Differential Coils ... Making

[moodz]

1.
Two strands of 19SWG ( 1mm ) insulated solid copper wire are twisted together at approx 1 turn per inch to form a single twisted pair strand. ( You can do this with an electric drill at one end of the pair and a nail on the fence at the other ... stretching it out to avoid kinks)

2.
Wind the twisted pair onto a suitable coil former. Length required will depend on diameter and number of turns.

3.
If one of the conductor pairs is A and the other B. Cross connect the A conductor from one end of the twisted pair coil to the B conductor of the other end . This junction is the "common". The remaining two ends are the differential ends of the coil.

4.
Connect the coil to the electonics using a 3 conductor lead ( 3 conductor mains flex works for me )

For reference ....
A 32 turn 22 cm dia coil has a decay of 24 us and requires a 1K damping resistor. In receive the coil is equivalent to a 64 turn mono coil.
A 16 turn 18 inch flat wound coil has a decay of 20 us and requires a 750 ohm damping resistor.


Simplified diagram of a flat wound coil with no twists below ... with thanks to Tesla.

[WM6]

1.
Two strands of 19SWG ( 1mm ) insulated solid copper wire are twisted together at approx 1 turn per inch to form a single twisted pair strand. ( You can do this with an electric drill at one end of the pair and a nail on the fence at the other ... stretching it out to avoid kinks)

2.
Wind the twisted pair onto a suitable coil former. Length required will depend on diameter and number of turns.

3.
If one of the conductor pairs is A and the other B. Cross connect the A conductor from one end of the twisted pair coil to the B conductor of the other end . This junction is the "common". The remaining two ends are the differential ends of the coil.

4.
Connect the coil to the electonics using a 3 conductor lead ( 3 conductor mains flex works for me )

For reference ....
A 32 turn 22 cm dia coil has a decay of 24 us and requires a 1K damping resistor. In receive the coil is equivalent to a 64 turn mono coil.
A 16 turn 18 inch flat wound coil has a decay of 20 us and requires a 750 ohm damping resistor.


Simplified diagram of a flat wound coil with no twists below ... with thanks to Tesla.

Hi moodz

is there any influence how to connect A and B connection wires from Tesla sketch to other PI parts? Can this two coupling points be swapped in connection?

[moodz]

Hi moodz

is there any influence how to connect A and B connection wires from Tesla sketch to other PI parts? Can this two coupling points be swapped in connection?

The A and B can be swapped ... I think .... the field induced pulse polarity would be reversed ... truthfully I have not done enough experimentation to verify effect.

moodz

[moodz]

18 inch flat wound differential coil.

To clarify the construction of a practical diff coil here is the visual detail below.

18 inch flat wound coil.

detail of connection of flex lead.

winding detail .... you can clearly see twisted pairs wound flat in basket weave to minimise capacitance. The black tape is ordinary PVC electrical ... there is no shielding.


moodz

[WM6]

Interesting coil, not seen yet.

Something like "Differential twisted pair spiral zig-zag coil". And for coil cable you use simple main power connection cable, never seen to yet on PI too. And pro connector.

Very inventive. Looks good - must be good. High Q for sure.

[WM6]

How to make twisted pair wire out of lacquer copper (Cul) wire:

http://www.electronics-radio.com/art..._pair_wire.php
http://www.evilmadscientist.com/article.php/twistedwire


Possible solution for moodz coil winding accessories.

[Geo]

Very good coil....
It is time to try it

Regards

[moodz]

How to make twisted pair wire out of lacquer copper (Cul) wire:

Possible solution for moodz coil winding accessories.

Hey ... thats almost exactly how I did it !!! I used a circle of MDF with odd number of holes drilled every inch along outer edge. Used bamboo skewers as spokes. Wind on the coil ... paint with varnish then cut skewers at base to remove coil.

[J_Player]

This is important... see the spokes.
These separate the wires to reduce capacitance.
Remember, capacitance is the enemy in a PI coil... and these spokes separate the windings to reduce capacitance to make a very fast coil.
Better than using litz wire....



Do this same as Moodz did so you can have a fast coil too...

Best wishes,
J_P

[WM6]

Better than using litz wire....

Hi J_P

You mean using litz wire only?
Using litz wire in differential coil construction suppress EMI.
It is worth to try litz wire as proposed instead of say (untwisted) speaker cable (see Tesla diagram) to minimize received noise.

Letter marks of connection wires on Tesla diagram do not fully suit to coil connection letters on schematic.
Maybe this for clarification:

[J_Player]

Hi J_P

You mean using litz wire only?
Using litz wire in differential coil construction suppress EMI.
It is worth to try litz wire as proposed instead of say (untwisted) speaker cable (see Tesla diagram) to minimize received noise.

I do not mean to use litz wire.
I mean to make the coil construction exactly as you see in the photo above.

I believe the Tesla diagram construction will cause unnecessary capacitance in the coil windings, but the Moodz construction using spokes will produce a very fast coil with low capacitance.

Best wishes,
J_P

[WM6]

I do not mean to use litz wire.
I mean to make the coil construction exactly as you see in the photo above.

I believe the Tesla diagram construction will cause unnecessary capacitance in the coil windings, but the Moodz construction using spokes will produce a very fast coil with low capacitance.

Best wishes,
J_P

Sorry for term confusion, it was my fault.
I have in mind "twisted pair", not "litz wire".
Mean twisted pair suppress EMI not litz wire.
Litz wire only suppress eddy currents in thick coil wire.
But if we make twister pair out of litz wire, coil can be even better.

[J_Player]

Sorry for term confusion, it was my fault.
I have in mind "twisted pair", not "litz wire".
Mean twisted pair suppress EMI not litz wire.
Litz wire only suppress eddy currents in thick coil wire.
But if we make twister pair out of litz wire, coil can be even better.

My understanding of a PI coil is you want to keep the capacitance as low as possible. This is to allow a fast time constant which is needed for finding small gold items in the ground.
One way to keep the capacitance low is to separate the conductors in the coil.
Litz wire is known to reduce the skin effect and proximity effect, but I think it could reduce the capacitance some small amount as well due to the extra separation distance in the outer insulation.

I would expect the advantage of using Litz wire would be seen as an improvement in switching time to produce a faster transient during the pulse fall time.
Another method is to make a "spider web" coil to separate the conductors, to reduce capacitance and EMI effects.
This is similar to the method Moodz used by inserting spokes in his coil to keep the conductors from lying tight against each other.

You might see an improvement if you use Litz wire when winding the coil in the method Moodz uses.
I think it would be worth a try if you have Litz wire available for a coil.
Some PI coil builders have reported good results when using Litz wire whether using the spoke design or simple flat coil.
This is often done in hand-wound coils used for radio receivers for the AM broadcast band and long wave.
See photo of a long wave receiver coil using Litz wire in a spider web design:

[Waikiki_Sweep]

I have some doubts regarding twisted pair coil design: high capacitance, 550v between wires.

Thank you for the great project. Can't wait to see it working.
I have some doubts regarding twisted pair coil design:

1. There is a huge capacitance between twisted wires.
If good coaxial cable has 16 pF/ft. then twsted pair has 30-40pF/ft or so.
Coil will have additional approx. 35pF x 60ft. = 2160 pF
You can find twisted-pair capacitance calculation here:
http://www.signalintegrity.com/lib/htm/TWIST.htm
It may help to reduce capacitance if wires will not be twisted
or coated thick insulation with low dielectric constant (teflon,polyethylene)

2. If flyback voltage can reach 1100V then you can expect 550V between
wires in twisted pair according diagramm below.
Regular magnet wire can survive 130V only.
So it requires much stronger insulation. At least 600V teflone coated wire.
http://www.bulkwire.com/wire-cable/p...nded-wire.html
Unfortunately this wire is silverplated so it inducing more eddy currents in the
wire then tinned wires. Teflon coated Litz wire will be best if it is possible to find.

[Aziz]

Fast Differential Coil

Hi all,

if you want a very low capacitance differential coil, just use a 300 Ohm twin lead antenna cable. ( http://en.wikipedia.org/wiki/Twin-lead )
Capacitance: 11.8 pF/m (meter - not foot!)

Wind the coil spiral like with polystyrene tape between windings (winding spacer, 1-2 mm thick). The voltage between each winding will be low so the energy stored in the interwire capacitance will therefore be low too. Only the two coil halves will see the full half flyback voltage. But the distance between them forming a low capacitance.

Connect the coil halves to a center-tapped coil.

Aziz