ARMD (ARMRADIO based Metal Detector) VLF IB PROJECT

[moodz]

In the meantime i have bread boarded the ARM board with power supply / blue tooth / rotary encoder / audio amp / LCD / TX board and just about to wire in the RX board.
So far so good .. no smoke and everything working.

[ivconic]

You are over analyzing the circuit Ivica .. there is no shoot thru ( or if there its not a problem ).

Just use a TC4428A chip to drive both sides of the bridge. Its available in DIP no smd. The worry about your driver is making me ill.

moodz

Explain then why Carl burned his set at the first attempt?


TC4428 I can not obtain, as I mentioned several times, nor the other lookalikes.
Besides I like to try with "simple" components. that can be obtained anywhere.
Is there a problem with the schematic?
That's why I posted it; to hear other opinions and remarks.
But nothing like ".... use this or that driver chip..."

[Carl-NC]

Your schematic looks fine. I personally like to AC-couple the TX clocks to prevent any chance of a latch-up fire. You only need to worry about shoot-thru if your clocks have too much overlap, that should be fairly easy to avoid.

My first attempt smoked because I let Q3 and Q4 avalanche, and I was using lower voltage devices everywhere else.

[ivconic]

Thanks!
Q9 and Q12 should prevent that, but also I can put ac coupling too.
I wanted to clear up on analog parts of the TX for now.
Next thing is the clock generation and as you said to avoid shoot-thru situation.
And again I am thinking rather on some rock solid analog solution instead the MCU only.
MCU can "choke", lag, "freeze" for so many reasons and then the damage is huge!
Maybe I will use MCU for basic clock generation but I would like to explore the possibilities how to add "protection" against shoot-thru, in case MCU goes wild for some reason.
Handfull of more componentes is not an issue, it doesn't matter to me as long as those are "simple" and available ones.

[ivconic]

This will work, minor value adjustments may be required.
The MCU can now go as crazy and when it want…
...
I might be wrong but I think you Carl mentioned that you will do the final version later with FPGA... which is good if you have the conditions.
I found, it seems to me, a very good solution that can be identically implemented with FPGA.
So along the way I accidentally ran into this: https://icestudio.io/#lk-download

[pito]

There is another problem = thermal-mechanical instability. High power is sufficient for dual box type detectors where the Tx and Rx coils are not close to each other. Test = DD coil , 12 V and current consumption 350 mA from the battery. The Tx coil itself was about 40 degrees warm, the balance of the DD coil was not stable wihle two box machine with the same current was ok.
I think less than 200 mA it is ok = for Tx can be used any audio amplifier.

My video post #61, 12V/70mA

[ArchibaldSTM]

ivconic, Just looking at the TX drive circuit makes me feel bad. Is it really impossible to do it simpler, using a proven scheme? For example, like the Quasar ARM . Why is a sine needed here? A meander is enough.

[ivconic]

Quasar Arm is half bridge, if I remember correctly.
Here I continue thinking on the subject of the proposed TX for the AMX project.
I don't care to make any TX at any cost.
But to develop the idea so that it can be completely done with discrete electronics.
And that it depends as little as possible on acrobatics in the code of some MCU.
Whether you going to generate pulses with the help of an fancy MCU with dubious code acrobatics or simply from two "dirty" NE555s; the same: the TX frontend must remain bulletproof against all possible excesses.
That schematic is only a proposal, not a finished thing.
I can already see the potential flaws.
The first is +5v. I will have to raise the voltage there.
Second... what happens when one of the pulses just disappears? Or both?
The truth-table for logic does not foresee such a case.
So an extra circuit is needed to take care of that case.
Works are in progress.
If it turns out well... I won't make it public!



There are interesting details, you can see here what's the AMX idea: https://www.geotech1.com/forums/foru...ts/amx-project

[moodz]

There is another problem = thermal-mechanical instability. High power is sufficient for dual box type detectors where the Tx and Rx coils are not close to each other. Test = DD coil , 12 V and current consumption 350 mA from the battery. The Tx coil itself was about 40 degrees warm, the balance of the DD coil was not stable wihle two box machine with the same current was ok.
I think less than 200 mA it is ok = for Tx can be used any audio amplifier.

My video post #61, 12V/70mA

I dont know what your experience is Pito however high currents and voltages are the normal in Pulse induction. So I am using a Minelab DD coil which has been tested and used extensively in Australian desert conditions.
If you think about it that in 40+ degrees celcius heat and full sunlight the coil is being exposed to around 40 watts of energy even if its not turned on.

I am using 2.82 amps RMS with a 0.4 ohm coil = around 3 watts of energy dissipated.
Bottom line is the coils dont drift and dont warm up due to TX energy.

moodz

[KingJL]

@moodz: What solution have you come up with for your requirement for a preamp with log gain characteristics?

[moodz]

Explain then why Carl burned his set at the first attempt?


TC4428 I can not obtain, as I mentioned several times, nor the other lookalikes.
Besides I like to try with "simple" components. that can be obtained anywhere.
Is there a problem with the schematic?
That's why I posted it; to hear other opinions and remarks.
But nothing like ".... use this or that driver chip..."

So I use a differential driver chip ( TC4428A ) .. when you power the circuit with no TX drive one or the other side will be randomly on and this will cause excessive current to flow in one leg if your through impedance is low ( eg 0.3 ohms ) and this will cause heavy amps to flow from the supply and mosfets will burn out due to current not avalanche.

The solution to this is just heatsink your mosfets and limit the current from the supply or use a more sophisticated driving scheme ( preferred )

If you use a current source instead of a voltage source for the supply ( which I think Carl did ) .. this is also a gotcha as the low voltage mosfets will blow / avalanche unless a protection scheme is in place.

moodz

[pito]

Pulse induction = is there any coil balance, how long does the pulse last and how long is the coil not energized?
No more comments.

[moodz]

@moodz: What solution have you come up with for your requirement for a preamp with log gain characteristics?

An interesting solution .... however every solution has its drawbacks .. still testing it.

so far it

1. Recovers the sine wave from below the noise floor.
2. Has a log output response eg 30 mv input = 1 volt sine wave / 300 mv = 2 volt sine wave 3000 mv = 3 volt sine wave
3. Its very simple
4. Its purely analogue

However ( BAD ) it only works at one frequency and not easily MCU controlled. The frequency can be changed but adds complexity.
moodz

[moodz]

Pulse induction = is there any coil balance, how long does the pulse last and how long is the coil not energized?
No more comments.

Even though your question is short ... the answer is long. Google pulse induction using IB / DD coils ... practically and successfully for many many years ( before monocoils I think ).

In mineral prospecting by they flew massive coils with kilowatts of power in them. It sounds like your experience is mainly in VLF.

moodz

Here is the system.

[ArchibaldSTM]

ivconic...Users of the Quasar ARM tried to make a sine wave. After several months of testing, we came to the conclusion that it was useless. The device parameters did not improve. The meander was left. By the way... In the Fortuna metal detector, the TX swing is done using a sine wave, but it is much simpler --- on the LM 386