Why like this?

[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.

[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.

[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)

[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.

[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]

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.

[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.

[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.

[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.

[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?

[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

[Altra]

Green, Try 12 to 14v on your TX. The TDI, GS5 and Goldquest all used current limiting and a higher supply voltage.

[green]

Increased PS to 12V. Tx on time 300us(current flat tops with higher coil resistance). Lower coil resistance still higher signal.

[green]

Labeled R7 L6 quarter, not correct. Look at Rx for quarter decay, Quarter is combination of the three RL circuits.

Quarter simulation https://www.geotech1.com/forums/show...210#post258210

[green]

Couple reasons higher coil resistance could cause higher signal. Circuit doesn't like higher Tx current for some reason(simulation doesn't care). Higher coil current takes longer to decay, not enough delay time(sampling when amplifier saturated). Any others?