Totem-pole gate driver VS Active pull down VS Fast Fet turn off

[dbanner]

Yeah, biorobots indeed. Those were great men willing to give their lives to save, well, probably the entire European continent. Now we say loonie tunes in power across Europe and you have to wonder
I'll take my dose of Boris Johnson any day, as crazy as that seems, than to live under any psuedo socialist super state. We saw how that ends.

Anyways, back to the topic please.

I'm breadboarding a front end pi circuit and guess what, the damn Chinese breadboard is a trap. Sheesh, I blew up two transistors before I realized the bus ain't no bus at all!!

I'll use pidgin..... Cheap Ting no good, shaft you in Bam Bam!!!

[Ferric Toes]

This has been some good reading.

So keeping MOSFET out of avalanche is a good thing.
Another way to do this is to reduce the peak coil current with a series resistor. This also decreases the coil Tau and allows longer TX on pulse (good for high conductive targets).

I ended up adding 10 Ohms in series with the coil on the Hammer-head I built. Without the resistor the detection was not quite stable.
Later did distance tests and found that detection distance didn't decrease with lower coil current. Possible due to MOSFET going into avalanche and first sample being cleaner.

Hi Waltr, Yes, that is a good way. I have also wound coils with thinner wire to decrease the coil Tau. The coil I have been using for the tests reported in this thread is wound with 1.0/0.2 wire, PTFE insulated, and the resistance works out, without cable, about 5ohms. With cable is is 5.8ohms. Adding 10ohms in series on the live side reduced the flyback from 600V to 300V and nice flat top Tx current pulse for the last 200uS of a 300uS total length. What I have done previously with such an arrangement, is increase the pulse repetition rate to end up with the same current draw as it was without the additional series resistance. This can help to make up for the lower S/N due to the reduced Tx current amplitude.

The avalanche mode in a Mosfet appears to be similar to that of a zener diode, which is well known to be noisy. I wonder if the noise stops immediately the device exits the avalanche region, or does it continue for a while?

Eric.

[waltr]

Yes, decrease peak coil current and increase pulse rate is a goo way to go.

[dbanner]

BTW waltr, you forgot the d at the end of goo..

Well, after much bemusement, I finally have a circuit that even l could be proud of. Can't see the nitty gritty, but at least it didn't go into meltdown.

I pause, I'm reminded of those experimental scientists, they had surely earned their keep. Keep up the good work guys, now only if I can analyze this waveform....

[green]

Yes, decrease peak coil current and increase pulse rate is a goo way to go.

Tests I've done, signal strength proportional to peak current. If current is reduced to half, how much increase in pulse rate should be needed to get S/N back to where it was before current was decreased.

[dbanner]

Well, there's duty cycle as well, it gets rather complicated The way to approach such a calculation is of course to use calculus. Then compare actual measured results.
There is no doubt in my mind that we are dealing with a collapsing electromagnetic field and as such one should be careful in not underestimating it's inherent effect. It may be non linear.

[waltr]

Tests I've done, signal strength proportional to peak current. If current is reduced to half, how much increase in pulse rate should be needed to get S/N back to where it was before current was decreased.

I increase series resistor (decrease peak coil current) just enough to prevent MOSFET avalanche for cleanest sampled signal.

Well, there's duty cycle as well, it gets rather complicated The way to approach such a calculation is of course to use calculus. Then compare actual measured results.
There is no doubt in my mind that we are dealing with a collapsing electromagnetic field and as such one should be careful in not underestimating it's inherent effect. It may be non linear.

No simple calculations since Peak current vs pulse rate vs integrator values vs sampling time are none linear.

Same with doing GB sampling.

[Monolith]

Tests I've done, signal strength proportional to peak current. If current is reduced to half, how much increase in pulse rate should be needed to get S/N back to where it was before current was decreased.

Waltr and dbanner give a good answers.

Maybe, the way to look at this is to make a simplified version, like an ideal version. Ideal TX, ideal target, ideal integrator etc. Then the ideal answer is simply: half the TX current = double the repetition rate.
Since an ideal world does not exist, we might simplify a very complex answer by choosing just the most important factors of the many, many that are involved.
Should we start with: what would be an ideal TX?

[dbanner]

An ideal TX would be the one that senses the target. That being said, I don't mean to oversimplify.
Drake's equation states.....
Yes waltr, but it isn't that obvious to those whom art not skilled in the art.
What may seem intuitive to some may fly straight over one's head.

I offer a solution.

An exponenential curve, lest we forget.
GB is merely an obstacle, to surpass.....

[dbanner]

Well, maybe not in the milky way, but my point being that one should not try to over analyze what is staring at you straight in the face.
No amount of code can get us out of this Quagmire.
Now we enter the realm of quantum physics.

[dbanner]

Now if I blast you with a series of equations you will accuse me of cutting and pasting. Well I can assure you, I'm not a cut and paste kind of guy.

[dbanner]

I challenge anyone to plot I vs V with such a complex intra dependent waveform. The closest I've seen is Mr. Green's analysis. Yet he fail to realize the obvious.

[dbanner]

If we ramp the current, notwithstanding the limitations of the devices we use, then the target ought to have been fully energized, to respond, in a manner which is percievable to detection, hence a metal detector. We know this to be rudimentary.however....

[Monolith]

Now if I blast you with a series of equations you will accuse me of cutting and pasting. Well I can assure you, I'm not a cut and paste kind of guy.

Well, here is a cut and paste. I cut out this small part, because there are many pages of it in the public domain. It is from the legendary genius who designed more commercially successful detectors than anyone in the world. Dave Johnson.

SPECTRAL CHARACTERISTICS OF THE TRANSMITTER
If the transmitter voltage were a square wave, say for instance like the Fisher CZ's, the voltage of the harmonics would be inversely proportional to the harmonic number. This puts about 90% of the voltage energy at the fundamental. The current waveform is a triangle wave, the integral of the the voltage waveform.
In a CCPI system, the current waveform is approximately a square wave-- that is, the first derivative of a triangle wave. Over a broad range, the voltage of the harmonics is proportional to their frequency. This relationship no longer holds at frequencies whose period is less than about twice that of the transmit pulse duration (Phases I and III). There is a null in the spectrum for harmonics which have a period about that of the transmit pulse duration, and the higher frequencies roll off at a 1/f rate.
The square wave (as used by the Fisher CZ) is good for finding objects within a somewhat restricted size (i.e. equivalent conductivity) range, for instance US coinage and aluminum pulltabs. The transmitted spectrum of a CCPI machine is particularly well suited to finding objects of unknown size-- for instance gold prospecting, which requires high sensitivity to tiny nuggets while retaining good sensitivity to larger nuggets as well.

[green]

This has been some good reading.

So keeping MOSFET out of avalanche is a good thing.
Another way to do this is to reduce the peak coil current with a series resistor. This also decreases the coil Tau and allows longer TX on pulse (good for high conductive targets).

I ended up adding 10 Ohms in series with the coil on the Hammer-head I built. Without the resistor the detection was not quite stable.
Later did distance tests and found that detection distance didn't decrease with lower coil current. Possible due to MOSFET going into avalanche and first sample being cleaner.

Been thinking detection distance should decrease. Tried adding resistance in spice, signal lower but looks like can sample sooner. Need to try with real circuit.