why coils shold be 330uH?

[Point]

titel /\ why shold the coils be 330uH? why not 1mH or 100uH?

greez Point

[kt315]

http://www.amazon.com/Inside-Metal-D...DDMSEQ5SG7TWCJ

Inside the Metal Detector offers hard-to-find information on the technology behind metal detectors. The basics of induction and eddy currents are explained, plus the major categories of BFO, TR, motion VLF, and PI are covered. Included are hands-on experiments and complete metal detector projects in every category. A list of resources includes web sites, books, magazine articles, and patents.

[Teleno]

titel /\ why shold the coils be 330uH? why not 1mH or 100uH?

greez Point

A lower inductance requires more current fore the same target signal.

A higher inductance causes a higher flyback voltage and is slower to decay.

In the end a compromise must be found for the voltage rating of the MOSFET and the current capacity your battery and the time constant of the targets you're looking for. These don't vary too much for portable equipment so the average inductace is around the 300uH - 400uH mark.

[kt315]

Foster used 1mH coil in his last projects and guessed a decay still stays sensitive to small gold. you MAY use ANY coil,
only report us what you see and found =)))

[FatBob]

Most homebuilt PI's use around 300 uh as this has been found to strike the best balance between
high current fast TX and sensitive RX. The TX wants to have a small inductance for fast high power
and RX wants a big inductance for more signal to work with.

A possible different solution would be to use a center tapped coil and run the TX to one side only
for a fast powerful transmit and receive with a differential input off the whole coil. With this setup
you might go to 450 uh for better RX.

The main reason to go to 300 uh is to see tiny gold. If your just looking for coins other values
will work just fine. In fact if you are looking for cannons and tanks a 1 mh might be a better choise.

[kt315]

The main reason to go to 300 uh is to see tiny gold.
---
sometimes people is reading but do not able to read. this discourage! if i write ERIC FOSTER USED 1MH FOR HIS LAST
GOLD DETECTORS so THAT IS THAT. Goldquest SS has 1mH coil.

Eric Foster: My Beachscan uses a coil of 1mH as does the C.Scope CS7 and CS6PI.
================================================== =
The CS6PI is similar circuitry to the Beachscan, both of which were made some years ago now. A later development is the Goldquest which has also gone through two or three versions.The major changes since the CS6PI is the ability to sample at 10uS, the addition of a variable SAT control, and a switchable two stage autotune filter, plus you can use DD coils for all but highly mineralised ground. What you need therefore is the addition of ground balancing to the latest Goldquest system. Hmmm!
See the latest Goldquest SS v3 details by clicking on the Surfscanner banner above.
Eric.

[Teleno]

Foster used 1mH coil in his last projects and guessed a decay still stays sensitive to small gold. you MAY use ANY coil,no
only report us what you see and found =)))

According tu users, CS6PI only goes down to a 17 or 18 uS delay. Not for small gold, but that's the sort of timing you may expect from a large inductance.

You may NOT any coil for all purposes.

[kt315]

According tu users, CS6PI only goes down to a 17 or 18 uS delay. Not for small gold, but that's the sort of timing you may expect from a large inductance.

You may NOT any coil for all purposes.

sometimes people is reading but do not able to read. 1mH coil has MORE DELAY. so THAT GIVES you GOOD reaction ON SMALL GOLD with 18us delay.

IS THAT CLEAR FOR YOU PEOPLE? or, i again have to give you a link on ITMD?? (RTFM eah?)

[Teleno]

sometimes people is reading but do not able to read. 1mH coil has MORE DELAY. so THAT GIVES you GOOD reaction ON SMALL GOLD with 18us delay.

IS THAT CLEAR FOR YOU PEOPLE? or, i again have to give you a link on ITMD?? (RTFM eah?)

Small gold has a small inductance. For the same resistivity that means a smaller time constant than big gold, FAR BELOW 18us. IS THAT CLEAR FOR YOU?

Cool down and learn some Physics.

[Qiaozhi]

In Eric's detectors, that use 1mH coils, the TX pulse rate is very high, and the TX pulse width is very small.
As far as I'm aware, these detectors are designed for use on the beach, and are good for finding thin gold chains. I'm not sure they're intended for hunting for gold nuggets per se.

The low pulse width is compensated for by the higher than usual inductance of the coil, and the higher pulse rate improves the signal-to-noise ratio.
It would be interesting to plot the results obtained using various inductances with different pulse rates and widths. Perhaps there's a sweet spot there somewhere.

[Davor]

That's perhaps because the whole PI world is completely absorbed in optimising detectors for nugget hunt. Some UXO detectors use even higher inductances than 1mH, but their main targets are small chunks of brass that are found in landmines. I guess for relic hunting values around 1mH and generous first sample delays would lead to very decent detection, not riddled with salts or soil irregularities.