Nexus coil

[Locator]

Hello,
Ι have Nexus standard MKII metal detector and i would like to construct a big coil like Ultima(50X80cm).
Any ideas?

Thank's in advance!

[dfbowers]

Hello,
Ι have Nexus standard MKII metal detector and i would like to construct a big coil like Ultima(50X80cm).
Any ideas?

Thank's in advance!

I may be wrong.. but what information I got.. Just for a base line, the 2 9” OO coils are 100 turns each. The TX coil is probably center tapped in an autotransformer configuration (140v P-P)? Put a scope on it and you will see.
Anyway, you would probably have quite a challenge to null the coil and keeping it that way.

[Locator]

With Nexus standard MKII i have a 13" DD coil which have 5 pin connector:
between pin 1 and 5 we have self-induction=1.4mH and R=22ohm(i believe transmitter)
between pin 2 and 3 we have self-induction=5.55mH and R=150 ohm(i believe receiver)
the ground of the cable is connected to the pin 4,also between 4 and 5 we have 380μH with R=4.8ohm
and between 1 and 4 373μH.
Is the pin 4 middle receiver of the coil or the ground is connected to it?
I believe that like the 13" DD coil is constructed and the big Nexus Ultima(50X80cm)coil.

Thank's for your time!

[AK48]

hello , all what i have for nexus brand

File0026.pdf

[AK48]

good schematics

Varna_sch.pdf

[Locator]

Hello,
I made some measurements today about 13" Nexus DD coil which i have and i found that it can not have any capacitor in series(in the coil)
because it would give a very high value of inductance and coil resistance.
Also i found that the pin 4 is the middle receipt of the transmitter coil.I made another coil with the same induction and i "took" a middle receiver...so
it gave me the same values! 380+380μH and in total 1.4μH.
If we assume that we can put the big(50X80cm) OO coil in the position of 13" coil and work without problem,
so the coils are the same internally and looks like the Varna vlf circuit!

[sirbedevir]

The TX and RX coils of the nexus mp v-2 device are completely identical: they have the same number of turns and are wound from the same thick wire. For example, the 13" 5.7 kHz head has 50-50 turns, 1.4 mH inductance, 0.5 or 0.6 mm diameter wire. The only difference between TX and RX is that the center tap of the TX is led out, and the resonance capacitor of the TX is molded in. What is the reason that the TX and RX are the same? After all, if they have a different number of turns or wire thickness, they could be tuned for resonance. I want to build a special depth finder head for my factory Nexus, so this question is interesting to me.

[sirbedevir]

For a 50" 5.7 kHz head, what kind of inductance, what number of turns, what thickness of wire would be ideal?

[sirbedevir]

I made a 110×100 cm search coil for nexus MP. It works well. In air test, a half liter beer can is 170 cm.

[Locator]

[Locator]

Can you give us construction details?

[sirbedevir]

The original 5.7 kHz search heads have identical TX and RX coils, with an inductance of about 1.5 mH and a resistance of 2 ohms. For such a large search head, this would mean 24 turns and 0.8 mm diameter wires. I made the coils this way, but it was not possible to zero them by positioning the TX and RX coils, the copper mass was too large. Therefore, the RX was made of 0.3 mm diameter wire with 24 turns, and the center-tapped TX was made of 8 pieces of 0.3 mm Litze wires with 24 turns. The RX has 1.8 mH inductivity. The TX has 2 A sinus current. I wound a hair-thin copper wire on each coil with a pitch of 1 cm and sprayed it with graphite spray, this is its shielding. The shielding does not reach the entire circumference, it is interrupted in a 1 mm ring. These coils could be nicely balanced by positioning and using a 5 cm diameter ring cut out of aluminum foil. The original nexus heads are carbon fiber laminates, but I made a fiberglass laminate instead. The template is made of XPS glued to a plasterboard sheet, coated with a layer of epoxy and many layers of mold release. I used vinyl ester resin for the lamination. I heat-treated it at 80 degrees C, then glued the TX and RX coils in. It was molded with hard PUR foam. Of course, it weighs more than the factory head, but it works well, it is not noisy, and you can search with it. A lot of work, it is easier to buy the factory one, and it also weighs less. But it was a fun game to make.

[sirbedevir]

Oh, I forgot that the transmitter needs a resonant capacitor for the 5.7 kHz frequency. Polyester film capacitor, about 560 nF, 250 V.

[sirbedevir]

[Gwil]

Post # 12: " I made the coils this way, but it was not possible to zero them by positioning the TX and RX coils, the copper mass was too large. "

This is interesting. The way I see it there are three signals that mught need nulling out:

(1 ) Inductive (of course)
(2) Capacitive
(3) Resistive

(Resistive coupling can be ignored assuming good insulation. )

It seems to me that an inductively coupled signal must go through zero as the coils are adjusted and the signal goes from In-Phase to Out-Of-Phase, so it should be possible to eliminate this component. Is this true?

Capacitive coupling can occur within the control box-coil cable if the cores are not screened from each other, or between the coils if they are not screened from each other.

So I'm wondering why the coils could't be nulled, and whether the unwanted signal was inductively or capacitively coupled, or both.
Maybe I'm missing something. I'd be grateful for an explanation.