PI Printed Circuit Coils.

[Anonymous]

Would there be any advantage using a PI printed circuit coil over a standard one, or are they just cheaper to make?

 

[Anonymous]

One would think the advantage would be consistency. I spoke to White

 

[Anonymous]

Hi Jeff,
Printed coils can be very good in certain circumstances. As Mr Bill says, consistency is one advantage. Modern pcb quality control would maintain inductance, resistance and winding capacitance within very tight limits. I have used pcb coils in certain industrial PI applications, but not yet in treasure hunting detectors. The reasons are these:- It is difficult to get low resistance windings with sufficient inductance. To get the inductance you would need a multilayer board which, in the UK, would work out considerably more expensive than a wire coil. You then have to shield the pcb coil, which I can do much more effectively with a wire coil by wrapping it with foil or fine pitch ribbon cable. With TH detectors a tolerance of plus or minus 10% in R, L and C causes no problems, so the greater consistency of a pcb coil is rather wasted. For pulse delays less than 5uS a pcb coil becomes much more attractive, but nobody is using delays this short for TH and for beach and salt water work it would be out of the question.
Eric.

 

[Anonymous]

Hi Jeff, Eric
I have done some PCB coils aswell and a good shielding can be done with Graphite spray.
1. Make one ekstra circle of copper at the outer edge of the PCB not connected to the coil and make a 1-2 mm gap, so it is not shortet.
2. Mount a ring of isolating tape on this ekstra ring of copper to shield it from non-conductive spray.
3. Then you spray the PCB copperside with normal non-conductive paint to isolate the windings of the coils and let it dry.
4. Remove the ring of tape.
5. From the little 2 mm gap in the outer copper ring you then place a 3 mm slice of tape to the center of the coil.
6. Then you spray it with a conductive paint like Graphite 33 (CRC Industries germany). Make sure it touch the outer circle of copper.
7. solder a wire from the outer copper circel to the shield of your cable.
8. Finally paint the hole finised coil in some nasty paint or two component epoxy or any water resistent goods.
On a single sided board you can use the backside of the board and place a piece of cobber tape to solder you connection to.
Mark

 

[Anonymous]

Eric,
I am trying to find a method of coil shielding that does not affect the flyback pulse-width of a PI machine. When I wrap the search coil with aluminum or copper foil the flyback pulse stretches out almost double it's normal length. If I wrap the coil with closely spaced wire things seem to work O.K. However, since I tend to try a lot of different coil configurations it can mean a lot of wire winding around coil perimeters. I remember you mentioned something about using lead foil in one of your earlier notes. Does it have insulation on one side to prevent the overlapping turns from shorting,or, does lead have some special property. I hope you can shed some light on this problem. Thanks for this forum. It has been extremely helpful and I have learned a great deal about a subject that has very little written source materials. Thanks for your time!

 

[Anonymous]

Hi Dan,
There are a few methods that give good coil shielding. One that I have used, is to use fine pitch ribbon cable. I tear of a strip about 1/2in wide which contains 19 conductors. One end of the ribbon is stripped and all the conductors joined with a piece of wire soldered across the ends. A tail about 3in long is left on the joining wire. The far end of the ribbon is left open. The ribbon cable is then helically wound around the search coil winding. Using the 1/2in width you can cover an 11in or 10in coil with the ribbon edges neatly butting up to one another. The ends are secured in place with adhesive tape. The tail from the ribbon screen is connected to the ground side of the coil.
Another way is to use multi-conductor cable that has a aluminium deposited mylar film screen and drain wire. I have some 26way cable of this sort and made a good coil. It's a bit tedious joining and sleeving the ends but we made a small pcb to make life easier.
Copper or aluminium tapes are too conductive and the eddy currents generated in them have the effect of lengthening the recovery time. I use a specially made lead tape which is OK due to its lower conductivity and you can solder a wire to it for connection to ground.
Adding a screen of these types increases the self capacitance of the winding considerably. I have used a spacing wrap between coil and screen to minimise this, but however you do it, it is a tedious operation.
Eric.

 

[Anonymous]

Eric,
Thanks for you prompt reply on coil shielding. I will try out your method of using ribbon cable. Sounds like a great idea. Thanks again!
Dan G.