My opinion on MOSFETs

[Teleno]

Thats one of my circuits, Q2 and a few other components around it act as a flyback blocking circuit (this is used in place of the typical 1k resistor and two diodes method). Another
user here called Moodz designed the circuit.

I haven't used this circuit for ages due to limitations. My detector now uses a micro to control Q1 and Q2, plus samples.

All right I've located the patent: http://ip.com/pdf/patent/AU2013101058A4.pdf

Constant current damping could possibly be achieved using a varistor (voltage dependant resistor) having a positive voltage coefficient (resistance increases with voltage), however, only negative voltage coefficient are commercially available now, but this might change in the future: http://iopscience.iop.org/0022-3727/...7_19_4_004.pdf

[Davor]

Maybe it is a worthy goal to obtain a self-regulating circuit that behaves like a varistor with positive voltage coefficient. There is a circuit similar to a tunnel diode, called a lambda diode, which is comprised of a complementary pair of FETs or NJF+PNP for greater steepness of a negative resistance slope. We'd just have to make it opposite

Having a similar thing that kicks in at lower voltages and is otherwise an open circuit would do it as well, and a clamp comes to mind.

[Teleno]

Maybe it is a worthy goal to obtain a self-regulating circuit that behaves like a varistor with positive voltage coefficient.

Any biased transistor, whether bipolar or (MOS)FET behaves as a constant current source/sink at the collector/drain.




Something like this could possibly be used instead of a damping resistor.

[Davor]

My thought exactly. You wish to have a high impedance ... thing ... at high voltages, and low predictable impedance at low voltages, and such contraption would provide acceleration of flyback without any active control. It would perhaps enable even some easily adjustable damping. Most obviously you'd have to set it to a desired end-impedance for perfect damping by some external control. That may lend itself nicely to a lap of microcontroller freaks for rigs with easy coil changes.

A BJT current source requires some bias, and BJTs impedance is always related to a current flowing through it. I'd seek a solution among the FETs as their channel impedance depends on Vgs rather than currents.