Getting ready to build the MPP

[green]

Mr. Green,
your tests are very interesting and accurate. Yours are the the most precise information about target TC that are publicly available.

There is one other subject about which it is difficult to find accurate information: The relation between target surface area and TX coil surface area.
A general rule of thumb says that a target with a surface area of 1% of the surface area of the TX coil, is about the smallest target that can be detected at the center of the coil.
Did you do any tests on this subject?

Not yet. I'm redoing my test rig right now. Looks like it might be an interesting test.

[Tinkerer]

Mr. Green,
your tests are very interesting and accurate. Yours are the the most precise information about target TC that are publicly available.

There is one other subject about which it is difficult to find accurate information: The relation between target surface area and TX coil surface area.
A general rule of thumb says that a target with a surface area of 1% of the surface area of the TX coil, is about the smallest target that can be detected at the center of the coil.
Did you do any tests on this subject?

[green]

----------------------------------------

After looking closely at the IRF740 and the IPD50R800 datasheets the COSS of the IRF740 at 10v is about 350pf while the COSS of the IPD50R800 is about 400pf at 10 volts. At 0 volts the IRF740 is about 1400 pf and the IPD50R800 is about 800 pf. The Vishay diode you mention has half the capacitance of an HER208 and it will help.

I often see comments about 'In the Coil Electronics' but I don't under stand how these can be implemented without the metals they contain becoming targets themselves. If the detector will be used to search for small gold it will need to sample below 10us and it seems the copper, solder, tin in the coil mounted electronics will impede its ability to do so.

If the objective is to keep capacitance low I do it by using a continuation of the 600 volt PTFE coil wire to create a 34" long twisted pair. It is very difficult to find coax that has lower capacitance than this configuration and in addition coax will require solder joints in the coil and this should be avoided in my opinion.

Just a thought.

Regards,

Dan

Hi Dan
I posted some solder targets awhile back. The solder targets have a short TC. I don't think a solder connection would effect detecting small nuggets. Try soldering two short pieces of your coil wire together and see if you can detect it. If you can tape it to your coil and see if it effects the detection distance of your gold targets. I'm with you, if you put enough stuff near the coil it will.

[baum7154]

hi Dan,
The COSS of the IRF740 is about 1400pf @ 10V so specs are definitely better for the IPD50R800. Also it allows me to build smaller as I will attempt to build the front end inside the coil housing to further eliminate capacitance of the feedline. Will give it a go and see what comes out.

----------------------------------------

After looking closely at the IRF740 and the IPD50R800 datasheets the COSS of the IRF740 at 10v is about 350pf while the COSS of the IPD50R800 is about 400pf at 10 volts. At 0 volts the IRF740 is about 1400 pf and the IPD50R800 is about 800 pf. The Vishay diode you mention has half the capacitance of an HER208 and it will help.

I often see comments about 'In the Coil Electronics' but I don't under stand how these can be implemented without the metals they contain becoming targets themselves. If the detector will be used to search for small gold it will need to sample below 10us and it seems the copper, solder, tin in the coil mounted electronics will impede its ability to do so.

If the objective is to keep capacitance low I do it by using a continuation of the 600 volt PTFE coil wire to create a 34" long twisted pair. It is very difficult to find coax that has lower capacitance than this configuration and in addition coax will require solder joints in the coil and this should be avoided in my opinion.

Just a thought.

Regards,

Dan

[Achillion]

--------------------------------------
What is your self resonant frequency with 36" of feed line? Depending on what feed line is used I would expect the SRF to drop to 1.050 mHz or so which is right about where all of my 300 uh to 335 uh spider coils end up with feeds. However once they are in circuit on my CHANCE PI they really end up at about an SRF of about 850 to 950 kHz.

Regarding the Infineon IPD50R800 it looks like the device has a COSS of about 400pf at 12 volts (DSS) but they show it has 23pf @ 100 volts Drain to Source. Is that better than the IRF 740?

Regards,

Dan

hi Dan,
The COSS of the IRF740 is about 1400pf @ 10V so specs are definitely better for the IPD50R800. Also it allows me to build smaller as I will attempt to build the front end inside the coil housing to further eliminate capacitance of the feedline. Will give it a go and see what comes out.

[Chet]

Hi kellogs
Silverdog sold the Chance PI boards sometime ago. I don't know if they are available now.
Chet

[kellogs]

Hi chet once again thanks for your help i will be ordering a kit from silverdog also where do purchase the chance pi from my next project is to build your oval coil grat to hear from you have a great day Kindest Regards kellogs.

[baum7154]

Did some measurements on the basket weave coil with the following results:

Coil base : 4mm PE 10"x8" oval
Conductor : 0.12 mm2 stranded 26AWG PTFE insulated
Inductance : 338 uH
Capacitance : 31.6 pF
Self Resonance Frequency : 1540 kHz

Planning to replace the IRF740 with an Inineon IPD50R800 and a fast diode (Vishay 5EWH06FNHM3) in series. Front stage to be build in the coil housing.
Mechanical side is also progressing. I used a telescopic carbon monopod from Benro. 3d printed the piece between the rod and coil (through Shapeways).
I want to experiment a bit with the pulse timing and developing this around a LPC 1768 uC (maybe as a future MPP 'shield')
Anyway, lots of ideas to keep me busy.

Cheers,

JCobb

--------------------------------------
What is your self resonant frequency with 36" of feed line? Depending on what feed line is used I would expect the SRF to drop to 1.050 mHz or so which is right about where all of my 300 uh to 335 uh spider coils end up with feeds. However once they are in circuit on my CHANCE PI they really end up at about an SRF of about 850 to 950 kHz.

Regarding the Infineon IPD50R800 it looks like the device has a COSS of about 400pf at 12 volts (DSS) but they show it has 23pf @ 100 volts Drain to Source. Is that better than the IRF 740?

Regards,

Dan

[Chet]

Hi kellogs

The Minipluse Plus is a good kit. The build manual takes you through the assembly stage by stage. I think you will find it easier than the Surf PI. The coils should be interchangeable. I don't know which is the best machine. Maybe someone on the forum knows which has the best performance.

I wish you a fast recovery and have a great day,
Chet

[kellogs]

Hi chet kellogs back on my feet again had some skin cancers taken out of my left leg.any way have been spending some time on the forum and i se a new project has evolved in the mpp i have been on to silverdogs web and he has a revised pcb ;d; i woud like to have a go at building one what are your thoughts mate look forward to hearing from you Kindest Regards kellogs

[Achillion]

Did some measurements on the basket weave coil with the following results:

Coil base : 4mm PE 10"x8" oval
Conductor : 0.12 mm2 stranded 26AWG PTFE insulated
Inductance : 338 uH
Capacitance : 31.6 pF
Self Resonance Frequency : 1540 kHz

Planning to replace the IRF740 with an Inineon IPD50R800 and a fast diode (Vishay 5EWH06FNHM3) in series. Front stage to be build in the coil housing.
Mechanical side is also progressing. I used a telescopic carbon monopod from Benro. 3d printed the piece between the rod and coil (through Shapeways).
I want to experiment a bit with the pulse timing and developing this around a LPC 1768 uC (maybe as a future MPP 'shield')
Anyway, lots of ideas to keep me busy.

Cheers,

JCobb

[Chet]

Chet, for some reason I missed reading your reply. Thanks for the info. Have you tried truck bed liner? I use it on my coils. When you notice wear, just apply another coat. Just check that it does not contain conductive material.

Hi multieagle
Thank you, I might give it a try.
Chet

[KRinAZ]

Hi all, & apologies for my absence! First time back & enjoyed catching up on this thread.Life takes us in unexpected directions sometimes, I have been (still am) camping at mining claims & prospecting (Chet was so right!). As my internet access is rare & I currently only have my ancient iPhone as a computing device, my posting capability is not so much.I WILL be back to this project again someday! - especially as I really wish I had a working MPP with me with the BWC (basket weave coil). I have been thinking on some mods to the coil design, will build a vacuum table for the coils, & will progress with the MPP. I see it's in revD now, cool.Cheers!

[multieagle]

Hi multieagle

Also you might ask if he can use a thicker ABS material or can provide skid plates. I think I will wear through the bottom of my coil housing quite quickly. The California gold areas are much harsher than grass in the parks. I go through at least one skid plate each year.

Regards,
Chet

Chet, for some reason I missed reading your reply. Thanks for the info. Have you tried truck bed liner? I use it on my coils. When you notice wear, just apply another coat. Just check that it does not contain conductive material.

[baum7154]

I would use the series diode as it has much lower capacitance that is in series with the much higher capacitance of the mosfet effectively lowering the system capacitance across the coil and raising the Self Resonant Frequency of the coil in the system.
The required damping resistance will then go up according to the formula: PI * Self Resonant Frequency (in mHz) * Coil Inductance (in microhenries) = Rdamping (in ohms)

Regards,

Dan