Vallon VMH3CS Mine Detector

[TH'r]

Hello Carl;
I too purchased one of the recently advertised Vallon VMH3's from England. They are being shipped with only their field cards. Would you share your manual with us? Thanks

Yes, the unit is complete with the carry case, manual, field card, and firing pin OTP (operational test piece). This unit does not appear to be as good as yours. At max sensitivity it will detect the OTP (which tells me it's working within spec) but a US nickel is only 9-10" or so, and it won't detect my 1x foil standard at all. The TDI easily beats this.

I think there is something out-of-whack on this unit. My foil standards are square 1"x1" pieces of foil layered in clear packing tape, with 1x being single layer, 2x has 2 layers, etc. I have 1x-8x continuous (helps identify target holes), plus 12, 16, 24, and 32x. The 1x is invisible. 2x has a negative response, that is, I get a signal as the target moves away from the coil. 3x has a weaker negative response, 4x a decent positive response and the rest look good from there. I'll try to play with it some more today.

Yeah, the Recon/ATX looks a lot like the Vallon. I also have an ATX so maybe I'll try it as well.

[Qiaozhi]

Hello Carl;
I too purchased one of the recently advertised Vallon VMH3's from England. They are being shipped with only their field cards. Would you share your manual with us? Thanks

I don't know whether this will be of any help, but here's a detailed brochure:
http://www.vallon.de/pdf/VMH3CS_Leaf...5CB2iqj1226624

Also, this company sends out a photocopy of the field instructions with their Vallon detectors, so it might be worth contacting them to see if you can buy a copy -> http://stores.ebay.co.uk/Rays-Tek-Su...p2047675.l2563

[Ferric Toes]

Hi Eric

I had a bad and a good night : a bad night because I didn't sleep well but also a good one because I think I found why I have so strange waveforms !
I have measured the voltage across the coil and everybody knows that V = L di/dt so di/dt = V/L.
You have written that the final current is 1 A. I see that the last 40us of the pulse are at 6V so it means 4mA/us (6/0.0015) or a 160mA variation in 40 us.
The pulse is 48.6 us long so after 8.6 us the current is 1000 - 160 = 840 mA. So in the first part we have about 100 mA / us average slope or 150V average voltage.
We have a sawtooth at about 300V decreasing in 8 us to 6V : that's about 150 V average or 100mA /us.
In the first 8 us, the current as a parabolic shape with a slope decreasing from 200mA / us to 4mA / us and the last 40 us a linear variation from 840 mA to 1A.

Maybe they do that by polarizing the Mosfet with a decreasing voltage from 300V to 6V by discharging a capacitor or something similar ...

Philippe

Hi Philippe,

It's a bit tricky to do, but if you can put a 0.1ohm resistor in the ground side of the coil, this will display the coil current waveform without the high voltage transients. What you should see is as the image on my post no.53. Under the cover that you have removed, the braid termination can be unsoldered by using a slim iron and lifting the wire out of the slotted terminal with slim taper nose pliars. I presume that lead free solder has been used as per regulations, but I found it easier to melt by applying a bit of tin/lead solder first. I also made a heat shroud out of thin brass to prevent any risk of damage to the thread into which the plastic cap screws.

Eric.

[Phiphi]

Eric

The image in your post 53 is exactly as the description I made in my previous post : an increasing current with decreasing slope during about 8 us up to 840 mA followed by a linear increase up to 1A for the last 40us.

Have you done the measurement I made by just setting a scope probe on the coil pins ? You should have obtained the same results than mine, I guess.

Philippe

[Phiphi]

I have made a simulation with PSPice trying to get the same current and voltage coil waveforms than those measured by Eric for the first one and by me for the second one.
Here is the schematic :



That's the classic one but the Mosfet source is polarized with a signal going from -300V to -6V in 8 us instead of a constant voltage.
The result is shown below



On top is the current in the coil increasing quickly to about 850mA in 8us with a parabolic shape followed by a linear increase to about 900mA in 40 us.
On bottom is the voltage across the coil with the first part with a sawtooth shape and the second one with the classic back-emf.

So now we have to imagine how to realize practically the source polarization.

[Qiaozhi]

In #342, KingJL posted a simulation with real components. There's an updated version in #355.
I also simulated something similar using ideal components in #362, and Teleno offered a monopolar version in #364.
KingJL's simulation looks the closest to the actual results.

[Phiphi]

In #342, KingJL posted a simulation with real components. There's an updated version in #355.
I also simulated something similar using ideal components in #362, and Teleno offered a monopolar version in #364.
KingJL's simulation looks the closest to the actual results.

OK, I see now there is much work on this topic ! But I don't understand why Eric was surprised by the voltage waveforms I have measured across the coil ...

Philippe

[Ferric Toes]

I have 2006 VMH3CS where it is easier to separate the two circuit boads. In the TX there are 8 IRF320 mosfets; four on the topside and four on the underside positioned just slightly offset to one another. It is not possible to see how they are interconnected as it is a multilayer board. It looks like three layers, as up against the light there is an internal ground plane layer that does not appear on the top or bottom. I must say that this is the most complex detector electronics that I have ever seen. The OPA 627A, which I reported as possibly being the RX preamp, has another IC on the underside with the number 4532A on it, which are N and P channel mosfets.

Eric.

[Ferric Toes]

OK, I see now there is much work on this topic ! But I don't understand why Eric was surprised by the voltage waveforms I have measured across the coil ...

Philippe

Hi Philippe,

Your waveforms are quite correct, but we approached the problem differently. I look at the unloaded TX waveform first to check the shape, amplitude, width and the frequency. I then look at the inductively loaded current waveform across a 0.1R resistor to check the turn on time and turn off time, as it is this that generates the necessary magnetic field and generates eddy currents in the target. At some point I also measure the flyback voltage but didn't examine the detail of its shape. The difference in turn off and turn on was new to me, but it make sense now we are certain that the turn on is assisted by the stored 250V flyback from the previous alternate pulse. When I use the x10 probe to give 20V/cm vertical scale, I can then see the minus/plus 6V alternate TX pulse along with 80V of the high voltage spike either way. By reducing the timebase to 10uS/cm I can clearly see the differences in shape of the two spikes. That was something I had not done before.

Eric.

[Ferric Toes]

These are the boards referred to in 443. The boards are plugged together and the first picture shows what you would see on top and bottom side when you turn it over in the plugged state. The second picture is what is inside (middle layers) when unplugged. The furry component is an inductor which is later versions is slightly smaller and mounted on the recessed end of that board.



The four IRFR320 mosfets are at the top of the board with the OPA627 preamp on the right near the yellow/orange twisted lead.



Four more IRFR320 mosfets on the underside and the 4532 P/N cmos gate almost under the OPA627.

Eric.

[KingJL]

Eric,
How did your field testing with the Vallon w/your home made round mono work out? Was there a noticeable difference with it vs the original vallon coil?

[Ferric Toes]

The Vallon coil is 11.5in x 6in and my coil is 10.5in circular. Measurements are take from centre to centre of the winding bundle. My winding should be 11in diameter but the winding jig was a bit too tight so I lost 1/2in. However, there was little noticeable difference in performance. Vallon make a 12in circular coil but quoted me 573 euros. The battery cap is 35 euros, so you can imagine what a new VMH3CS would cost. It is a shame that other manufacturer's coils don't work as I have quite a selection. I am going to wind up a 15in coil at some stage, as that should show a noticeable improvement.

What do you make of the 8 Mosfets and the ring core transformers? I surmise that the Mosfet's gate drive is via the transformers.

Eric

[KingJL]

What do you make of the 8 Mosfets and the ring core transformers? I surmise that the Mosfet's gate drive is via the transformers.

Eric

I am still studying that... It seems the arrangement is different between the MOSFETs on the bottom vs the top. The 4 on the bottom of the board (the side without the torroids) are arranged in pairs with the drains connected together and sources tied together thru resistors to the same point for each pair (can't tell where the gates are connected), whereas the 4 on the top are arranged in pairs with the source connected to the drain and gates tied together for each pair. The torroid transformer windings appear to use awfully heavy gauge wire for use a pulse transformrs. There needs to be a means of generating the HV for driving the leading edge of the TX for ~10us. I had assumed that the HV kick was from capturing the HV flyback, but the TX voltage waveforms seem to refute this... capturing the HV flyback reduces the voltage significantly from the 250v that is indicated by the TX coil waveforms. Also capturing the flyback of 1 amp coli current will only kick start to about 750 ma. I suspect there is an addition HV being generated to kickstart at least 1 MOSFET in each TX cycle to 1A with the other MOSFET working off LV with total resistance set to limit current to 1A. I agree that the gates are most likely transformer driven. My experiments with driving MOSFET gates with pulse transformers suggests a 2:1 ratio with a minimum 500uH in the primary (preferably 1mH).

It keeps getting curiouser and curiouser!!!!

[KingJL]

I am assuming that the yellow and orange wires at the top right of the board with the torroids are the wires that go to the coil. Or maybe I shouldn't assume!

[Ferric Toes]

I am assuming that the yellow and orange wires at the top right of the board with the torroids are the wires that go to the coil. Or maybe I shouldn't assume!

Yes that is correct. The red and blue wires are the supply volts. That connector plugs into a third pcb which is basically a distribution board with cable runs to the control switch, display/push button, and data/headphone connector. There appears to be a small switch mode psu on that board also. Maybe for the LED display.

On that third board there is a second 8 pin connector where the raw battery supply comes in (red and blue wires also) and the coax cable to the coil goes out from that. The coax links by pcb tracks to the orange/yellow wires, with the orange being the coax braid.

Eric.