Nexus Standard SE

[Nexus]

The Sovereign has a preamp in the coil which is powered off the TX magnetic field. I don't think this was done in later FBS designs, although maybe in the CTX which they call "FBS2". I haven't done much with it. In all the BBS/FBS designs, the TX coil is just a coil, typically 500uH. You can directly measure it.

- Carl

I get it. It works this way in the Minelab's coils because the TX is of a very low inductance. Our TX are usually of 10-20 times higher inductance 5 - 10 mH.

[Nexus]

For an inductor: V = -L.dI/dt, so for a coil with low R, a fixed voltage gives a current ramp up/down at a fixed rate, thus a square-wave produces a triangle.

Search the forum for Minelab coil dissections/analysis, the circuit diagrams are there, even PCB layout files if you want to copy them to make your own coils for those machines. The circuit was just a pre-amp, and a bridge rectifier is used to turn the coil-drive square-wave into a DC power supply for the amplifier.
Edit: here's a couple, there are others:
http://www.geotech1.com/forums/showt...inelab-pre-amp
http://www.geotech1.com/forums/showt...Sovereign-Coil

Ok. That is the case. I didn't know the Minelabs TX were of a low inductance and low R. Didn't bother to measure them.