My opinion on MOSFETs

[GT Blocker]

I was of the understanding that a diode between the coil and the Mosfet reduced the capacitance because the coil no longer see's the Mosfet directly.

Hey Mickstv,

I'm pretty sure that the capacitance of the coil, cable, and MOSFET are considered as a whole. Someone correct me if I'm wrong here. "Back in the day", I think it was Reg who added a diode (15ETH06) between the R11/coil junction (the L1 damping resistor and coil junction) and the MOSFET. A resistor (56k) runs from the diode/MOSFET junction to V-. There is deffinately a thread here somewhere about it. Anyone remember where?

Is that what you mean? I'm speaking about the HHd only here.

[GT Blocker]

Davor,

I appreciate your response and now have a better understanding of my misguided assumptions. CW dectors are an animal apart from PIs. I appologize for gittin uppity.
In the PI world, small targets mean small capacitance is a must have in the food chain.

I did get a bit snarky. Sorry about that. ___ GTB

[Davor]

I don't mean to come off surly, but come on. Sit back and learn for once.

I enjoy this forum and most of the communication going on here. I also have some experience from professional radio (transmitters etc.) that may come of some use here and there. Sitting back is what I mostly do during winters as my shack is at a somewhat remote place. Now it is the way I relax in between writing some big piece. As for my detectors, I'm focused to CW detectors as they meet my requirements better than PI, but I tend to keep my options open. My next build will most probably be a simple watertight PI for diving purposes.

I did not realise you got offended, so I'll let you be and wait for some results (as most people do). Good luck.

[mickstv]

I was of the understanding that a diode between the coil and the Mosfet reduced the capacitance because the coil no longer see's the Mosfet directly.

[GT Blocker]

CMF10120D

Hey Tec,

I'll just go ahead and buy one of these suckers (CMF10120D) from Mouser. Should be interesting. What are the options for increasing the coil voltage if this puppy works out? I never thought I'ld see a MOSFET at 1200v/134w and 63pF. Hope it works as well as it sounds. Thoughts?

GTB

[GT Blocker]

You'll do what you must.

True enough, sir. As will you.

(You never did answer the above question(s)...although you post an average of 100 posts per month)

May I see a photo of a detector that YOU have built? I wonder about the validity of your opinions. Perhaps wrongly so. I don't know. It just seems to me that you have many opinions and few credentials to back the play. When do you work in your lab if you are posting at least three times PER day? 1600+ posts in 16 months? Really?

I don't mean to come off surly, but come on. Sit back and learn for once.

My opinion,
GTB

[Davor]

Water makes some difference if you let it permeate your braid, otherwise a simple weave offers less capacitance than coax, and for all practical purposes shielding as offered by coaxial is of less significance at PI bandwidth.

You'll do what you must.

[GT Blocker]

OK

Ok, Davor,

"For purpose of connecting your rig to a coil you actually do not need a coax. Much easier build is a simple weave."

I thought we had covered this in my Litz wire thread (http://www.geotech1.com/forums/showt...itz-wire-thing {of note, post No.12}). Re-read both your and my posts concerning this matter.

Just for fun, Davor, how, exactly, would you go about sampling at or below 6uSec? Your target is 4 grains @ 80% pure gold. It lies behind 6" to 8" of solid rock (perhaps mineralized rock). Gonna go for the "simple weave"? No need for coax? Opps, I forgot to mention, you're underwater while you're looking for that gold and so is your detector. Good luck with the "simple weave, no coax" thing. What is your plan for sub-6uSec sample rates?

Just me....

GTB


__________________________

Solutions shall forever triumph over opinions. GTB

[Davor]

...I am building my own Litz wire coax...

For purpose of connecting your rig to a coil you actually do not need a coax. Much easier build is a simple weave.

[GT Blocker]

Hey Tec,

Nice to hear that I am not completely crazy.

I let the 'magic smoke' out of my first HH a few weeks ago. Rather than diagnose and repair a well worn horse, I decided to pull out a new board and start over. Since I am useing 1% resistors, tantalum caps, and a host of other tweaks (wrong thread), I felt I should go for the lowest capacitance possible from the coil to the front end. To that end, I am building my own Litz wire coax, Litz wire coil, and looking for the least capacitance MOSFET I can find.

I hadn't thought about the break down voltage. Very interesting observation on your part.

I'll let ya all know how this works out as it moves along (waiting on parts, testing, etc.)

If anyone else has thoughts, step in..

GTB.

[6666]

Tec is that the right number IRF2405 ?
they seem to be only good up to 55 volts

[Tec]

Hey all,

I had to dig into this old post to get some thoughts about the Coss thing. I am under the belief that the capacitance of A: the coil + B: the cable + C: the MOSFET Coss = the capacitance which leads to the damping resistor value. I am also under the belief that reducing the capacitance in ANY of the three will result in potentially faster sample times.

Am I correct in this?

If one ignores the cost of the MOSFET I am considering (WOW$$), it looks promising for my Litz coil and Litz coax attempt at very short delays and small targets.

The MOSFET I am going to try is the Cree CMF10120D: 1200v, 24a (49a-pulsed), 134w, Coss 63pF. ~$21 usd.

[ATTACH]24535[/ATTACH]

The Coss 'savings' over an IRF740 is -157pF. I am unfamiliar with how the other specifications of the part compare to the IRF740 and as to how they relate to the needs of faster sample times.

So, what do you folks have to say (other than the cost!) about this "Silicon Carbide N-Channel" MOSFET? And for that matter, the role that MOSFETs play in the bigger picture of fast sample times?

I'll try one out anyway, just to see what I can get for shorter timings.

Thanks, GTB

Hi,

I'm interested to know if your mosfet speeds things up. Yes I agree it will, because the capacitance is the most important bit.
Also it has a high break down voltage so, there will be higher peak voltage in the spike.

Here's what I think.
When switch-off occurs, the power in the magnetic field is lost rapidly at the following rate : voltage x current. The quicker the change in magnetic field the better/smaller the target can respond.

Capacitance momentarily lowers the voltage (slows the discharge rate) and stores some energy, only to push it out later in the form of oscillations.


Something I am doing at the moment is using a single loop. The lowest possible inductance and capacitance.
With high current.
With no damping resistor it rings at 13MHz on switch off... so this is all due to the mosfet capacitance.

Tec

[GT Blocker]

Another question about MOSFETs, and an expensive one at that...

Hey all,

I had to dig into this old post to get some thoughts about the Coss thing. I am under the belief that the capacitance of A: the coil + B: the cable + C: the MOSFET Coss = the capacitance which leads to the damping resistor value. I am also under the belief that reducing the capacitance in ANY of the three will result in potentially faster sample times.

Am I correct in this?

If one ignores the cost of the MOSFET I am considering (WOW$$), it looks promising for my Litz coil and Litz coax attempt at very short delays and small targets.

The MOSFET I am going to try is the Cree CMF10120D: 1200v, 24a (49a-pulsed), 134w, Coss 63pF. ~$21 usd.

CMF10120D.pdf

The Coss 'savings' over an IRF740 is -157pF. I am unfamiliar with how the other specifications of the part compare to the IRF740 and as to how they relate to the needs of faster sample times.

So, what do you folks have to say (other than the cost!) about this "Silicon Carbide N-Channel" MOSFET? And for that matter, the role that MOSFETs play in the bigger picture of fast sample times?

I'll try one out anyway, just to see what I can get for shorter timings.

Thanks, GTB

[Davor]

Midas, I think you are onto something. Coss is usually higher with MOSFETS with lower Rdson, and in fact Rdson is ridiculously small compared to the coil resistance already, everything below 1/10 of coil resistance is insignificant and it is not going to improve anything by getting any lower. It is the off state that generates pulse, not the puny Rdson.

Tec, you have ringing, and it surely messes up your reading. Trouble with ringing is that resonance changes due to ground proximity effects, and you get false reading. Hairy parts are moving left and right and samples are wandering, just as Midas says.

So far I thought Rdson is ridiculously small for MD purposes, so what?!? Now I know better. I'll seek some older MOSFETs for my builds. Thank you.

[Midas]

Thanks for the replies, I've posted the graphs in nice PNG format.

These are the actual coil spikes, on the coil (50V per division)

Notice one has a slight ring right at the beginning, this I believe is what makes it very sensitive to wet sand, and makes a huge difference..
This ring will occur regardless of coil damping,..increase or decrease the damping, the ring will still be there..
THe different mosfets required different damping resistance..the damping resistance seems more to do with mosfet than coil.. The fast mosfet needed more damping (250 ohm), the slower mosfet needed 330 ohm.

damping is selected based on what is happening after the amplifier..which shows up any slight ringing. Again, these graphs are of the coil spike at the coil.

Hi Tec,

Don't forget you have to look at the output capacitance (Coss) specification of the mosfet as well, lower is better. From the damping resistance you required it sounds like your 'faster' mosfet has a greater output capacitance which is what is slowing the whole system down. The sensistivty to wet sand is probably because with the extended decay curve your MD is no longer taking the samples in the correct place.

Midas