Coil Saturation in a PI?

[bklein]

I may be totally mistaken (again) but changing the resistor in series with the coil also changes the TX voltage across the coil so the actual 5T (or any T) current is different. A better test (for me) would be to eliminate the series resistor and instead increase TX voltage. This makes it reach (previous) 1T the desired current sooner. It takes just as long to reach the 1T current level but it is higher because the 99% 5T goal current will be higher with higher TX voltage. (Correct?)
Correct me if I’m wrong again but from what Carl said earlier, the start of the pulse causes eddy currents in the target just as the release of the pulse does (not to the same degree). Some settling time is necessary at the start optimally before the pulse end occurs. How to figure this - I haven’t a clue. So there’s this as well as a possible damping resistor value change necessary if the TX voltage changes.
So back to your test of coil current vs depth, although it didn’t pan out for you, several are saying that a higher voltage battery pack on the TDI is worth it. So why....?

[waltr]

Green,
The charts you posted are about what I expected from theory and my experiments.
A low TC target (nickle) didn't increase response with long TX times but a quarter did up to 2-3
TC and same with the Ike dollar.

As was posted above an increase of TX time from 2 to 3 target TC increases TX current (magnetic field) by less than 2%. So a TX pulse of 2 target TCs is a good trade off between target response and total detector current draw.

Bklein,
A series resistor limits current into the coil and decreases the time it takes to reach peak current. At this point in time the Voltage across the coil is reduced by the ratio of the coil's resistance to the series resistance of the R & MOSFET.
This lower Voltage (at peak) and lower current (magnetic field in the coil) is what helps prevent MOSFET avalanche in the MOSFET during fly-back and also helps reduce stray current spikes through out the detector's circuits. This is why a series resistor can help 'stabilize' many of the hobby build PI detectors which increases the detection distance.

The TDI is a professional designed and build unit. It has many more parts and sure the PCB lay-out is better than the hobby build PCBs.
From the schematics of the TDI I have seen there is a 2Ohm, 25W series R (this seems to dissipate a lot of power). Also the RX has a two stage pre-amp. My feeling is that higher Voltage on the TDI is useful due to other factors than simply the coil's magnetic field strength (current through coil due to higher Voltage). Also if the timing diagram on the TDI schematics is correct, then the TX pulse time is only 100us. This does not seem to be optional for high TC targets but more than enough for low TC targets.

It is very hard to directly compare different detectors

[dbanner]

The flyback voltage seems always problematic, hence the need for limiting the current, which implies slightly greater pulse width for achieving the desired peak saturation of the target, not so good for targets with small TC, the opposite is true for the inverse proposition. so in the end it always boils down to designing a front end for a specific target or group of targets in mind. Large cannon balls and ship anchors aside.

[green]

I may be totally mistaken (again) but changing the resistor in series with the coil also changes the TX voltage across the coil so the actual 5T (or any T) current is different. A better test (for me) would be to eliminate the series resistor and instead increase TX voltage. This makes it reach (previous) 1T the desired current sooner. It takes just as long to reach the 1T current level but it is higher because the 99% 5T goal current will be higher with higher TX voltage. (Correct?)
Correct me if I?m wrong again but from what Carl said earlier, the start of the pulse causes eddy currents in the target just as the release of the pulse does (not to the same degree). Some settling time is necessary at the start optimally before the pulse end occurs. How to figure this - I haven?t a clue. So there?s this as well as a possible damping resistor value change necessary if the TX voltage changes.
So back to your test of coil current vs depth, although it didn?t pan out for you, several are saying that a higher voltage battery pack on the TDI is worth it. So why....?

Change in signal strength should be proportional to change in battery volts if nothing else changes. 6 to 12 volts doubles signal. 12 to 15 volts increases signal by 25%. One division on chart is 26%, three divisions is double. With a 8inch coil, 25% increase in signal increases detection about 3/4inch at 16inches. Doubling signal increases detection about 2inches at 16inches.

We have coil on TC and target TC. Looks like Tx time on should be 2 times target TC. What is the procedure for selecting the best coil TC once Tx on time has been defined?

[Carl-NC]

Carl,
Isn't the Coil Tau = L/R. Which is why increasing the series R decreases the Coil Tau.

Oops, yes, typo... tau = L/Rs.

[eclipse]

Green,

The TDI is a professional designed and build unit. It has many more parts and sure the PCB lay-out is better than the hobby build PCBs.
From the schematics of the TDI I have seen there is a 2Ohm, 25W series R (this seems to dissipate a lot of power). Also the RX has a two stage pre-amp. My feeling is that higher Voltage on the TDI is useful due to other factors than simply the coil's magnetic field strength (current through coil due to higher Voltage). Also if the timing diagram on the TDI schematics is correct, then the TX pulse time is only 100us. This does not seem to be optional for high TC targets but more than enough for low TC targets.

It is very hard to directly compare different detectors

The TDI SL doesn't have the 2 ohms resistor and the TX is 100uS

[waltr]

The flyback voltage seems always problematic, hence the need for limiting the current, which implies slightly greater pulse width for achieving the desired peak saturation of the target, not so good for targets with small TC, the opposite is true for the inverse proposition. so in the end it always boils down to designing a front end for a specific target or group of targets in mind. Large cannon balls and ship anchors aside.

Adding series R decreases the coils Tau which means the coil reaches Peak Current sooner. This is an advantage for all targets and allows shorter TX pulse times since the peak current is reached sooner.
Yes, it does boil down to the full design and optimizing to the desired targets.
This is why I do not bother with trying to build a very fast coil, I will not be looking for small, or any size, gold nuggets.

The TDI SL doesn't have the 2 ohms resistor and the TX is 100uS

Ok, thanks.
Is there an accurate schematic available for the TDI SL & Pro?

I only have the one with lots of errors (op-amps and transistors back-wards) that I found on this forum.

[Carl-NC]

I continue to object (as I have in the past) to some of the descriptions of how things work. We are not trying to saturate the target during the TX-on time. That would imply that we are trying to build up a to maximum eddy current. When the coil is energized it will induce eddy currents in the target, but those eddy currents are in opposition to the eddy currents that are created by the turn-off (flyback) event. Residual on-time eddy currents will therefore subtract from the turn-off eddy currents and reduce overall detectability.

Instead, we want the on-time eddy currents to die out to zero. To do this, you would extend the TX-on time not only long enough for the TX current to flat-top, but also for the target eddies to decay to 0. If the coil tau is, say, 150us, and you are looking for small nuggets, then 450us (3 tau) might be enough. If you are looking for Atocha silver bars then maybe you need 10s or 100s of milliseconds because the target tau will extend eddy settling long past the coil tau.

I have long questioned how much of a difference this really makes, especially with targets that have a tau shorter than the coil tau. As you extend the TX on-time to minimize the subtractive effect, you also extend the off-time flyback due to the higher coil current and so the sample delay must be extended. It is a trade-off, and I'm not sure where the trade-off cross-over occurs. For high-tau targets it may make more sense because the off-time peak of the target's eddy response occurs at 1*tau (the target tau, not the coil tau) so if that tau is 1ms it makes no sense to try and push the sample delay to 15us where there is little signal anyway. You want to sample at/near the peak.

[ivconic]

I enjoy reading questions, answers and objections here.
Very educational.
At the end real book could be made from most of the posts from this topic.
Thanks to all the parties involved!

[Qiaozhi]

I have long questioned how much of a difference this really makes, especially with targets that have a tau shorter than the coil tau. As you extend the TX on-time to minimize the subtractive effect, you also extend the off-time flyback due to the higher coil current and so the sample delay must be extended. It is a trade-off, and I'm not sure where the trade-off cross-over occurs. For high-tau targets it may make more sense because the off-time peak of the target's eddy response occurs at 1*tau (the target tau, not the coil tau) so if that tau is 1ms it makes no sense to try and push the sample delay to 15us where there is little signal anyway. You want to sample at/near the peak.

Same here. I did some experiments a while back that lead me to the same conclusion.
We've been discussing this for many years:
https://www.geotech1.com/forums/show...025#post143025
https://www.geotech1.com/forums/show...226#post256226

[green]

I continue to object (as I have in the past) to some of the descriptions of how things work. We are not trying to saturate the target during the TX-on time. That would imply that we are trying to build up a to maximum eddy current. When the coil is energized it will induce eddy currents in the target, but those eddy currents are in opposition to the eddy currents that are created by the turn-off (flyback) event. Residual on-time eddy currents will therefore subtract from the turn-off eddy currents and reduce overall detectability.

Instead, we want the on-time eddy currents to die out to zero. To do this, you would extend the TX-on time not only long enough for the TX current to flat-top, but also for the target eddies to decay to 0. If the coil tau is, say, 150us, and you are looking for small nuggets, then 450us (3 tau) might be enough. If you are looking for Atocha silver bars then maybe you need 10s or 100s of milliseconds because the target tau will extend eddy settling long past the coil tau.

I have long questioned how much of a difference this really makes, especially with targets that have a tau shorter than the coil tau. As you extend the TX on-time to minimize the subtractive effect, you also extend the off-time flyback due to the higher coil current and so the sample delay must be extended. It is a trade-off, and I'm not sure where the trade-off cross-over occurs. For high-tau targets it may make more sense because the off-time peak of the target's eddy response occurs at 1*tau (the target tau, not the coil tau) so if that tau is 1ms it makes no sense to try and push the sample delay to 15us where there is little signal anyway. You want to sample at/near the peak.

https://www.geotech1.com/forums/atta...0&d=1570406043 changed L1 Rser to 2R for 150us coil TC. Changed target to the nickel, closer to small nuggets. Ran a simulation. Don't see why 450us might be enough. Signal at 150us on time is lower than the other on times(less current at turn off)signal looks to be close to the same for the other on times. I have been thinking we were trying to make Tx on time 2 or more times the target TC not the coil TC. What am I missing?

[Qiaozhi]

And there's this ->
https://www.geotech1.com/forums/show...739#post221739

[Carl-NC]

I have long questioned how much of a difference this really makes, especially with targets that have a tau shorter than the coil tau.

This is not quite right, when I wrote this I had in my head the turn-on tau. The coil tau we are talking about in this case is the flyback tau, which is 0.5*L/Rd, where Rd is the damping resistor. Ferinstance, L=300uH & Rd = 680 yields a flyback tau of 220ns. The coil flyback voltage will peak at 220ns (1tau) and then (roughly-eventually) decay to zero with this tau. The target will see a peak induced EMF that is lagging the peak flyback according to its own tau and decay from there.

So I should have said, "especially with targets that are out to perhaps 100x the tau of the coil."

Again, this is speculation but is based on some tests I did many years ago. I am currently grinding out the math on this and will have a numerical solution soon.

[Carl-NC]

Same here. I did some experiments a while back that lead me to the same conclusion.
We've been discussing this for many years:
https://www.geotech1.com/forums/show...025#post143025
https://www.geotech1.com/forums/show...226#post256226

Thanks, I would have never found this. In quickly scanning through it I see one of my flubs:

The TC doesn't depend on surface area, surface area just affects the strength (amplitude) of the response.

This is absolutely WRONG. There is an edge effect so that when the target is smaller than the incident magnetic field the tau will be dependent on target area. When the target is larger than the field then making it larger doesn't matter. One of my goals in life is to derive the skin depth equation factoring in boundary conditions.

[green]

This is not quite right, when I wrote this I had in my head the turn-on tau. The coil tau we are talking about in this case is the flyback tau, which is 0.5*L/Rd, where Rd is the damping resistor. Ferinstance, L=300uH & Rd = 680 yields a flyback tau of 220ns. The coil flyback voltage will peak at 220ns (1tau) and then (roughly-eventually) decay to zero with this tau. The target will see a peak induced EMF that is lagging the peak flyback according to its own tau and decay from there.

So I should have said, "especially with targets that are out to perhaps 100x the tau of the coil."

Again, this is speculation but is based on some tests I did many years ago. I am currently grinding out the math on this and will have a numerical solution soon.

Read your formula and wondered if it was correct, then remembered decay is twice as fast if critical damped?