The coil

[morganton]

A multi-turn pot should not be necessary, but you'll have to experiment yourself as I've not tried this is practice. Also, you'll need to be careful to keep the MOSFET from avalanching.

Thank you.

[morganton]

Swapping R7 with a 3.3K pot

A multi-turn pot should not be necessary, but you'll have to experiment yourself as I've not tried this is practice. Also, you'll need to be careful to keep the MOSFET from avalanching.

Since your reply to my question about R7, KRinAZ did some mods to the miniplus and swapped out R7 with with a 3.3K pot. http://www.geotech1.com/forums/showt...eotech-members post 7. I am in the process of beginning to build the miniplus from Silver Dog's kit and wanted to make the R7 switch too so I could use several different size coils.

As you indicated in your reply, there may be a concern about the MOSFET avalanching and there was some discussion about it too under KRinAZ's build thread. I think, if I read KRinAZ's post correctily, http://www.geotech1.com/forums/showt...che#post206414, post 239, he did not think it was a problem.

Can I safely replace R7 with a 3.3K pot without making any other changes to basic build (and without incorporating KRinAZ's other changes) or is there something else I need to add?

Thank you.

[geoscash1]

Can I safely replace R7 with a 3.3K pot without making any other changes to basic build (and without incorporating KRinAZ's other changes) or is there something else I need to add?

Thank you.

If you plan to use different coils, I personally wouldnt change the stock settings or parts. The mpp is excellent as is and will work fine with different coils. If you plan to nugget hunt then modifying is the way to go.

I have done all of Kyle's mods as well, I have 3 modified versions of the mpp, each using a little different set of the mods.

I would recommend building a stock version first, trying it out and seeing how you like it. if it doesn't fit the bill at that point then you can always pull the board and add the mods later.

If you will be looking for nuggets specifically then all of the mods together are best along with the fast 3dss coil.

I tried a switch on the TX using 2 resistors on a sp/dt switch, one position set for approx. 34us and the other at the stock setting of 58us. It made the board (mine) very touchy. when switched, the threshold would go crazy for about 20 seconds before settling down and then the delay would seem to act weird and the mosfet would get quite warm, I don't know if it was that particular board or not but I didn't do any more testing with it and removed the switch and replaced with a trimmer and set it to leave it.

That's just my 2 cents for what its worth.

ps - yes, you can just change R7 if you want, it wont hurt anything..

[morganton]

Thanks for your reply, geoscash1. I have built the original Baracuda and the TGSL and they work great for coin hunting with 9" coils. I also built the Hammerhead, but haven't used it much. I wanted to try something different for my next detector to use around where I live in Northern California to explore the dredge tailings. Whether I'd actually find anything--who knows, but it gives me something to piddle with. This thread caught my eye over a year ago, since I wanted to be able to use both fast and slow coils with the miniplus I'm getting ready to build. I don't want to do all of KRinAZ's mods (although I know he got excellent results) because I'd probably end up in an endless loop of trying to get it to work. But simply adding a pot for R7 is easy enough. Thanks for your input that it wouldn't probably damage the MOSFET.

[Qiaozhi]

Since your reply to my question about R7, KRinAZ did some mods to the miniplus and swapped out R7 with with a 3.3K pot. http://www.geotech1.com/forums/showt...eotech-members post 7. I am in the process of beginning to build the miniplus from Silver Dog's kit and wanted to make the R7 switch too so I could use several different size coils.

As you indicated in your reply, there may be a concern about the MOSFET avalanching and there was some discussion about it too under KRinAZ's build thread. I think, if I read KRinAZ's post correctily, http://www.geotech1.com/forums/showt...che#post206414, post 239, he did not think it was a problem.

Can I safely replace R7 with a 3.3K pot without making any other changes to basic build (and without incorporating KRinAZ's other changes) or is there something else I need to add?

Thank you.

I would follow geocash's suggestion of building the MPP as described in the Build Document first, before making any modifications.

[morganton]

I would follow geocash's suggestion of building the MPP as described in the Build Document first, before making any modifications.

Ok, thank you.

[bbsailor]

Everyone building coils,

Please do not ignore the interrelationship of the MOSFET parameters, coil cable, coil parameters (inductance, resistance, inter-winding capacitance), TX pulse width, RX time and these effects on the effective damping resistance value that determines the ultimate discharge slope of the discharge time constant that determines how fully the target is stimulated. The longer you must wait for the TX pulse to be fully damped so the RX circuit can be turned on the more the eddy currents in the target are decaying and the weaker its signal will be. That is why for short TC targets like gold nuggets and some jewelry you need very short delays but you also need a discharge slope that is closer to vertical but this can only occur with a higher value damping resistor.

To obtain this higher value of damping resistor the following things have been proven to work.

1. Change from a mono coil to a well balanced DD coil so that the RX circuit works independently from the TX circuit. In a mono coil while the voltage is above the clamping diode voltage of about 0.7V the input resistor to the first amplifier stage is effectively in parallel with the damping resistor (Rd) thus making the discharge slope less vertical down to 0.7V and then its the value of Rd alone.

2. While the flyback is above the avalanche voltage of the MOSFET, it is being clamped (flat topped) and extends the delay time slightly.

3. Higher flyback voltages contain more energy that needs to be damped thus a lower value of Rd must be used. If there is too much energy, the heat in Rd changes its resistance slightly higher and may not be optimum.

4. If there was no capacitance in the TX circuit the discharge TC would be almost vertical. Any capacitance in the TX circuit retains some of the flyback voltage and creates a discharge oscillation that must fully dissipate before the RX circuit can be turned on. This takes a little more time... longer delay!

5. Op amps go into saturation governed by the gain of each stage, level of the input voltage, supply voltage to the op amp and ultimately the time it takes for that op amp to come out of saturation, again causing a longer delay.

6. The target characteristics can point to an optimization that can help build a PI machine with TX and RX characteristics and coil size and style optimized for that target. The TX discharge time constant should be 5 times faster than the target TC to fully stimulate it. 3TC is 95% effective, 4TC is 97& effective and 5TC is 99.5% effective. Here is where the Rd value is critical for obtaining a good enough stimulation (discharge TC slope) of the target to allow it to be detected while there is still some eddy currents left to detect (delay time). Different target sizes require different coil sizes to optimally detect them.

7. Ground conditions may not allow the optimum characteristics but should allow the user to vary the TX pulse energy to not overstimulate the ground but this may reduce the optimum conditions for seeking the desired target.

What we need, as microprocessors increasingly govern the TX and RX circuits, in a PI machine is to use these microprocessors to generate a ground profile so the user can optimize the TX and RX parameters as well as the coil sweep speed to automatically rather than manually optimize a PI machine and coil for specific targets. Points 1 through 6 are still valid for manually optimizing and ultimately requires compromises by fully understanding the interrelationship of all these variables.

I hope this helps?

Joseph J. Rogowski

[geoscash1]

Everyone building coils,

Please do not ignore the interrelationship of the MOSFET parameters, coil cable, coil parameters (inductance, resistance, inter-winding capacitance), TX pulse width, RX time and these effects on the effective damping resistance value that determines the ultimate discharge slope of the discharge time constant that determines how fully the target is stimulated. The longer you must wait for the TX pulse to be fully damped so the RX circuit can be turned on the more the eddy currents in the target are decaying and the weaker its signal will be. That is why for short TC targets like gold nuggets and some jewelry you need very short delays but you also need a discharge slope that is closer to vertical but this can only occur with a higher value damping resistor.

To obtain this higher value of damping resistor the following things have been proven to work.

1. Change from a mono coil to a well balanced DD coil so that the RX circuit works independently from the TX circuit. In a mono coil while the voltage is above the clamping diode voltage of about 0.7V the input resistor to the first amplifier stage is effectively in parallel with the damping resistor (Rd) thus making the discharge slope less vertical down to 0.7V and then its the value of Rd alone.

2. While the flyback is above the avalanche voltage of the MOSFET, it is being clamped (flat topped) and extends the delay time slightly.

3. Higher flyback voltages contain more energy that needs to be damped thus a lower value of Rd must be used. If there is too much energy, the heat in Rd changes its resistance slightly higher and may not be optimum.

4. If there was no capacitance in the TX circuit the discharge TC would be almost vertical. Any capacitance in the TX circuit retains some of the flyback voltage and creates a discharge oscillation that must fully dissipate before the RX circuit can be turned on. This takes a little more time... longer delay!

5. Op amps go into saturation governed by the gain of each stage, level of the input voltage, supply voltage to the op amp and ultimately the time it takes for that op amp to come out of saturation, again causing a longer delay.

6. The target characteristics can point to an optimization that can help build a PI machine with TX and RX characteristics and coil size and style optimized for that target. The TX discharge time constant should be 5 times faster than the target TC to fully stimulate it. 3TC is 95% effective, 4TC is 97& effective and 5TC is 99.5% effective. Here is where the Rd value is critical for obtaining a good enough stimulation (discharge TC slope) of the target to allow it to be detected while there is still some eddy currents left to detect (delay time). Different target sizes require different coil sizes to optimally detect them.

7. Ground conditions may not allow the optimum characteristics but should allow the user to vary the TX pulse energy to not overstimulate the ground but this may reduce the optimum conditions for seeking the desired target.

What we need, as microprocessors increasingly govern the TX and RX circuits, in a PI machine is to use these microprocessors to generate a ground profile so the user can optimize the TX and RX parameters as well as the coil sweep speed to automatically rather than manually optimize a PI machine and coil for specific targets. Points 1 through 6 are still valid for manually optimizing and ultimately requires compromises by fully understanding the interrelationship of all these variables.

I hope this helps?

Joseph J. Rogowski

Thank you bbsailor, much appreciate the excellent advice and information. Sometimes we get caught up in the target and forget the important details of the build. Some of it may be greek to some of us less informed or electronics noobies but it gives us some place to begin and start educating ourselves on some of the technical processes that make these detectors tick.

Geo