Vallon VMH3CS Mine Detector

[FatBob]

Those MT3608 converters are less than $1 on Ebay but remember

"The peak current output current is no more than TV university"
Whatever that means? (It's in all the listings).

[KingJL]

Clues.....I can confirm that the coil is a single mono. I cut a dud one open to have a look at the coil cross section. I can also confirm that the coax braid and the coil shield are not connected to either +3.7V or 0V battery supply rails, whereas the case shield and other metal parts are connected to the supply 0V rail, as is the headphone ground. The coax braid and coil shield sit at +10V with respect to battery negative (0V) and the bipolar +6V and -6V TX pulses sit symmetrically on the +10V.

Eric.

I like clues!!! And really doesn't change my view so far... different signal and power grounds are common. sounds like an up-boost power supply from 2+ to 20 V with circuit ground at 10V. But that is reallay a WAG!

[KingJL]

I had also looked back over the previous posts, and determined by calculation that the drive voltage cannot possibly be restricted to 6V, and needs to be about 300V (or thereabouts) to get the coil current up to 1A in 10us.

Would you mind posting the LTSpice files?

Done... Try these.
vallon_spice.zip
Let me know if I missed something!

[Ferric Toes]

I suspect that's not the case, as earlier you stated that the coil is single stranded with an outer mesh shielding, and the connector is 7-pin with the centre pin as live. All other six connectors are wired together to form an outer shield, and connected to the coax braid.

In which case, an H-bridge arrangement would be using the shield as the live connection on one polarity. Therefore I was wondering if the coil itself is actually two loops in anti-phase (probably bipolar wound). But ... if that was true, then your homemade coil would produce a unipolar, rather than a bipolar pulse.

There's a puzzle here to be solved .....

Clues.....I can confirm that the coil is a single mono. I cut a dud one open to have a look at the coil cross section. I can also confirm that the coax braid and the coil shield are not connected to either +3.7V or 0V battery supply rails, whereas the case shield and other metal parts are connected to the supply 0V rail, as is the headphone ground. The coax braid and coil shield sit at +10V with respect to battery negative (0V) and the bipolar +6V and -6V TX pulses sit symmetrically on the +10V.

Eric.

[Qiaozhi]

I have tried to model the TX based on Eric's descriptions/observations at the beginning of this thread. The model indeed requires 4 mosfets, but not as an H-Bridge.
The parameters described by Eric:
Measured voltage at the coil = 6Volt,
Measured current: 1amp at 10usec... near constant for the next 40 usec,
250V flyback, transmit pulses 530 usec apart alternating
[ATTACH]36934[/ATTACH]
The only thing that i don't remember him stating was a HV kickstart pulse the same polarity of the main TX pulse, but I can see no way to get that 1 amp coil current within 10 usec without it.

I had also looked back over the previous posts, and determined by calculation that the drive voltage cannot possibly be restricted to 6V, and needs to be about 300V (or thereabouts) to get the coil current up to 1A in 10us.

Would you mind posting the LTSpice files?

[Ferric Toes]

For up-converters, it's worth looking at hobbyist modules like those made by Pololu and others. (I have plans for one of these 5 Volt output modules to reduce battery pack size)
Example:
http://www.hobbytronics.co.uk/batteries

There are quite a few variations on the market, but the one I chose uses the MT3608 converter. This runs at a high frequency of 1.2MHz which I thought would give less chance of interference than others that run at lower frequencies. Up to 2A of output current is more that adequate.

I have machined a small housing for the board, which I will wrap in copper shielding tape and ground to the electronics in the detector (my viscosity meter) as there is bound to be a bit of radiation from the inductor. I didn't notice any effect on the first test though.


Eric.

[KingJL]

I suspect that's not the case, as earlier you stated that the coil is single stranded with an outer mesh shielding, and the connector is 7-pin with the centre pin as live. All other six connectors are wired together to form an outer shield, and connected to the coax braid. In which case, an H-bridge arrangement would be using the shield as the live connection on one polarity. Therefore I was wondering if the coil itself is actually two loops in anti-phase (probably bipolar wound). But ... if that was true, then your homemade coil would produce a unipolar, rather than a bipolar pulse. There's a puzzle here to be solved .....

I have tried to model the TX based on Eric's descriptions/observations at the beginning of this thread. The model indeed requires 4 mosfets, but not as an H-Bridge.
The parameters described by Eric:
Measured voltage at the coil = 6Volt,
Measured current: 1amp at 10usec... near constant for the next 40 usec,
250V flyback, transmit pulses 530 usec apart alternating

The only thing that i don't remember him stating was a HV kickstart pulse the same polarity of the main TX pulse, but I can see no way to get that 1 amp coil current within 10 usec without it.

[Qiaozhi]

All I have gleaned from a quick look at the boards is that the bipolar TX is probably an H bridge arrangement as there are four Mosfets close together.

I suspect that's not the case, as earlier you stated that the coil is single stranded with an outer mesh shielding, and the connector is 7-pin with the centre pin as live. All other six connectors are wired together to form an outer shield, and connected to the coax braid.

In which case, an H-bridge arrangement would be using the shield as the live connection on one polarity. Therefore I was wondering if the coil itself is actually two loops in anti-phase (probably bipolar wound). But ... if that was true, then your homemade coil would produce a unipolar, rather than a bipolar pulse.

There's a puzzle here to be solved .....

[Teleno]

Maybe you want to revue your calculation?
Is the signal amplitude not proportional to the turns of the coil? not the inductance.

Yes, the signal in the 1500uH coil is sqrt(1500/300) times stronger. Sqrt(5)= 2.25

[Skippy]

For up-converters, it's worth looking at hobbyist modules like those made by Pololu and others. (I have plans for one of these 5 Volt output modules to reduce battery pack size)
Example:

http://www.hobbytronics.co.uk/batteries

[Tinkerer]

For lack of any schematic I'll venture a guess.

The magnetic field is proportional to the ampere-turns. Inductance is proportional to the square of the no. of turns. Given two coils (300uA and 1500uA) with the same diameter, the 1500uH coil at 0.44 amperes creates the same field as the 300uH coil at 1 ampere. ( sqrt(300) x 1A = sqrt(1500) x 0.44A )

The smaller current in the 1500uH compensates for the larger time constant and the transients of both coils reach the sampling state mor or less with the same delay. To compensate further, the receive singnal in the 1500uH is 5x stronger.

So I think it's perfectly possible to measure low conductivity targets due to the lower current taking less time to decay and the higher sensitivity allowing measurements with a slightly longer delay.

Maybe you want to revue your calculation?
Is the signal amplitude not proportional to the turns of the coil? not the inductance.

[6666]

One thing I have learned is that it is easy today to power things off 3.7V Li-ion batteries. I have just received an up converter the size of a postage stamp where you can put any voltage above 2V in and get an adjustable voltage out of whatever you want up to 20V. I have it running a detector right now that requires 15V and it seems to work a treat. Prior to that I was using 10AA batteries!

Eric.

Hi Eric would you mind telling us which up converter you have, as its good to know what really works, thanks.

[Teleno]

... how this detector can detect such low conductivity targets while using a 1.5mH coil?

For lack of any schematic I'll venture a guess.

The magnetic field is proportional to the ampere-turns. Inductance is proportional to the square of the no. of turns. Given two coils (300uA and 1500uA) with the same diameter, the 1500uH coil at 0.44 amperes creates the same field as the 300uH coil at 1 ampere. ( sqrt(300) x 1A = sqrt(1500) x 0.44A )

The smaller current in the 1500uH compensates for the larger time constant and the transients of both coils reach the sampling state mor or less with the same delay. To compensate further, the receive singnal in the 1500uH is 5x stronger.

So I think it's perfectly possible to measure low conductivity targets due to the lower current taking less time to decay and the higher sensitivity allowing measurements with a slightly longer delay.

[Ferric Toes]

Eric - It has just dawned on me that you have the Vallon completely in pieces! Does that mean you've been able to probe the circuit and discover how this detector can detect such low conductivity targets while using a 1.5mH coil?

Sorry to disappoint, but I am just stringing black boxes together. I can remove the whole electronics with wiring loom and controls connected and just transplant into different cases without knowing any of the circuit details.

The main electronics is in the grey box and consists of two SM boards of equal size piggy backed together. Both sides of the boards are loaded with components and can't be unplugged from each other. They are probably multilayer boards from the component density and would be a nightmare to unravel the details. It is all programmed in firmware too. All I have gleaned from a quick look at the boards is that the bipolar TX is probably an H bridge arrangement as there are four Mosfets close together. Where the RX preamp is, I do not know. Everything is covered in a coating of laquer so that even probing for voltages or waveforms involves penetrating this layer.

I do not have any faulty modules, otherwise I might probe deeper, but I am happy at the moment just shifting the black boxes around to make a detector that is easy to use on land and in water.

One thing I have learned is that it is easy today to power things off 3.7V Li-ion batteries. I have just received an up converter the size of a postage stamp where you can put any voltage above 2V in and get an adjustable voltage out of whatever you want up to 20V. I have it running a detector right now that requires 15V and it seems to work a treat. Prior to that I was using 10AA batteries!

Eric.

[Qiaozhi]

Eric - It has just dawned on me that you have the Vallon completely in pieces! Does that mean you've been able to probe the circuit and discover how this detector can detect such low conductivity targets while using a 1.5mH coil?