Zeroing DD coil with single loop

[Mullihaka]

There are instructions for construction of a DD coil, which seems to have an interesting feature: nulling with a single loop of wire. The article is in Russian, so I'm not sure if I'm correct. Have a look at http://www.metdet.ru/IB/, select sensor30.zip and open the Word doc.

Can someone please tell, if the single loop shown on page 10 is really used for nulling? If yes, I'd be grateful to have a rough translation of the nulling procedure. Also the possible benefits and drawbacks using this method would be nice to know, if described in the document.

I wonder, if this method could be used with any DD coil. It looks promising, as moving only a single loop to null the coils would not be so accurate work. The required movement should be the much larger than moving the whole coil.

All hints appreciated,

MH

[porkluvr]

The single loop is only effective as a fine adjustment. You must first already be very close to null with the basic physical geometry.
Babelfish can translate Russian into English. It's not perfect, but better than only having pictures to look at. Here: http://babelfish.yahoo.com/translate_txt

With a 150 word limit, you must do it in little chunks. Paste the translated text back into the original document to minimize the number of applications you need on your screen at one time.

It looks like the search head in the article has a phase compensating RC network. The RC components values would be specific to one particular detector. The heading at the article's beginning seems to confirm that there is a "subject" detector.
I speak not from practical experience, only regurgitating what has been said before, but you should probably count on having to use a combination of proper placement, adding or removing a turn or three from one or the other coils, AND THEN using the placement of a loop of wire for a final tweak!

Don't plan on a single loop of wire to be a cure-all. You still must work up to the final null intelligently and methodically. Then, with everything pretty much fixed in place (and a rough null already obtained), with luck, playing with placement of a single turn could be icing on the cake. Good luck!!

[Qiaozhi]

The nulling of coils is a common question in the forums, and there is often much emphasis placed on getting the residual voltage to be as close to zero as possible. BUT - this is most often not the most desirable situation. In fact, most coils that are adjusted to give minimum residual voltage fail to work in practice. So why is this?

It very much depends on the detector you are planning to use. The GEB and DISC circuitry usually requires an initial phase-shift between the TX and RX coils, otherwise the detector will be unusable. It is also important to note how an original coil reacts to ferrous and non-ferrous targets. Does the RX signal phase-shift left or right, and what happens to the amplitude? You will also find that commercial coils will have something like 15mV residual voltage, and this is not because detector maufacturers are lazy when nulling their coils.

For example, the Tesoro detectors require an initial phase-shift between TX and RX of about 120 degrees. A concentric coil will phase-shift left for all targets (ferrous and non-ferrous), and will have an amplitude increase for non-ferrous and a decrease for ferrous. The situation is slightly different for a DD coil, but the result is the same.

This is why you cannot simply null down to nV, and expect to have produced a perfect coil. It just doesn't work that way. First you must adjust the coils to achieve close to zero volts at the RX coil, but only approximately. Then carefully move the coils apart to achieve the correct phase-shift between TX and RX.

[kt315]

garrett coils have NOT single loop, to your attention. these guys use MORE perfectly tune way. you must learn more productive tuning way, not tesoro not whites not fisher but garrett.

Anatolij,
from Lithuania
fresh perfect technology and electronics for you and for all

[Qiaozhi]

garrett coils have NOT single loop, to your attention. these guys use MORE perfectly tune way. you must learn more productive tuning way, not tesoro not whites not fisher but garrett.

Anatolij,
from Lithuania
fresh perfect technology and electronics for you and for all

Garrett coils have some passive components in the search head to achieve the correct tuning. Or are you referring to something else?

[kt315]

yes. click on link, wait 10 seconds, then save a file
http://slil.ru/27026602

[Rega]

Hi kt315,
it tell me file not found, there is a copy somewere??

Thanks

[WM6]

it tell me file not found, there is a copy somewere??

In fragment here:

[Rega]

Thank you WM6!
I hoped it was something of specific for tesoro coil DD.
I have build a bandido tesoro with a dd coil and It is unstable in all metal and normal tune.

[WM6]

I hoped it was something of specific for tesoro coil DD.
I have build a bandido tesoro with a dd coil and It is unstable in all metal and normal tune.

It is about russian detector Koshei and not about Tesoro.
But some tunning solutions can be usable to tunning Tesoro DD coil too.
Your coil is probably wrong shielded or wrong wired to pcb.
Here DD coil wiring and tunnig schematic:

[Tinkerer]

DD coil fine tuning

It is about russian detector Koshei and not about Tesoro.
But some tunning solutions can be usable to tunning Tesoro DD coil too.
Your coil is probably wrong shielded or wrong wired to pcb.
Here DD coil wiring and tunnig schematic:

The TINKERERS discriminating PI, uses a DD coil. For precise adjustment, a single turn adjustment coil is added.
A precisely adjusted coil works perfect on mild ground.
However, for aggressive ground, a further adjustment has been added.

The current in the adjusting single turn coil is being controlled by an on board variable resistor.
Since this resistor has to handle high currents, a Mosfet is being used as the resisting element.
This makes it possible to compensate for a highly conductive environment like salt water, or a highly reactive magnetic mineral ground.

More pictures about the DD coil assembly are to be found at: http://www.geotech1.com/forums/showthread.php?t=15467

[porkluvr]

uh-oh

Tinkerer, the zener is reversed in your PMOS transistor.

That's not quite as bad as when I drew a gate arrow facing the wrong direction.

[WM6]

The current in the adjusting single turn coil is being controlled by an on board variable resistor.
Since this resistor has to handle high currents, a Mosfet is being used as the resisting element.

Interesting solution. Thanks!

[Tinkerer]

Tinkerer, the zener is reversed in your PMOS transistor.

That's not quite as bad as when I drew a gate arrow facing the wrong direction.

Thanks porkluvr,

You are right, the source should be on the +15V.
Also the R 24 resistor should be on the ground. It could be reduced to 1 Ohm.

This circuit allows the signal to be compensated about 1V positive or negative.

Tinkerer