Chance PI coil...

[Tinkerer]

This a repost of a post in the Tech Forum - Damping Resistor

Ran coil #3 (an original CHANCE pattern coil) this AM on my CHANCE PI and with an adjustable network set at 1360 ohms this coil would run at a Guard Interval setting of 11, but never at the minimum of 10. Had to unwind a turn off the coil (now it is 368 uh) to get the solder joints out of the coil in order to get it to run at 11. Discrimination and sensitivity were not good. Large Gold Ring was represented at about 75% up the scale. This coil definitely needs shielding at the low GI settings.

Decided to put the prescribed 390 ohm damping resistor in and see how it behaved. The lowest Guard Interval it would run at was 18. At this setting it would not see 8.8 grain dental gold. It does see a 10.8 grain piece of dental gold at 1.5". Discrimination on a 6mm wide men's 10K wedding band is odd. Face on to the coil it displays 75% up the scale, but on edge it is solidly on the left end as Gold. Detection distance on the ring face on is about 9" and on edge to the coil is about 7"

This coil does not serve my purpose to find small gold. While it is quite fast it badly needs shielding but when that is applied it will lose much of it's speed. With it's broad flat profile even with 8 mm dielectric shield spacing it will be much diminished. A spider coil like #1 in post 48 of this thread is much better for my purposes.

Dan

The 8mm dielectric spacing sounds good, interesting to see results. In fact we have no numbers about the difference it makes for the spacing. It might be interesting to do some tests with 6, 8, 10, 12mm spacing.

There are other things that slow down the coil. Like solid wire compared to litz wire. An alternate solution would be tinned multi strand insulated wire.

[baum7154]

[QUOTE=baum7154;184152]First of all I have not tried to detect a 1/4" X 1/4" target of either foil or aluminum can. 1/4" X1/4" is 1/16 of a square inch. I have detected a 1/4 square inch target of aluminum can and it is a great target for testing, measuring 1/2" X 1/2". Perhaps the language I used caused some confusion. Sorry if it did. If you are detecting targets 1/4" X 1/4" of aluminum can that is impressive. My most difficult standardized target so far is the 1/2" X 1/2' .001" thick aluminum foil target.

Just performed detection distance on a 1/4" X 1/4" piece of .004" thick aluminum can and my CHANCE PI with #1 coil and 1100 Rd sees the sample at 2.5" near coil center. It does not see a 1/4" X 1/4" piece of .001" thick aluminum foil probably due to being too short of a decay for it. I used a 1/2" X 1/4" piece of .001 thick foil and folded it to a 1/4" X 1/4" square .002" thick and it is seen at 1.5".

Dan

[green]

Green is there a schematic on the web for what you are useing ?

I forgot that I had changed to a 3 amp instrument amp. As Tinkerer says there are a lot of schematics. Including schematics I've been using on the bench. I'm just learning. They work for testing. Need additions and improvement and whether they they will work in a PI detector I don't know yet. I've been thinking about more gain for the PI detector.

[baum7154]

The 8mm dielectric spacing sounds good, interesting to see results. In fact we have no numbers about the difference it makes for the spacing. It might be interesting to do some tests with 6, 8, 10, 12mm spacing.

There are other things that slow down the coil. Like solid wire compared to litz wire. An alternate solution would be tinned multi strand insulated wire.

I have not used Litz wire and the wire in my #1 coil is 24 AWG PTFE stranded silver plated wire. I did note that dropping wire size has a big effect on capacitance and I will next wind the same coil with either #26 PTFE insul or #28 PTFE insul if I can find it.. The silver plating does cause more eddys but I think that as wire size drops that effect is diminished.

Dan

[baum7154]

The 8mm dielectric spacing sounds good, interesting to see results. In fact we have no numbers about the difference it makes for the spacing. It might be interesting to do some tests with 6, 8, 10, 12mm spacing.

There are other things that slow down the coil. Like solid wire compared to litz wire. An alternate solution would be tinned multi strand insulated wire.

Intuitively the greater spacing will reduce the capacitance and it should work. My current coil does not have shielding other than by its own geometry so it would make a reasonable guinea pig for some spacing tests. Let me think about how this might work out and maybe we can begin to document the effect.

Dan

[6666]

Thanks for the info on the 2 and 3 amp instrument amplifier.

[Tinkerer]

Intuitively the greater spacing will reduce the capacitance and it should work. My current coil does not have shielding other than by its own geometry so it would make a reasonable guinea pig for some spacing tests. Let me think about how this might work out and maybe we can begin to document the effect.

Dan

Below is a calculator that gives you the inductance and capacitance of wire. There you can see what influence the thickness of the cable and the spacing between has, for different dielectric. One of the best dielectric is air.

You are right about the eddy currents in the wire. It does make a difference. When we get down to the last few us, every little bit counts.
bbsailor wrote some very good details about how to squeeze the last few us out of a traditional PI.

http://www.mogami.com/e/cad/electrical.html

[Tinkerer]

Here is a good explanation for increasing capacitance. Just the opposite of what we want but we can look at it as what to avoid.

http://www.murata.com/products/emico.../cap_en14.html

[baum7154]

The murata paper confirms that the direction we want to go is greater shield spacing. Good info!

[Tinkerer]

The murata paper confirms that the direction we want to go is greater shield spacing. Good info!

Intuitively, more space results in less capacitance. However, when it comes to dielectric, it gets interesting.

Better dielectric, INCREASES the capacitance, according to the Murata paper. With the Mogami calculator the better dielectric DECREASES the capacitance.

To complicate things a bit further, PVC, a common cheap cable insulation, has a dielectric constant that varies widely with temperature. Black coils, used in the hot sun, get quite warm.

By the way, I like your coil winding on the former. What is the measured inductance? How many turns? Inner and outer diameter etc.

[Tepco]

Another interesting Russian “basket” design, see pages 5,6,7, text is on Russian but Google can handle it.

[kt315]

not interesting no STRONGLY RECOMMENDED FOR CHANCE basket coil . not nerve me tepco! READ SOMETHING BEFORE YOU ONLY MEAN TO MAKE ALL DETAILS!

Ïðèáîð ðàáîòîñïîñîáåí òîëüêî ñ êàòóøêàìè, èìåþùèìè ìàëóþ ïàðàçèòíóþ ¸ìêîñòü. Ïàðàìåòðû êàòóøêè, ñ êîòîðîé îòðàáàòûâàëàñü ïðîãðàììà: L = 400 uH, R = 1,7 Ohm. Áûë èñïîëüçîâàí ïðîâîä äèàìåòðîì 0,8 ìì, êàðêàñ áûë èçãîòîâëåí èç îðãñòåêëà òîëùèíîé 4 ìì. Æåëàòåëüíî èñïîëüçîâàòü áëèçêóþ ïî ïàðàìåòðàì êàòóøêó, õîòÿ, êàê ïîêàçàëè èññëåäîâàíèÿ, ïðèáîð óäîâëåòâîðèòåëüíî ðàáîòàåò è ïðè çíà÷èòåëüíûõ îòêëîíåíèÿõ ïàðàìåòðîâ êàòóøêè îò óêàçàííûõ.

[Tepco]

Did you tried this coil design, and if you did, what is wrong with it? I mentioned this as interesting design approach, maybe not specifically for Chance. Also, please you read this topic, this is all about making different coil type for Chance, in order to improve performance, because “original” coil turns out to be most problematic part of this detector, limiting it's usability.

[baum7154]

By the way, I like your coil winding on the former. What is the measured inductance? How many turns? Inner and outer diameter etc.

Coil #1 - The coil former is 8" OD -6" ID .093 thick polycarbonate machined with 37 slots .400" deep inside and out staggered to the midpoints.
Wire is 24 awg 19 strand silver plated PTFE 600 volt insulated. 32 turns gives 335uh at about 2 ohms including 33 inches of coil wire continuation twisted at 3 turns /inch as a feedline. This technique eliminates all solder joints in the field of the coil. Metal held in close proximity to the feed line is not detected.
Polyethylene rod and sheet is used to fill out the coil profile per the photos in this post before 2 layers of fiberglass are applied. This coil is not flooded with foam and it is not separately shielded. System ground is connected to the outermost winding wire end to provide self shielding effect. The coil is not sensitive to a hand placed on the coil with CHANCE PI set wide open on sensitivity.

Take a look at the photos in this thread to see more detail.

Regards,

Dan

[baum7154]

Did you tried this coil design, and if you did, what is wrong with it? I mentioned this as interesting design approach, maybe not specifically for Chance. Also, please you read this topic, this is all about making different coil type for Chance, in order to improve performance, because “original” coil turns out to be most problematic part of this detector, limiting it's usability.

I have not tried this coil though it looks to be very low capacitance. Flooding it with foam gives it enough structure to be fiberglassed for protection. This may be even better for small gold detection. Would have to build and test one to know if it is.

Dan