Vallon VMH3CS Mine Detector

[Koala]

Coil looks odd plus 7 pin connector. Could it be two coil.

[Carl-NC]

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.

[Ferric Toes]

Coil looks odd plus 7 pin connector. Could it be two coil.

Some time ago, I bought six Vallon coils and shafts on ebay. These were ones that had been scrapped by the Army as faulty. Four of the coils were good with the faults being in the shafts or connectors. One of the dud coils was cracked where it had been whacked against a rock, so that one I cut open to see what was inside. It is a single coil around the periphery with the crossbar empty. The wire is solid single strand with an outer mesh shielding, but I have yet to measure the wire gauge. The 7 pin connector has the centre pin as live to the coax core and all six outer pins are wired together forming an outer shield and connected to the coax braid. The thin coax is coiled like a spring within the telescopic shaft. A lot of thought went into the design of this mine detector and my first impressions are that it will make a good top end hobby unit. For normal use as such it is almost indestructible and totally weather and dust proof.

[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.

[mickstv]

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

[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.

[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?

[daverave]

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

[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.

[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.

[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!

[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/showt...ulse-Induction

[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.

[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!

[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.