MPP revE need help

[Albdelafar]

Hello I did a new test today with my new coil and my 680ohm potentiometer used as damping resistor.I know it is not a good idea but I have nothing else to do it….
I bought resistors online yesterday with a new o'scope and a 100x probe but I have more than a month to be delivered….
So I will try like that and if it burns it doesn't matter.
So I read my signal in TP3 with a can of soda and that's what I see at 0.2 V / div (10x probe) and 20us time / div.
Can you please confirm that the signal corresponds to what is expected ?
Thank you !!
Alban

[Albdelafar]

Your coil is a mono coil, which means that the same coil is used for both transmit and receive. There is not one wire designated TX and the other RX, as shown in your diagram.

The damping resistor goes between PL1 pin2 and PL1 pin1 (or pin3).

For Tx and Rx i think I understood. In my scheme i have two wire for TX and RX because of my connecting cable between PCB and coil are made to be adaptable for all mono and dual coil. I solder RX and TX pin on the coil connector for my mono coil.
My mistake is the connection of damping tool i have to connect the 0V with RX or TX cable (RX and TX are the same because of my mono coil)

[Qiaozhi]

I need a 2w resistor for damping the high voltage it's right ?
Can you confirm that my montage is good for damping calculation and o'scope measurement please ?
Thank again for your help this forum is really helpfull !!
[ATTACH]49762[/ATTACH]

Your diagram is very confusing. It looks like you've taken the two wires from the coil and shorted them together.
However, I think you're confused by the fact that the MPP can use either a mono coil or a two separate TX and RX coils.
Your coil is a mono coil, which means that the same coil is used for both transmit and receive. There is not one wire designated TX and the other RX, as shown in your diagram.

Forget about the electrostatic shield for the moment. Connect one wire from the coil to PL1 pin2, and the other wire to PL1 pin1 (or pin3). Then short PL1 pin2 to PL2 pin2.
The damping resistor goes between PL1 pin2 and PL1 pin1 (or pin3).

[Albdelafar]

I need a 2w resistor for damping the high voltage it's right ?
Can you confirm that my montage is good for damping calculation and o'scope measurement please ?
Thank again for your help this forum is really helpfull !!

[Qiaozhi]

Thank you waltr for your reply. I know resistor calculation rules but i have just 1000ohm CMS resistors and no 10k potentiometer.
I need to invest i think...
i will try to continue whitout optimal damping resistor and come back later to adjust it.

Just fit a 680R damping resistor for the moment. You can check how well it's working by looking at the preamp output on the scope, and then either fit a higher or lower standard value if necessary.

[Albdelafar]

Thank you waltr for your reply. I know resistor calculation rules but i have just 1000ohm CMS resistors and no 10k potentiometer.
I need to invest i think...
i will try to continue whitout optimal damping resistor and come back later to adjust it.

[waltr]

The resistor values in the Damping Tool can be changed slightly.
For example: the 2k could be 2.2k or 1.8k and the tool will still do what you need. That is find the optimal damping R value and this is done after adjusting the pot then remove the 'Tool" and measure what the resistance across the Tool is.

What you need from the Tool is a resistance of about 200 to 2k Ohm. Since you are learning electronics calculating the Tool's resistance range is a very good excise.

[Albdelafar]

Thank you Qiaozhi I will build a new coil with an internal radius of 125 mm and 25 turns to be as close as possible to 400?H.
For my potentiometer, I buy this :

https://fr.farnell.com/bourns/3590s-2-102l/potentiometre-1k/dp/1700941

It is given for 2w, 1500 VAC minimum and an insulation resistance of 500 VDC I hope it could survive because I do not have the necessary resistances for your damping resistance tool. If it burns, I will buy the resistors to build your damping tool but I do not know how long I will wait before being delivered ...

[Qiaozhi]

ok thank you for your explaination !!
So do you think i need to build a new coil if 199?H is not enough for this detector ?
I have a 1kw motor winding with very fine wire i can try to do something with it.
Because of covid-19 i can't buy anything and i need to find something to do or i will be crazy

Try 21 turns of the 1mm wire for an inner radius of 125mm. This should give an inductance of 297uH.
22 turns will give 324uH. Either should be ok.

The damping resistor will probably need to be 680R. You should not simply use a potentiometer for adjusting the damping resistor value, as it will most likely burn out due to the high flyback voltage. You should use a damping resistor tool (see attachment) to determine the correct value.

[Albdelafar]

ok thank you for your explaination !!
So do you think i need to build a new coil if 199?H is not enough for this detector ?
I have a 1kw motor winding with very fine wire i can try to do something with it.
Because of covid-19 i can't buy anything and i need to find something to do or i will be crazy

[Qiaozhi]

Thank for you reply Qiaozhi
To easy project are not enought interesting for me. I used this coil calculator before but it doesn't give to me the same results that i find on others online inductance calculators. I don't know why perhaps you can give me the reason why ?
This calculator give me a 450mH coil inductance and it's why i choosed to build my coil like this at begining of my project.

For the paper to help understand how metal detector work i tryed to read it but it's sometimes hard because of my english level

You may have input the diameter of the coil into the calculator instead of the radius?
The Geotech calculator gives 199uH for 17 turns of 1mm wire on a 125mm radius.

The online calculator that you used is not suitable for air-cored coils. It is for calculating solenoids.

[dbanner]

mH - millihenry

uH - microhenry

[Albdelafar]

Thank for you reply Qiaozhi
To easy project are not enought interesting for me. I used this coil calculator before but it doesn't give to me the same results that i find on others online inductance calculators. I don't know why perhaps you can give me the reason why ?
This calculator give me a 450mH coil inductance and it's why i choosed to build my coil like this at begining of my project.

For the paper to help understand how metal detector work i tryed to read it but it's sometimes hard because of my english level

[Albdelafar]

Thank you for your reply.
I made two short videos to explain what i see.

https://drive.google.com/open?id=1QJEvvEUSJPZFHfgCtf45Mfw1rl7KIYjI

https://drive.google.com/open?id=1SRmvyIoSLPX2WzBqtbM9aGaSF2POxLQw

Sorry for my poor english level i do my best
I will do a video for the damping resistor signal too.
Before to do step 6, do you thing i need to modifie the tx pulse width and rate ? i use the mono coil recomandations with pulse width of 100?s and pulse periode of 1ms.

https://drive.google.com/open?id=1kF3J5g4LnnBkDv_ZMdi6ApbHpqhoYGeW

EDIT with good links and damping resistor video ....

[Qiaozhi]

There is a coil calculator on the forum here -> Coil Calculator

A metal detector is not the easiest project to start with if you're trying to learn electronics.
In order to gain a better understanding of how metal detectors work, you will find this useful -> Inside the METAL DETECTOR - Second Edition - Published 2015