Vallon VMH3CS Mine Detector

[Asgard]

Hi Eric, thanks for the reply.

This sounds the same as the Square wave Pulse Induction detector Deemon is/was building, link below.

http://www.geotech1.com/forums/showt...ulse-Induction

That's right !

We are working on this technology for some time (and Tinkerer willy.bayot and me) at the beginning there was Demoon, Tinkerer and Aziz, Minelab has recently inspired by this technique ...
There have found their sources on this forum but also australianelectronicgoldprospectingforum

Eric, normally if we are right you will not find damping resistance.

Alexandre

[Ferric Toes]

That leaves ~250pf for the coil... about what I would expect for a 1.5 mH coil made with 23AWG enamled copper.

I just found the exact shielding material Vallon used. It is Scotch 24 Electrical Shielding Tape - Tinned copper wire braid. I see there is some on ebay for $7.0. I have a reel but never really used it as it is hard to hold in place on a coil. I prefer adhesive backed tape. The shield adds a bit of capacitance too.

[KingJL]

I am suspecting some sort of voltage doubling pulse forming network technology... one for the positive side and one for the negative side each with a ~50/1000 usec pw/prt. We used that sort of thing back in my radar days. That would explain the short rise time and constant current.

[KingJL]

...
The resonant frequency of the coil and associated cable is 170kHz which is very low... I measured 330pf just for the cable...

That leaves ~250pf for the coil... about what I would expect for a 1.5 mH coil made with 23AWG enamled copper.

[ivconic]

Things "getting curiouser and curiouser"! There is definately some hidden magic going on... 3 D cells, 6V TX, 1.5 mH - 3 ohm coil which should yield ~500 usec tc, yet coil current 1 A in 10 usec. I want some of that!

Exactly my thoughts!
30 hours with such drain... must be very good those batteries!

[Ferric Toes]

Did you note how long it takes for the voltage, as measured at the coil, to decay to [let's say] 1V?

That remains to be done along with many other unknowns.
The resonant frequency of the coil and associated cable is 170kHz which is very low. With coils that I made for sampling at 10uS, I would try to achieve a f.res of 400-500kHz. with cable.
The coil wire is AWG23 enamelled copper with a mesh screening wrapped round. Bundle wound coils with this type of wire are not usually good for short delays. The coax is lap wound which usually makes for high capacitance and with the coiled cable in the telescopic shaft I measured 330pf just for the cable.
Somehow all this appears to work well.

[mickstv]

Hi Mick, Yes, I managed to put in a 0.1 ohm resistor in series with the coil. What I see is a rectangular pulse of 0.1V which means that the pulse current is 1A; very simple. The current rises exponentially in 10uS to the full value and then is substantially flat until the cutoff at 50uS. As stated before, this is a bipolar system so both positive and negative pulses are the same. Looks like there must be a current limiter otherwise you would have a sawtooth shape with that amount of inductance.

Eric.

Hi Eric, thanks for the reply.

This sounds the same as the Square wave Pulse Induction detector Deemon is/was building, link below.

http://www.geotech1.com/forums/showthread.php?20140-Square-wave-Pulse-Induction

[KingJL]

Things "getting curiouser and curiouser"! There is definately some hidden magic going on... 3 D cells, 6V TX, 1.5 mH - 3 ohm coil which should yield ~500 usec tc, yet coil current 1 A in 10 usec. I want some of that!

[Ferric Toes]

Would be interesting to open it and see how it is damped.
Enclosure looks pretty tight, i guess there is not much room inside for robust electronic?
As low power design it looks very interesting for seeing how is done.
What exactly would be the power drain?
Considering that it is powered with only 3 cells and designed for serious business.

There must be at least two circuit boards in the case. I have one that came attached to some coils parts that I bought about two years ago. It looks like the audio and display driver and it still had the vibrator attached that is somewhere in the handle. All is very miniature and surface mount parts.
I am not going to attempt to open my good unit until I get a maintenance manual.
I did however manage to measure the current drain which is 340mA, This is higher than I expected and it looks as though there is no attempt to recover energy at switchoff.

[Ferric Toes]

Hi Eric, have you had a look at the TX current yet ? is it triangle or square wave ?

Hi Mick, Yes, I managed to put in a 0.1 ohm resistor in series with the coil. What I see is a rectangular pulse of 0.1V which means that the pulse current is 1A; very simple. The current rises exponentially in 10uS to the full value and then is substantially flat until the cutoff at 50uS. As stated before, this is a bipolar system so both positive and negative pulses are the same. Looks like there must be a current limiter otherwise you would have a sawtooth shape with that amount of inductance.

Eric.

[daverave]

reminds me of a garrett ATX....could both be similar technology ???

[Qiaozhi]

At the end of each pulse is a back emf spike of 250V nicely damped.

Did you note how long it takes for the voltage, as measured at the coil, to decay to [let's say] 1V?

[ivconic]

Actually it was quite easy as there is a cover that you can unscrew near the coil swivel. Underneath are two pins that connect the coax to the coil winding and it was easy to get the scope probe on. Result; it is pulse induction as I know it. The drive voltage to the coil is a 6v rectangular pulse 50us wide on both polarities. At the end of each pulse is a back emf spike of 250V nicely damped. The mystery remains as to how they get high sensitivity to small nuggets. Fully GB'd on ironstone a 0.3gm nugget gives a good signal at 1in and just detectable at 2in.

No more tests tonight - my wife is frowning at me.

Eric.

Would be interesting to open it and see how it is damped.
Enclosure looks pretty tight, i guess there is not much room inside for robust electronic?
As low power design it looks very interesting for seeing how is done.
What exactly would be the power drain?
Considering that it is powered with only 3 cells and designed for serious business.

[mickstv]

Hi Eric, have you had a look at the TX current yet ? is it triangle or square wave ?

[Ferric Toes]

Hi Eric, nice to hear from you. I recently borrowed a Scheibel MIMID mine detector and was surprised at the performance, especially given its narrow long coil. I was far more interested in the package design and I didn't investigate how it worked. Had to return it, but now looking to buy one.

Hi Carl, Yes, I got quite enthused with the Vallon unit and decided it was time to return and discuss it on Geotech. I have Sheibel AN19/2 PI unit which I was not impressed with as it has no ground compensation and the coils were unshielded resulting in instability on a salt wet beach. The coil cable was unshielded too and if loosly wrapped around the top aluminium shaft section would give false signals. I hope their later units are better as I would not want to go mine hunting with the AN19/2. I am looking for a maintenance manual for the Vallon, but I guess I will have to try and get one from the manufacturer as there seems to be nothing on the internet.