at a common point. It is a VLF, and the common point is going at the shield wire.
c: I checked the windings and are ok (fat to Tx, and thin to Rx)
d: How am i looking at the signal to check for a null?? I connect the transmit winding at the detector (turn it on) and looking at the receive winding with the Op.Amp out of the socket.
I used a semi-conductive tape of unknown firma (i had at my Lab), not the 3M Scotch 13. Do you think the tape to have any other constitutive that makes the coil useless?
What are u say if i use a large piece of foil and to use the semi-conductive tape to connect the foil with the ground wire ????
Regards

Hi Geo,
" First time that i wrap the coil with foil it worked fine but latter i had unstable
operational because i used pieces of Al and it had bad conductive."
Fist time I wound a VLF coils I had same bad idea...using pieces and separate strips.
But when understand why it was falsing everytime I move the coil then I understand it
would be a unique strip of aluminium.
Foil could be used for VLFs but it's a bit tricky work and require special attentions.
I'm quite expert now so here are my suggests.
When use aluminium foil to shield:

1. measure thickness if you can: a good quality micrometer (10^-5 meter resolution) required
2. less thickness = less problems ; I use mylar-AL >=30micron but some thinner foil are also
easy to find and inexpensive ---> try very bad kitchen rolls , some are really thin!
3. always leave the gap ! (MIND THE GAP!) if you short shields --> big problems !
4. there are two ways...I know
-wrapping around as spiral on wounded coil -> I use this kind, need a "roll" of aluminium
-middle of the strip --> wire on the middle of say 1inch height strip then carefully enclose
best as you can over top side, using EVER a single strip to do everything
5. be very careful when try to insert shielded coil in housing...if something moves wrong
you could have problems on field testing
6. to avoid too movements between wounded roll shield I use this method:
spray all the shielded coil with TRANSPARENT acrylic varnish very lightly ---> this permits
me of manipulating the coil after 15-20 minutes before a complete dry without worring about
too movements of shield turns; then after 1 hour everything become rigid !
This method is good for mylar and conductive tape also if you feel unsafe to move e.g.
minor shaping before inserting in the housing.
This varnish is very cheapy for me (and very useful for other things also): 3.5eur for a 300ml can
7. In tx-rx vlf need two shield cables to start at aluminum strips then connecting at the
same cable shield that go to the pcb.
8. inter-insulation between different shields is mandatory
9. Soldering is possible but at higher temperatire and with right solder/flux but really don't needed

If you can't find suitable thickness foil try to find conductive mylar that is very good!
It's not so simple to obtain but I've used both with PI coils and VLFs and work very good
if you don't overlap too much turns. Shield contact can be made wounding conductive side
to extern facing you (not the wire), then wire-wrapping a silvery-looking (plated) rigid
copper wire (diameter under 0.5 mm) like the ones used in telephone lines or in some
ethernet cables of cat-5 : it doesn't oxidate too much (like naked copper) and makes electric
perfect contacts.
Also phosporous bronze wire(if you have) is very good to make wrapping. But not so easy to find.

I leave always 5mm gap both for PI and VLF and not more or less ---> more capacitive problems
in some wet ground etc, less mean be very careful of shorts don't happen when epoxy came in
place etc.

Also copper-tape is good for shielding but expensive and I know as good only 3M type.

Avoid naked jewelery copper-tape, I've tested too --> too thick for shielding purposes.

Also litz wire is good one candidate too ! I've used with success in some experiments. But
require a lot of time and efforts...it's endly boring to do this way if you have small litz.
Results are very good anyway using 7 conductors litz .10 mm to .20 mm diameter and lot of turns,
but not for everyone - belive me!

Graphite is simple. I don't like spray this stuff...so I don't use. But I had and worked
good. Is fast, easy but you can't do things bad ! You have only one chance - must spray from
30cm or more far away, in a uniform pattern with rotating the jet. Conductivity is made by
eye ! I suggest try to spray on paper sheets some different spots...then measuring resistivity
for different "tones" --> darker more conductivity
Ever use breather, mask , gloves etc to prevent BIG PROBLEMS!

I adbandoned this method because
of these complications...don't anyway want to talk about NICKEL-SPRAY. This stuff can
kill very easy. Don't use if you don't know what you are doing.

I've a brush silver-varnish also...but isn't good for this stuff and expensive too.

One could experiment with say acrylic laquer and graphite powder...temerary...but may work!

Lead foil is good for PI, don't know on VLF. Difficault to find. Only surplus stuff on ebay...

Never tested computer bags (conductive plastic silvery/gray looking). Seems they are good too in
VLF coils. Other stuff could be useful: I found some big potato chip bags are good too ! But the
big problem is removing oil not eating potatoes...without damaging the plastic here !

Also some helium-like ballons are good, I've tested !!! They are very good...but I can't find so
easy because most companies use now cheaper stuff , full plastic !
What ever ??? Well, there are many other things one could use but don't remember now...

"I connect the transmit winding at the detector (turn it on) and looking at the receive winding
with the Op.Amp out of the socket."

So without op. amp. preamplifier ? OK. If you get 200mV steady without any target near...well you
probably have a too conductive path between TX and RX. The shield, acts like "core" that
chain your Tx coil to the RX one. Shield must be weak enough to avoid coupling between TX-RX, just
to stop E field, not to chain magnetic induced signal.
This depends also on how much power the TX section delivers ! have you a push-pull stage on TX or
something to increase tx power delivered ???
If so, RX align could become critical and shielding could become a coupling between.

"When i place a metal object near the coil (1...2 cm near) i see at oscilloscope a phase shift
but not an increase of the amplitube(width) of signal (maybe shield stop it). I measure the resistance
of tape and i find 14Kohm from one side to the other of Rx coil and 34Kohm at the Tx coil. "
That seems not bad ! In DD coils crytical point are overlappings...so I think you have a local problem there.
I mean resistance in the overlapping area of shields is much lower and there would be major coupling.
"I used a semi-conductive tape of unknown firma (i had at my Lab), not the 3M Scotch 13. Do you think the
tape to have any other constitutive that makes the coil useless?"

WELL, WELL, WELL a bell rings in my head now!!!
I suggest checking this tape conductivity per inch at different points to make sure it's uniform this way
and also in thickness. If it's low quality product could be not uniform here and also other thing can be
present.
I had a silvery looking tape some months ago...and seems it contains iron oxides and not only aluminium !
I've tested using my PI detector after measuring resistivity as described (and seemed OK for this!) then
when I putted near my PI...wow ! 1cm^2 sounded very loud...at 3cm ! My hypothesis is that some chrome or
other stuff like iron oxides were present like in old cassette tape ! ::crazy::
Well, they don't sell to me as a "shielding tape"...if I say I need some silver-ribbon...to make shields
on metal-detectors someone could think I'm crazy for real.

Be aware of this ! If you have a PI unit test small pieces of you conductive tape then let me know.

Hope this helpful.

Best regards,
Max

[Geo]

Hi All.
Thanks to All .
I Fixed the problem . I removed the semi-conductive tape and i wrapped around the windings a insulation tape.
After it i cut a piece of foil 80x2.5 cm i wrapped it at length of the windings. I wrapped agan around the windings a insulation tape. Of course i let a gap of 1cm. At the end of the foil i put a naked wire (about 3 cm) on the foil and i tied it tight with semi-conductive tape on the foil and became a fix body. The other end of the wire i soldered at the common point. I made the same for the other winding. Now the coil works perfect (very stable) but when i touch the coil it gives a signal . Why ????? It is full shielding (0.2 Ohm), who knows .
Ohh... who knows what semi-conductive tape was it????
My Regards

[Geo]

Ohhh I forgot about "nulling the coil".
First i connected the Tx winding at the detector who is the Anker SS60 with 4 transistors (2 push-pull at bridge connection). The Tx has the tuning capacitor in series with the coil so the voltage across the coil is very High (110 vP-P).
As Reg said i connected a load at the Rx winding (a resistor 2K2 parallel with the winding). Now i connect the scope parallel with the winding and try to null the output signal. The minimum signal that i took was 15mV. I Fixed the windings with silicone and the coil is OK . A 32 cm DD coil
Thanks to All

[detectoman]

ok geo you solver these problems, others subject
signal static in oscilloscope, at touch cover coil whit fingers, is for short internal shock proximity whit aluminium statics- wires, shield and circuit coils- in middle whit coton o paper tape, to keep proximity extreme sensibility imbalance
apologies my bad" english
detectoman

[Reg]

Hi Geo,

I am glad you got it figured out.

If you get time, you might check out the semi-conductive tape to see if you can tell why it didn't work.

Is there a piece of metal or wire embedded in the tape? You might also see if the tape is magnetic by taking a very strong magnet and placing it near the tape. Does the tape stick to the magnet?

There has to be some reason the tape didn't work.

I have used 3M's tape and it appeared to work fine with no problems. However, I only tried it on a PI also.

Cheers,

Reg

[Geo]

Hi Reg. The problem was not at the tape (as i saw later when i destroyed the shielding with tape) but at a bad connection (i made o short-connection at the Rx coil ) Sorry but don't strike me BIG mistake
Now i have not other semi-conductive tape so i made the coil with foil and it work perfect. The problem with foil as you know is that we can't solder a wire at it. I solved this problem (for the time, i don't know later) as follow:
At the end of the foil i put a naked wire (about 3 cm) on the foil and i tied it tight with semi-conductive tape on the foil and became a fix body
Best Regards