Autotransformer coil

[Skippy]

Thanks for posting the Racer schematic, Pechkata.

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Quote: "Some of the [schematics] have a coil in parallel to the one with central tap"

This may be the bucking coil that is typically wound close to / over the RX winding in a concentric coil design.

[Altair]

The coil part of the aqua sound schematic is incorrect also the 2n2222 driving the coil should be a 2n2907. See the Nautilus 2B schematic

Do you have some others info about how to do the Aquasound coil?

[pechkata]

Thank you all for all the material you provided me. I have only one question tho about some of the schematic you posted: some of them have a coil in parallel to the one with central tap. I assume it works as a secondary? Like there are both autotrnasformer and "proper" transformer in a single coil?

Yes - it's true...

I am also attaching a schematic without a demodulator...

[Altra]

Do you have some others info about how to do the Aquasound coil?

Yes, I made all of the Aquasound coils and circuit boards for over 15 years, What would you like to know?

[Altair]

Yes, I made all of the Aquasound coils and circuit boards for over 15 years, What would you like to know?

In the past I built a coil for Aquasound based on the information I was able to find and putting a bit of personal interpretation into it; The drawing represents how I made it. However, I think it does not exactly respect the original coil. I managed to balance it, the sensitivity is good, the discrimination is also good but the salt cancellation does not work and, despite the shielding, it is terribly sensitive to the capacitive effect. I'd love it if you could get me on the right street.

[Altra]

I will start a new tread for the Aquasound.

[Pep_T]

UPDATE: I built the circuit in fig 9.11 of ITMD4 and it works! I changed the capacitor accordingly but I have some not too good resoults.

- 3.3nF: Oscillations at 100kHz, 15Vpp, slightly distorted waveform
- 33nF: Oscillations at 39kHz, 15Vpp, pure sinusoidal waveform (based on just looking at it it of course)
- 330nF: Oscillations at 11kHz, 5Vpp, pure waveform but too low in amplitude

The amplitude starts decreasing right after 33nF, even with 47nF you can already see some attenuation. Why is that? I tried and change the value of the base resistors of the transistors without any luck.
Power supply is 12V, transistors are 2n3904, with hFE measured of 285 and 280.

The coil is a Nokta IM28 (sorry I think I called it LM28, that was wrong), with a TX induction of 125uH per inductor, 500uH total, considering that they are in fact tightly coupled.

Any suggestions? Thanks!

[Carl-NC]

Could be resistive losses in the coil. Try reducing R3.

[Pep_T]

That was it, thank you for the quick fix. I had to reduce it down to 220r in order to achive 13Vpp. And the coil resistance is just above 2 ohms.
BJTs are still cold to the touch so I got down to 100r. To my surprise the oscillations dies out in about 5 seconds.
I will stick with the 220r.

Well, Thanks again, I will post here in case of any developements with the TX side of my project

[Rumcajs]

Hi, Pep_T,

please look the HEATHKIT (Radio Shack?) GD-48 and GD-348 metal detector schematics, or MICRONTA metal detector sch. All use auto transformator in the TX.

Electromagnetic waves are attenuated by a factor of 5 or 8 in the soil, so it is not worth increasing the voltage of the resonant circuit beyond a certain limit.

Have nice day

rumcajs

[Detectorist#1]

For PI designs it is clear that more TX power and less RX noise are useful. But for VLF designs I haven't find posted sure answer how more TX energy increases the possibilities. Carl have to know the answer.

[Detectorist#1]

From energy side - no TX energy - no RX energy. But where is the true?

[pechkata]

For PI designs it is clear that more TX power and less RX noise are useful. But for VLF designs I haven't find posted sure answer how more TX energy increases the possibilities. Carl have to know the answer.

https://www.youtube.com/watch?v=DU90a-ejogI

[Detectorist#1]

And what is the answer from my question? If the voltage of the Tx coil is doubled, how this will increase the depth of a target's recognition in VLF metal detectors?

[Carl-NC]

With all metal detectors the strength of the TX field is proportional to the ampere-turns of the TX coil. If you double the voltage then the current doubles (assumes R=0), the TX field doubles, and the target signal doubles. All of this produces about a 10% increase in depth.