Mosfet Selection / Damping Resistor

[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.

[bklein]

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

Barry

[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).

[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.

[Aziz]

Here is the capacitance vs Vds (IRF740):

[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

[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.

[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).

[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

[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.

[hobbes_lives]

I believe the diode I used was a 1N4937, and I believe my damping resistor is somewhere around 470 ohms. I'll have to double check these values later, as this is all from memory at the moment.

The numbers on the diode are unreadable, but I'm 99% sure it's a 1N4937. The damping resistor value is actually 750 ohms, so I was way off on that.

[jon-ecm]

So far i can offer a sixpack of beer to anyone who can prove that series diode or active dumping can work faster than normal circuit under normal conditions (put aside uS pulses and sub uS sampling etc, we are not going to build UWB stuff, just normal detectors)

I don't drink but I'll do some tests with my oscilloscope and post pictures when I get home, I can't come to logical conclusion why it wouldn't work.

If results don't add up I'll put a piece of steel in the coil when I test it without a diode. :P j/k

[porkluvr]

I won't enter the fray about a series diode. It seems apparent that there are pros and cons either way. I have a design in the works that generates an interleaved waveform (such as 1:3, 1:4, 1:5, etc.). Two FETs are used, with one FET transmitting the single pulse and the other FET transmitting the multipulses. With two series diodes and two different values of series resistances I can have two different levels of coil current from a single coil. The diodes serve the purpose of isolating the two series resistance from the coil when their associated FET is not active, but that is not exactly related to your previous discussion about series diodes. My point is, limiting coil current with series resistance can enhance flyback speed. (See where I mention the CS6-PI below.)

Something that has been bothering me (how do I put this in words...) is that Teflon wire seems to be a trap.

What is needed for a faster coil is to use stranded wire. Stranded wire (as compared to a single conductor of equivalent gauge) reduces eddy currents as a pulse is transmitted.
Tin plating seems to work better than silver because silver gives higher conductivity between individual strands, causing them to act more like a single conductor.
Teflon insulation is highly desirable because of its low dc.

Ok... we all know these things.

But a huge problem arises because it is next to impossible to find tin plated, stranded, Teflon insulated wire. At the temperatures necessary to apply and cure Teflon insulation, individually tinned strands will fuse together. If you want stranded tin plated wire, you had better look for some other insulation besides Teflon.

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 don't know, just a guess. Eric does not reveal all his cards.

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??)

"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?

I wonder if bifilar wound coils like Moodz has been playing with might be something in the right direction. Or, maybe that would explain why Minelab uses (as I have read) Litz wire in some of their PI coils. Either method seems like drastic action to me but maybe that's what is required.

[hobbes_lives]

Well, I'm already using a series diode in a Surf PI Pro as one of my mods. I can sample down around 10uS, as opposed to 15uS stock. I'm not where I can verify the components right now, but I believe the diode I used was a 1N4937, and I believe my damping resistor is somewhere around 470 ohms. I'll have to double check these values later, as this is all from memory at the moment.

[Tepco]

Damping resistor is of course adjusted to optimum value every time, and this is still ALWAYS slower. Adding some high resistance "bleeder" across D-S terminal, but low enough to discharge Cds between pulses is mandatory. Diode forward drop is too small, no significant pulse width change is needed to compensate. And if pulse width is changed significantly, dumping resistor value must be too. At least in "normal" PI conditions. That is 200-500uH coil, 50-200uS pulse. Maybe this can work whit very short pulses, or very high voltage switches when breakdown voltage is not reached in process, but under normal conditions this is a bad idea. Another bad one is "active dumping", attempt to slow down MOSFET turn-off time in order to dissipate more energy on it. And this is why:

Ideal ringdown condition is pure LRC circuit, adding any nonlinearity to it (breakdown condition) will make it just slower. Adding even more nonlinearity, in form of diode or active dumping will just make things even worst. (I know US patent is given for this, John Hopkins university and active dumping, but will we ever see it in any MD, without being more complex than MD itself, and will it work?)

So far i can offer a sixpack of beer to anyone who can prove that series diode or active dumping can work faster than normal circuit under normal conditions (put aside uS pulses and sub uS sampling etc, we are not going to build UWB stuff, just normal detectors)

OK i will have to buy brewery if i'm not right on this, but this is tried so many times...