Why like this?

[bbsailor]

The benefit to flat-topping is to reduce the target's reverse-eddy current right at turn-off. Adding series resistance does this, but it also decreases the peak current so there is a trade-off. I suspect better flat-topping benefits high conductors the most.

The normal current rise Time Constant is at 3 TCs or at about 95 percent of max current when the current rise slope is more horizontal than vertical. If you turn the current off at 1 TC or about 63 percent, you are turning off the current while it is still rising and canceling some of the spent energy and not fully stimulating potential targets. That is why I always thought that the normal turn off time was at or near 3 TCs of the current rise. Given all recent developments, is this still valid for PI designs?

Joseph J. Rogowski

[green]

The benefit to flat-topping is to reduce the target's reverse-eddy current right at turn-off. Adding series resistance does this, but it also decreases the peak current so there is a trade-off. I suspect better flat-topping benefits high conductors the most.

Added a US quarter to simulation. Adding resistance still lower signal. Maybe I'm doing something wrong?

[Carl-NC]

The benefit to flat-topping is to reduce the target's reverse-eddy current right at turn-off. Adding series resistance does this, but it also decreases the peak current so there is a trade-off. I suspect better flat-topping benefits high conductors the most.

[Altra]

There has been discussions on series resistance in the past. Eric Foster was using 15-33 Ohm in series? The idea was to speed up the coil to be more sensitive to short tc targets. It also acted as a current limit causing the tx current to flat top, which I don't remember the benefit. Maybe Eric or Carl can explain.

[billr]

green
Thanks for the heads up on Spice. I'll give it a try. I did have access to Every Circuit online but found it not too reliable in practice.

The total series resistance has a lot to do with the coils Tau.
When I was building my Hammer Head II and experimenting I found that adding 5 Ohms in series with the coil actually increased the detection distance. Best answer I have is: Tau = L/R , therefore if series resistance (R) is larger, then the coil's magnetic field levels out sooner and before the end of the TX Pulse. Otherwise, if the magnetic field is still increasing when the TX Pulse ends then Target eddy currents will NOT be as large due to increasing and decreasing magnetic fields cancelling.

Hi waltr
Thanks for that. You explained this better than I did.
I may try adding resistance to the best coils I have and see what it does.

[green]

The total series resistance has a lot to do with the coils Tau.
When I was building my Hammer Head II and experimenting I found that adding 5 Ohms in series with the coil actually increased the detection distance. Best answer I have is: Tau = L/R , therefore if series resistance (R) is larger, then the coil's magnetic field levels out sooner and before the end of the TX Pulse. Otherwise, if the magnetic field is still increasing when the TX Pulse ends then Target eddy currents will NOT be as large due to increasing and decreasing magnetic fields cancelling.

Wondering why increasing coil resistance would increase signal, tried with spice simulation reply #18. Didn't increase with what I tried, maybe missing something.

[waltr]

Try the 1.8 coil on the MPP Beach only achieves 9.5" range.

Try the 3.2 coil on the MPP and the range improves to nearly 11.5"

The total series resistance has a lot to do with the coils Tau.
When I was building my Hammer Head II and experimenting I found that adding 5 Ohms in series with the coil actually increased the detection distance. Best answer I have is: Tau = L/R , therefore if series resistance (R) is larger, then the coil's magnetic field levels out sooner and before the end of the TX Pulse. Otherwise, if the magnetic field is still increasing when the TX Pulse ends then Target eddy currents will NOT be as large due to increasing and decreasing magnetic fields cancelling.

[green]

9.5 inches? This must be when the signal is strong yes? I still consider the detection range up to where it is just discernible.

Surf and Barracuda using the same coil with a resistance of 1.8 ohm pick up a signal at 14" air test. Try the 1.8 coil on the MPP Beach only achieves 9.5" from reply #1. S/N would need to be increased 8times with MPP with a 200mm coil to detect nickel at 14inches like with surf.

[green]

I would suggest learning spice if you haven't already. Can change component values. Step a component value to see change. Lot quicker than building a circuit and should be similar except for noise level. Simulation target TC=10us(US nickel)

[billr]

Check out one of these on Bangood or Aliexpress a handy tool to give you an idea of your L etc:
LCR T4 Mega328

Thanks Surf. I'll look around for one. I fell out with Bangood some years back.

[billr]

This all started off with me trying to get the greatest range of detection I could from a given coil. I start with https://www.geotech1.com/forums/atta...5&d=1600272466
Example: MPP detects a nickel at 9.5inches. S/N would have to increase 8times to detect the nickel at 14inches with a 200mm coil.

9.5 inches? This must be when the signal is strong yes? I still consider the detection range up to where it is just discernible.

[surfdetector]

I have no idea as I don't have anything to test them other than use the coil calc as a starting point. All I can go on is the resistance.
The Surf does well without any damping resistor. It losses a bunch of range with one fitted. The Cuda works well with the 330R. 470 seems to work OK with the MPP. Not much change with a 680R fitted.

Check out one of these on Bangood or Aliexpress a handy tool to give you an idea of your L etc:
LCR T4 Mega328

[green]

Thanks all for the replies.
This all started off with me trying to get the greatest range of detection I could from a given coil. It now feels like I may be able to exploit an unknown error I made in the builds. Hence the question. What caused it?
The coil with 1.8 ohms is different in design to the 3.2 coil. My thinking is I may be able to increase the 1.8 to 3.2 by adding more winding's and detect a nickle at 17" or more with the detector still happy to work as it did before. 17 inches would to me at least be quite some range for a coin.
Of course there would be drawbacks but I learn best when I ask myself. What if?

This all started off with me trying to get the greatest range of detection I could from a given coil. I start with https://www.geotech1.com/forums/atta...5&d=1600272466
Example: MPP detects a nickel at 9.5inches. S/N would have to increase 8times to detect the nickel at 14inches with a 200mm coil.

[surfdetector]

That is to better match the coil that I am using.

[billr]

The TX pulse width of the machines should ideally be 5x the TC of your coil. Are your TX pulse widths all the same or do they vary?

Cheers

On your video step 3. I noticed you adjusted the TX pulse width to 100us and the PP to 1ms instead of 400us and 8ms. What was the reason for that?