Vallon VMH3CS Mine Detector

[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.

[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.

[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

[Asgard]

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.

some survival blankets that have the distinction of having their apparent conductive face and a very low metal thickness (1 to 2 μ)
One can measure 1 meter to 10ohm

They remain above the skin effect in the frequencies we use (although analyze the pulse frequency mode with a spectral analysis that shows the harmonic in the 100 Mhz)

I worked a long time on this subject, you can also use it .:

http://fr.aliexpress.com/item/1x-30m...883373575.html

If you also want to reduce the capacitance on your machine they must change the mosfet heatsink which varies too much capacitance, you must directs you to this kind of thing:

http://www.amecthermasol.co.uk/datas...c%20Series.pdf

It is an antenna effect! with the human body, with the ground card, with the casing etc! it's even worse when you have some very flyback!

It would also be good to change the mosfet and driver !

Alexandre

[Asgard]

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.

This is normal for this system, the capacitance is not a problem, the section of the wire must be large to minimize the resistance line, which will allow later a better energy recovery, since it does there will be no losses by joule effect (there are more damping)

The coil resonates with either its internal capacitance or so with the addition of an external capacitance to adjust the frequency of resonnance during the pulse.

It is a pulse / VLF

Alexandre

[greylourie]

According to the manufacturer around 30 hours of use out of a set of batteries. And another report shows similar figures. (http://www.arl.army.mil/arlreports/2010/ARL-TR-5282.pdf)

[ivconic]

...If you also want to reduce the capacitance on your machine they must change the mosfet heatsink which varies too much capacitance, you must directs you to this kind of thing:
http://www.amecthermasol.co.uk/datas...c%20Series.pdf
It is an antenna effect! with the human body, with the ground card, with the casing etc! it's even worse when you have some very flyback!
It would also be good to change the mosfet and driver !Alexandre

I noticed that on some older designs.
I wonder; if such heatsink is isolated from the drain (electrically insulating but thermally conductive washer put in between) and further connected to ground: would it be any positive effect?

[Asgard]

I noticed that on some older designs.
I wonder; if such heatsink is isolated from the drain (electrically insulating but thermally conductive washer put in between) and further connected to ground: would it be any positive effect?

Everything is an area of question!

There are 3 problems at this level, the capacitance, the antenna effect for EMC, and thermal noise.

Over the rds mosfet is high more it heats, so you have a MOSFET with low RDS but also low capacitance, etc ... (many parameters are important) It's very difficult to find it.

We must dissipate this energy, dissipation pad of the mosfet has a surface and this surface will be close to the metal grounded card more capacitance will be great and will vary depending on many physical parameters. (Mass of the case, mass of the ground plane, with large flyback, no mercy!)

We must therefore avoid any metal sink with a large surface parrallel this mass to avoid the capacitive effect).

Unfortunately when a metal heatsink is good, this surface is large ! we must avoid to dispel with metal or electrical conductive materials!

The antenna effect will be more reduced ... and the electrcal noise output from the preamp will be greatly improved. etc....

The fact that Eric is not ground plane on the card at this level that the capacitance decreases relative to the sink, it is a chance, regarding the goldscan 5c case are plastic so it will also in the right direction, but using is kind of materials would have felt a huge noise difference after the preamp .... Best dissipation and almost no antenna effect!

Alexandre

[Ferric Toes]

This is normal for this system, the capacitance is not a problem, the section of the wire must be large to minimize the resistance line, which will allow later a better energy recovery, since it does there will be no losses by joule effect (there are more damping)

The coil resonates with either its internal capacitance or so with the addition of an external capacitance to adjust the frequency of resonnance during the pulse.

It is a pulse / VLF

Alexandre

I do not see any evidence that the pulse is causing the coil to resonate as in a 'pulsed VLF', but it is early days yet and I need to do some mods to my measuring setup.

[Ferric Toes]

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

At the moment I have two problems that prevents me from seeing what is going on at a low level i.e. when the flyback has died down to a volt or less. One is that being a bipolar pulse, my scope has trouble separating the pulses and the timebase triggers on both so that they appear superimposed on the screen. Normally I would trigger externally from another point in the detector's pulse generator, but I do not want to open up the detector at this point. The other thing that is easier to solve is that the scope overloads if I turn the sensitivity up to look at voltage levels of a volt or less. This is because there is a 250V flyback at the termination of each pulse and I need to put in a diode clipping network to get rid of that. I am using a x10 attenuation probe so the scope is seeing 25V. By the way and the scope is a Tektronix 2445A. If anyone knows a way of triggering off only say the positive pulse of a bipolar pulse train, let me know. The only access pins I have to the electronics are the two terminal pins in the coil housing. Maybe it could be done with diodes but I need to be careful not to upset the coil voltage waveform that I want to observe.

[Asgard]

I do not see any evidence that the pulse is causing the coil to resonate as in a 'pulsed VLF', but it is early days yet and I need to do some mods to my measuring setup.

Thank you Eric, like us informed.

I'm happy to finally talk to you, I have spent the past 10 years analyzing your work, I did a great job of retro engineering, I completely rebuilt the following models GoldQuest ssv 3 aquastar DeepStar 3 (last model Sami aquastar) goldscan 5B and 5C, some differences between versions of goldscan, which finally gave the TDI but that is far worse than your last goldscan 5C exit of your factory.

Meanwhile, I changed many things in your detectors principes, I added PIC32MZ type of microcontroleurs for timing, I think you have to improve substantially the analog part, we are doing a lot of extra things with digital .

Unfortunately although the results is exeptional, a new technology consumes me much mind! I continue to develop models (not commercial) and repair your metal detector and upgrade, I think by having seen more than a hundred of all kinds worldwide.

It is unfortunate that you are completely stopped and forsaken the patterns of goldscan at "White's" I would dream to take your job .... Now that Carl has left nobody continue your work ...

Without wishing to boast, and Isay something before it is sure, I think I'm the only one resumed and continued your work since your absence ...

Too bad....

Alexandre

[Chet]

At the moment I have two problems that prevents me from seeing what is going on at a low level i.e. when the flyback has died down to a volt or less. One is that being a bipolar pulse, my scope has trouble separating the pulses and the timebase triggers on both so that they appear superimposed on the screen. Normally I would trigger externally from another point in the detector's pulse generator, but I do not want to open up the detector at this point. The other thing that is easier to solve is that the scope overloads if I turn the sensitivity up to look at voltage levels of a volt or less. This is because there is a 250V flyback at the termination of each pulse and I need to put in a diode clipping network to get rid of that. I am using a x10 attenuation probe so the scope is seeing 25V. By the way and the scope is a Tektronix 2445A. If anyone knows a way of triggering off only say the positive pulse of a bipolar pulse train, let me know. The only access pins I have to the electronics are the two terminal pins in the coil housing. Maybe it could be done with diodes but I need to be careful not to upset the coil voltage waveform that I want to observe.

Hi Eric
You might try using the external trigger input jack connected to a pickup loop or another mono coil placed in the field of the Vallon coil. This worked for me on another bipolar detector.
Since the 1.5 mH coil is a slow coil; could it be that it is compromising for speed by enhancing the magnetic field with the additional turns in the coil? This would provide more eddy currents into the small firing spring or firing components of a mine. Also the short 50 usec transmit pulse should be adequate for these small parts.
Glad to see you back, have a good day,
Chet

[Davor]

Acceleration of current rise may be achieved with some high voltage discharge, either by discharging a capacitor (voltage approach), or by discharging a cored inductor (current approach). With bipolar Tx this voltage may be extracted from a flyback as a sort of snubber/damping network. In that case there is a partial recuperation mechanism involved. That may explain why that machine complaints when coil resistance is high.

[Ferric Toes]

Thank you Eric, like us informed.

I'm happy to finally talk to you, I have spent the past 10 years analyzing your work, I did a great job of retro engineering, I completely rebuilt the following models GoldQuest ssv 3 aquastar DeepStar 3 (last model Sami aquastar) goldscan 5B and 5C, some differences between versions of goldscan, which finally gave the TDI but that is far worse than your last goldscan 5C exit of your factory.
Meanwhile, I changed many things in your detectors principes, I added PIC32MZ type of microcontroleurs for timing, I think you have to improve substantially the analog part, we are doing a lot of extra things with digital .
Unfortunately although the results is exeptional, a new technology consumes me much mind! I continue to develop models (not commercial) and repair your metal detector and upgrade, I think by having seen more than a hundred of all kinds worldwide.
It is unfortunate that you are completely stopped and forsaken the patterns of goldscan at "White's" I would dream to take your job .... Now that Carl has left nobody continue your work ...
Without wishing to boast, and Isay something before it is sure, I think I'm the only one resumed and continued your work since your absence ...
Too bad....

Alexandre

Thanks for the complements. I'm sure you have learned a lot in the 10 years and also made many improvements to my designs. However, things move on and not always in the way we expect. I still have my workshop and all my test gear and have used the past five years developing, in an on and off way, a specialised industrial PI detector which is now in use in several countries around the world. On Monday a unit will be shipped out and heading for Canada. I take it easy now and don't look for work where I am under any pressure. When you reach 77, the years go by much quicker and sometimes I will go under the radar for a while doing different things and then something fires up the interest in hobby detectors with slightly different technology and I'll be back, like now.

Eric.

[Qiaozhi]

At the moment I have two problems that prevents me from seeing what is going on at a low level i.e. when the flyback has died down to a volt or less. One is that being a bipolar pulse, my scope has trouble separating the pulses and the timebase triggers on both so that they appear superimposed on the screen. Normally I would trigger externally from another point in the detector's pulse generator, but I do not want to open up the detector at this point. The other thing that is easier to solve is that the scope overloads if I turn the sensitivity up to look at voltage levels of a volt or less. This is because there is a 250V flyback at the termination of each pulse and I need to put in a diode clipping network to get rid of that. I am using a x10 attenuation probe so the scope is seeing 25V. By the way and the scope is a Tektronix 2445A. If anyone knows a way of triggering off only say the positive pulse of a bipolar pulse train, let me know. The only access pins I have to the electronics are the two terminal pins in the coil housing. Maybe it could be done with diodes but I need to be careful not to upset the coil voltage waveform that I want to observe.

Looking at the user manual, there doesn't seem to be an obvious way of successfully triggering the scope without using the TX oscillator as a source. If you had access to a digital storage scope, it would be a simple matter of either using the Single Trigger Mode, or pressing the Run/Stop button to freeze the display.