Looking for a solution to improve slow coil electronically to let it decay fast and keep R-dump high

[Waikiki_Sweep]

From memory it was about 4-5mm. The shield material is aluminum braided wire which I removed from some TV cable called RG6 quad. The wire showed next to no response when waved over the coil.

Good for you! 4-5mm spacer will add a centimeter to my coil thickness I will brake my arms to move such coil in water.

[mickstv]

Interesting. How thick that spacer between coil windings and shield and what material of shield and spacer?

From memory it was about 4-5mm. The shield material is aluminum braided wire which I removed from some TV cable called RG6 quad. The wire showed next to no response when waved over the coil.

[Waikiki_Sweep]

No, it is shielded.

Interesting. How thick that spacer between coil windings and shield and what material of shield and spacer?

[mickstv]

Looks like your coil has no shield. It is toooooooooo fast! Without shield it chatters in salt water in my case.

It is interesting idea to lower TX voltage. I saw that when batteries depleted my detector runs more smoothly I was thinking it was coil fiberglass sucking salt water inside and stabilizing work but lower voltage may be in charge too.

I would like to make shorter TX impulses and rise pps. rate. First it may let coil to decay faster, second it will let to detect more targets with overage coil speed.

No, it is shielded.


Lower voltage = earlier decay so the first sample no longer sees the flyback decay curve.

[Waikiki_Sweep]

On my test bench I have a PI set up with the following specs.

TX is 180us, using a constant current source (TX about 7 volts) The coil is 10 inch using litz wire.

Looking at the flyback it decays to zero in about 3-4us.

After the blocking fet decay is about 5-6us, then after 100x amp first sample is about 10us. Then I recover losses from the lower gain frontend in the post integrator stages.



So maybe try lowering your TX voltage.

Looks like your coil has no shield. It is toooooooooo fast! Without shield it chatters in salt water in my case.

It is interesting idea to lower TX voltage. I saw that when batteries depleted my detector runs more smoothly I was thinking it was coil fiberglass sucking salt water inside and stabilizing work but lower voltage may be in charge too.

I would like to make shorter TX impulses and rise pps. rate. First it may let coil to decay faster, second it will let to detect more targets with overage coil speed.

[mickstv]

I will sample at 15uS but my problem is that coil is slow and decay in 25-35us because I have to skip coil to shield spacer to make coil thin to have less drag in water. So I have to try “forced damping” stuff or other ways to let bad coil to decay within 15 uS or less.

On my test bench I have a PI set up with the following specs.

TX is 180us, using a constant current source (TX about 7 volts) The coil is 10 inch using litz wire.

Looking at the flyback it decays to zero in about 3-4us.

After the blocking fet decay is about 5-6us, then after 100x amp first sample is about 10us. Then I recover losses from the lower gain frontend in the post integrator stages.



So maybe try lowering your TX voltage.

[Waikiki_Sweep]

Simple don't sample at 3us, sample at 8-10us like normal.

I will sample at 15uS but my problem is that coil is slow and decay in 25-35us because I have to skip coil to shield spacer to make coil thin to have less drag in water. So I have to try “forced damping” stuff or other ways to let bad coil to decay within 15 uS or less.

[mickstv]

Long live monocoils, but real question is, even if someone can make 25cm shielded coil sampling at 3uS this way, on what terrain, except free air it can be used? Probably not in salt water, and some sort of Star Wars ground balance system will be needed...

Simple don't sample at 3us, sample at 8-10us like normal.

[Tepco]

This is why i'm figured out this “forced damping” stuff, to counter-bang the coil. If you inject energy this way into the circuit, impedance issue does not apply anymore, you have separate active source to counteract. This can truly “speed up the coil electronically” as proposed. Not sure how target will respond to this, but at first glance response wont be hurt, actually can improve. Someone have to build it, looks promising, most probably workable.


Long live monocoils, but real question is, even if someone can make 25cm shielded coil sampling at 3uS this way, on what terrain, except free air it can be used? Probably not in salt water, and some sort of Star Wars ground balance system will be needed...

[Waikiki_Sweep]

Good old story about impedance matching, maximum power transfer is when circuit is loaded with one particular resistance value to burn energy fastest. If you reduce this resistance, energy will remain stored in coil magnetic field and charge of parasitic capacitance inside coil for longer period of time. Like in Chance, during short circuit phase, energy remains stored, no flyback, just slow dissipation due to circuit losses. This is not like power rail, or capacitor, to discharge it faster by shorting it to draw maximal current, circuit itself will not generate maximal current to short circuit. Actually even real capacitor (with real ESR and internal resistance) shorted with some “ideal zero resistance superconducting short” will discharge slower compared to say, real few or few tens milliohm short, bang and arc, but this may not be best analogy for this case. Analogy with motors or relays is not good too, diodes etc will clamp flyback peak, but this is very slow way to get rid of excess energy, useless for detectors. Connect one diode across your coil and measure what happens. No one use diodes to damp coils this way. With resistor in series, you will find fastest response with resistor value about equal to normal damping value.

Unfortunately it looks reasonable. I will try to see it on oscilloscope.

If there is no way to dump slow coil faster so probably I need low inductance TX which will decay faster. But with low number of turns it will bad for use monocoil as RX because of low sensitivity.
So it may require separate RX. I see "Garrett sea hunter mark II" is using low inductance TX (somewhere 110uH) and separated not balanced RX to keep sensitivity up.

Other solution is spiral coils. Because spread between windings it has lower inductance with higher number of turns. It may help somehow to keep using monocoil.

[Tepco]

So low damping resistance will prevent flyback to start releasing energy? I am so stupid to understand it. As I remember diodes in opposite direction (zero damping resistance) on relays or electric motors shorting and sinking that energy of flybacks.

If 1 kOhm is proper damping resistor for my coil why use overdumping with 100 Ohm resistor for a short period of time will slow decay?

Oops. I have fixed polarity of Shottky diode here: [ATTACH]29003[/ATTACH]

Good old story about impedance matching, maximum power transfer is when circuit is loaded with one particular resistance value to burn energy fastest. If you reduce this resistance, energy will remain stored in coil magnetic field and charge of parasitic capacitance inside coil for longer period of time. Like in Chance, during short circuit phase, energy remains stored, no flyback, just slow dissipation due to circuit losses. This is not like power rail, or capacitor, to discharge it faster by shorting it to draw maximal current, circuit itself will not generate maximal current to short circuit. Actually even real capacitor (with real ESR and internal resistance) shorted with some “ideal zero resistance superconducting short” will discharge slower compared to say, real few or few tens milliohm short, bang and arc, but this may not be best analogy for this case. Analogy with motors or relays is not good too, diodes etc will clamp flyback peak, but this is very slow way to get rid of excess energy, useless for detectors. Connect one diode across your coil and measure what happens. No one use diodes to damp coils this way. With resistor in series, you will find fastest response with resistor value about equal to normal damping value.

[Waikiki_Sweep]

With signal levels below 6 increments on the VDI no. Above that it becomes more reliable. However there is still much to be learned about how this detector behaves when targets are presented to it end on versus broadside and full insight into the source code will help a lot. I believe this information will be coming our way here.

Yes. It is unclear. I worried that Chance measuring only conductivity of targets so iron pieces of different size, alloy, shape and orientation can mimic gold, aluminum and other metals.
To discriminate iron reliably there is a phase reading is necessary because color and ferric targets shifting phase in opposite direction.
But for phase reading you need balanced TX/RX coil instead of monocoil on Chance.
Minelab detectors based on the same patent as Chance like Excalibur, Sovereign, CTX3030, ... , all using balanced coils and discriminating iron perfectly.
Also in patent US5506506 used to create Chance you will see a balanced TX/RX coil.
And creator of Chance has refused to continue to work with it becauase of "problems in schematic" (I suspect use of monocoil)

[Waikiki_Sweep]

No, you will underdamp coil this way, with low resistance value, making it even slower. Due to LC circuit nature, coil will not release energy any faster with any other resistance value other than optimal. Low resistance value will tend to short it and prevent energy release.

So low damping resistance will prevent flyback to start releasing energy? I am so stupid to understand it. As I remember diodes in opposite direction (zero damping resistance) on relays or electric motors shorting and sinking that energy of flybacks.

If 1 kOhm is proper damping resistor for my coil why use overdumping with 100 Ohm resistor for a short period of time will slow decay?

Oops. I have fixed polarity of Shottky diode here:

[baum7154]

Yes 1.2.1. And you told me that gold and iron makes low tone on your Chance as well as on my Chance too.
Also I have tried gold rings with different "Iron rejection" modes and I can see that it reducing sensitivity to small gold rings too.

Is there a way to discriminate iron with Chance reliably?

With signal levels below 6 increments on the VDI no. Above that it becomes more reliable. However there is still much to be learned about how this detector behaves when targets are presented to it end on versus broadside and full insight into the source code will help a lot. I believe this information will be coming our way here.

[Tepco]

It is true, slow coil may release energy slow.

But with using different damping resistors we can see a different decay curves.

Idea is to bring coil voltage into preamp input range with low resistance load as fast as we can than use higher dumping resistance to read samples.

Probably we just need to connect additional damping resistor for a short moment before first sample.
How about 1k dumping resistor and 100 ohm resistor in line with Shottky diode.
When flyback is more than 200mV diode is open and damping is 100 Ohm. Once it goes below 200mV diode will close and damping will stay 1 kOhm.

No, you will underdamp coil this way, with low resistance value, making it even slower. Due to LC circuit nature, coil will not release energy any faster with any other resistance value other than optimal. Low resistance value will tend to short it and prevent energy release.