Mosfet Selection / Damping Resistor

[hobbes_lives]

I won't enter the fray about a series diode.

It's not a fray, just a courteous discussion.

I read that Eric Foster used (or uses) PVC insulated wire in some of his PI coils, but maybe he has some filler material in between individual turns to help reduce the higher dielectric constant.

I've read previously where Eric had good results with 30AWG Kynar wire-wrap wire.

One other thing he did (in his CS6-PI, notably) is use a series resistance (but no series diode) between the tx FET and the coil. That series resistance limits the value of peak current that the coil will charge to. That probably reduces overall depth of detection, but it also enhances flyback speed. He also used appropriately short TX pulses so as to not waste battery power. He did NOT use large gauge wire. That is another trap... and I know about traps. (Save the large gauge wire for monster size coils??)

I read where Eric used, I believe it was a 33 ohm resistor, in series with the coil on one of his detectors. I thought I saw it in a schematic on this site, but I'm not sure.

"Fast" and "deep" are subjective terms, but doesn't it seem apparent that "fast" coils and "deep" coils are somewhat mutually exclusive? We can have one or the other, but how can we have the best of both worlds?

Perhaps not so mutually exclusive. Eric has mentioned in the past that he believes that lowering the current, sampling sooner and using a faster pulse rate to average out more of the noise may be a viable solution. (I may not be recalling this exactly, but I think that's the gist of what he stated.)

I wonder if bifilar wound coils like Moodz has been playing with might be something in the right direction.

I want to experiment with something along the same lines, if I can ever find the time.

[Altra]

Porkluvr,

"One other thing he did (in his CS6-PI, notably) is use a series resistance (but no series diode) between the tx FET and the coil."

I believe the series resistor was to speed up the coil (T=L/R). As I recall Eric talks about it in an old post on the PI tech forum. He also used a series resistor in the Whites PI1000, 2000 and 3000. It adds problems of noise and instability as the resistor ages.

Regards Mark

[porkluvr]

"I believe the series resistor was to speed up the coil (T=L/R). As I recall Eric talks about it in an old post on the PI tech forum. He also used a series resistor in the Whites PI1000, 2000 and 3000. It adds problems of noise and instability as the resistor ages.

Regards Mark

YES.

Must be sure and use... 1W, maybe 2W resistor or whatever is necessary, it depends... . Make sure the resistor doesn't get too warm (same goes for the damper and also the receiver input resistors imho). But that's not all...

This brings up a question that was posed in post 6 in this thread: "is there any real disadvantage to letting the voltage peak so high besides dealing with it on the rx?"

ans: YES. You need to also deal with it in the transmitter.

Besides observing power dissipation in a resistor (perhaps by judging how much heat is being generated with a "touch test") wouldn't it also be prudent to pay attention to the voltage rating of any resistors in the transmit circuit?

I would be careful about using a high voltage FET and then pushing that device into breakdown. Maybe the active device can withstand the repeated stress, but what about everything else in the transmit path? And, what about if you decide to install a jack for an auxilary pinpoint probe. It should be switched, shouldn't it? Where will you find a switch that won't compromise performance??

Err, I just now realized that I should probably throw in a couple of drain bleed-off resistors in that tx sub-circuit that I showed earlier... 100kΩ~1MΩ, or something like that?? Those nodes would go high (about 400V with the IRF740) and stay high, without a bleed-off (because of the series diodes being there).

[hobbes_lives]

Err, I just now realized that I should probably throw in a couple of drain bleed-off resistors in that tx sub-circuit that I showed earlier... 100kΩ~1MΩ, or something like that?? Those nodes would go high (about 400V with the IRF740) and stay high, without a bleed-off (because of the series diodes being there).

You'll need to experiment with the value. I'd suggest to try 100k to start with. You may need to go lower. I'd select a value that will cause the voltage to drain down to a nominal level just before the next pulse starts.

[Aziz]

Hi all,

if you use a drain diode to block the mosfets capacitance, you must not use a drain bleed-off resistor! The voltage at drain should stay as high as possible to lower the mosfets capacitance. Look into the data sheet "capacitance vs. drain to source voltage".
Look, how the capacitances fall into a lower level.


So there is no need for the drain bleed-off resistor.
Aziz

[Aziz]

Here is the capacitance vs Vds (IRF740):

[porkluvr]

Hi all,

if you use a drain diode to block the mosfets capacitance, you must not use a drain bleed-off resistor! The voltage at drain should stay as high as possible to lower the mosfets capacitance. Look into the data sheet "capacitance vs. drain to source voltage".
Look, how the capacitances fall into a lower level.


So there is no need for the drain bleed-off resistor.
Aziz

Bu-Bu-Bu-Bu-Buh...

You are so right. Thanks, Aziz.

[WM6]

Jon, the MOSFET operates also as damping element because at breakdown it dissipates energy.

Yes, but those inner dissipation (including breakdown) is, as is, mean uncontrolled.

What we need is controlled breakdown. To reach this MOSFET inner dissipation is of small use.

So we need outer damping resistor: to get control over breakdovn, to shape breakdown curve in such way that we get useful signal prominent as much as possible.

Do we need damping resistor? Yes, we need it: for better results (sensitivity).

[bklein]

What if you used a local fet to momentarily short the coil after the TX pulse?
TX, short, release, RX.

Barry

[hobbes_lives]

Hi all,

if you use a drain diode to block the mosfets capacitance, you must not use a drain bleed-off resistor! The voltage at drain should stay as high as possible to lower the mosfets capacitance. Look into the data sheet "capacitance vs. drain to source voltage".
Look, how the capacitances fall into a lower level.


So there is no need for the drain bleed-off resistor.
Aziz

The reverse-biased diode will have a capacitance that is so much lower than the MOSFET that when you calculate the series capacitance of the diode and MOSFET, the capacitance of the MOSFET has little bearing on the total. The reason to use the bleed-off resistor is to get the voltage stored across the MOSFET down to a reasonable level prior to turning the MOSFET on again.

[Tepco]

!!!!!!!!!!!!IMPORTANT!!!!!!!!!!!!!!!!
First of all, i have to apologize to this forum about my previous writings, concerning series diode-MOSFET performance, and part, or all of my explanation here may be wrong. I did measured this dozen times, but in setup and under conditions when this trick cannot make any improvement (due to other tricks already used). As you may guess from my previous posts all my PI' s are based on cascode connected switch whit healthy 2KV flyback. But i failed to measure this under ordinary conditions (Surf-like detector etc). So it is not correct that under some conditions some improvement cannot be achieved doing this. I will not become advocate for this solution, changes are small, this will not make revolution in PI, but i was wrong claiming this will always be slower. In my current setup (not MD, trying to measure something else) i have too much noise and layout issues, and i don't have DSO at hand, but at first opportunity i will post waveforms under different conditions, coils etc to make comparison, and to see what can be gained (or loosed) doing this.
Until then, don't listen to me (we are not saints really). And, BTW, forums like this are ideal place to make mistake, get criticized and corrected, this is why i'm here.

Another part is, from previous post, using another MOSFET to short the coil. Not going to work, you need to get rid of all energy stored in the coil, shorting it this will not happen. Shorting the coil in part of TX pulse (burst) is the part of multipulse technology, see relevant Minelab patents etc. Sorry, at least i should be right on this one. Appologise accepted-YES-NO CANCEL?

[mikebg]

Yes, but those inner dissipation (including breakdown) is, as is, mean uncontrolled.

What we need is controlled breakdown. To reach this MOSFET inner dissipation is of small use.

So we need outer damping resistor: to get control over breakdovn, to shape breakdown curve in such way that we get useful signal prominent as much as possible.

Do we need damping resistor? Yes, we need it: for better results (sensitivity).

WM6, to realize fast TX network we need fast dissipation of energy stored in reactive elements L and C. That means we should achieve high momental power of dissipation
p(t)=u(t).i(t)
at damping process.
At breakdown interval with RF740 we have constant u(t)=Vbr=400V. If we have coil current 4A at moment t=0 (when switch turns off), this gives momental power of dissipation p(0)=400x4=1600W. When breakdown ceases, too litle energy remains in tank circuit for exponential dissipation (with damping resistor). Instead damping with MOSFET breakdown, German designers use VDResistor or HV Zener diode to realize damping with breakdown process:
http://www.pulsdetektor.de/
At exponential interval of damping process, we can not increase the power of dissipation because the resistance for critical damping is independent of flyback voltage. That's why breakdown damping is very important for fast TX network.

[Tepco]

This is not actually correct.In order to achieve 4A current in typical 300uH coil, you will need about 120uS pulse without much ohmic losses etc.(IRF740 will reach breakdown voltage whit 20-30uS pulse under similar conditions.) At this point energy stored in coil will be about 2.4mJ, (or 2.4 milliwatts per second) coressponding to 1.2W power at 500pps. And when it is released in typical 1uS pulse, peak power will be (ideally) 2.4KW. Dumping and breakdown rating of IRF will only influence that time. Actually, device whit higher breakdown voltage (whitout you can reach that limit) and lower capacitance will reduce this time, and even increase "peak power" ever more. Point is, you cannot just multiply breakdown voltage and coil current as posted here. (what you will get whit IXYS 4KV rated device then ,16KW or what?). But this "peak power" is just a math, and not so important compared to other issues (energy stored in coil and speed of release of is).

German designers did use all sorts of things in those detectors, including 4Ohm RdsON mosfets, way wrong dumping resistor values etc, only neon sign to dump the circuit is missing... I built one, another member of this forum built another, and, actually, when all boobietraps from schematics are eradicated, these are quite good, useful and nice built detectors.

At exponential part of ringdown process, (it is not actually exponential, this is RLC not LR condition and R is usually quite nonlinear) you can actually increase peak power avoiding breakdown and any nonlinearity related to it, dumping resistance is dependent on flyback voltage when breakdown is reached. But, peak power aside, optimal circuit will be one that can get rid of coil energy as soon as possible, and start sampling, what we trying to measure here are millivolts, after hundreds, or even thousands volts of flyback "bang". As far as i realized, name of the game here is speed, not peak power, coil voltage or current, pulse width or frequency,these just have to be optimized in right manner for maximum speed under given (coil) conditions.

Someone correct me please if i misfired something here.

And, i have to make another post related to those pro&con series diodes. I didn't wanted to raise that issue (but i did) and finally i had few hours free to make some measurements again, under different (breakdown or nonbreakdown conditions,diferrent coils etc) and i believe results will be interesting to anyone, and how i get managed to be half -right and half-wrong on it. That is for now, got to work...

(I believe most typical problem for most of us on this forum is typical lack of time to devote to this hobby)

[bklein]

At exponential interval of damping process, we can not increase the power of dissipation because the resistance for critical damping is independent of flyback voltage. That's why breakdown damping is very important for fast TX network.

This may be well accepted fact, but I don't see how it matters. Maybe you guys could explain it a bit more. If I short the coil after TX I would think the dissapation is at highest rate. If the shorting is done right at the coil you could isolate the feedline and its capacitance. You could be left with ringing afterwards that a traditional damping resistor would quell, but I wonder if at that point you could utilize a variable resistance/gain circuit or other means to normalize it. (?)

[mikebg]

This may be well accepted fact, but I don't see how it matters. Maybe you guys could explain it a bit more. If I short the coil after TX I would think the dissapation is at highest rate. If the shorting is done right at the coil you could isolate the feedline and its capacitance. You could be left with ringing afterwards that a traditional damping resistor would quell, but I wonder if at that point you could utilize a variable resistance/gain circuit or other means to normalize it. (?)

Bklein, the correct question for damping process in time domain is:
TO RING OR NOT TO RING?
I think the damping resistor should be designed using frequency domain, because for an amateur radio designer, the term PI means wide band metal detector.
http://www.geotech1.com/forums/showp...5&postcount=24