New PI technology proposals.

[simonbaker]

Aziz,

Just a quick note regarding the high flyback voltage you develop using the center tapped coil arrangement. As I see it, the driven 75uH coil is clamped by the avalanche effect of the MOSFET to about 400 volts. The other 75 uH coil as it is connected in series and bifilar has an effective inductance of 225 uH ( total both coils of 300 uH ) is unclamped other than the loading resistor so this part of the coil will develop higher voltages. This also means that the decay curve will be a lot less as the majority of the coil is unclamped. You can prove or disprove this on the standard mono driven circuit by using a MOSFET which has an avalanche of 800 volts or more.

So your circuit is showing that it is the unclamped coil that is giving the gains in reduction of delay.

Stefan

Does MOSFET ever go into breakdown? I thought that was avoided by design.

-SB

[simonbaker]

Hi friends,

you may play around with different simulation conditions with the enclosed simple simulation model for MONO coils. The model consists of 4 independend simulation experiments for comparing two systems:
System 1: Normal coil (left section), System 2: Splitted coil (right section)
with target (top section), without target (bottom section).

You should change only the .param commands to change the parts values. Otherwise, you would have 4 times more work.

Normally, I have 10nT, k=0.01 target parameter to simulate small targets. In the example below, the target modelled is exaggerated to see more differences.

Please see how to use LTspice in the documentation of LTspice.

The simulation file is a .rar file containing all necessary files (remove .zip from the file name before extracting).

Aziz

Thank you Aziz!

Interesting result is the Voltage-time-product (see pictures above):
The integral U(t)*dt (t->infinite for one pulse) for both damping voltages seems to be equal, in which it represents probably the exposed magnetic energy per ampere.

What is U(t)? If same as V(t), wouldn't we expect integral V**2(t) to be equal, since V**2/R is power?

Also, doesn't that confirm a point that I think you said earlier -- higher voltage dissipates faster -- the squaring really helps. Basically we want "flyback voltage" to be really high because that means mag field is collapsing really fast. Someone mentioned ignition circuit -- a spark gap would probably be great if it didn't ring (or destroy anything; or disturb whales).

I'm sure that is old news. Idea of distributed resistors in coil is main topic, and question is: what is theoretical max improvement possible?

I'll be following developments...

Cheers,

-SB

[Stefan]

Aziz,

Just a quick note regarding the high flyback voltage you develop using the center tapped coil arrangement. As I see it, the driven 75uH coil is clamped by the avalanche effect of the MOSFET to about 400 volts. The other 75 uH coil as it is connected in series and bifilar has an effective inductance of 225 uH ( total both coils of 300 uH ) is unclamped other than the loading resistor so this part of the coil will develop higher voltages. This also means that the decay curve will be a lot less as the majority of the coil is unclamped. You can prove or disprove this on the standard mono driven circuit by using a MOSFET which has an avalanche of 800 volts or more.

So your circuit is showing that it is the unclamped coil that is giving the gains in reduction of delay.

Stefan

[Aziz]

Hi friends,

you may play around with different simulation conditions with the enclosed simple simulation model for MONO coils. The model consists of 4 independend simulation experiments for comparing two systems:
System 1: Normal coil (left section), System 2: Splitted coil (right section)
with target (top section), without target (bottom section).

You should change only the .param commands to change the parts values. Otherwise, you would have 4 times more work.

Normally, I have 10nT, k=0.01 target parameter to simulate small targets. In the example below, the target modelled is exaggerated to see more differences.

Please see how to use LTspice in the documentation of LTspice.

The simulation file is a .rar file containing all necessary files (remove .zip from the file name before extracting).

Aziz

[KingJL]

I'd think the graphite would have to be highly conductive for shielding, but less conductive for the inter-winding resistance. That balance may be hard to accomplish, but perhaps possible.

That was with tongue-in-cheek...so to speak!

[simonbaker]

Spice files

Hi Aziz:

Sorry to keep bothering you.

Can you post the latest ".asc" LTSpice file that you are using (and any special components needed).

I have trouble understanding some details from the pictures.

Thanks!

-SB

[hobbes_lives]

If flyback can be damped faster with lots of resistors in coil, why not make a coil with flat spiral windings, use wire with no insulation, embedded in a fairly conductive layer of graphite? Infinite number of resistors!

That's an interesting idea. Perhaps use an etched PCB coil painted with graphite?

Yeah, and if done very carefully, you could accomplish shielding at the same time! Two birds with one stone... so to speak!!!
J. L. King

I'd think the graphite would have to be highly conductive for shielding, but less conductive for the inter-winding resistance. That balance may be hard to accomplish, but perhaps possible.

[KingJL]

Hey Aziz:

If flyback can be damped faster with lots of resistors in coil, why not make a coil with flat spiral windings, use wire with no insulation, embedded in a fairly conductive layer of graphite? Infinite number of resistors!

Maybe already done....

Cheers,

-SB

Yeah, and if done very carefully, you could accomplish shielding at the same time! Two birds with one stone... so to speak!!!
J. L. King

[ZED]

whoops

In post 49 i stated that at the start of the transmit there was no flyback signal,i now believe this to be wrong.With the fast current growth upto 45 amps within 10 u/s there must be a high voltage signal [flyback or flyup ] induced in the receive coils at startup,because the two receive coils are inductively balanced it is not seen in the picture.
Sorry Aziz!

Zed

[Aziz]

Hi Carl,

could improve a twisted-pair cable (with insulation) wound to a coil and then made to a center-tapped coil the differential signal?

Aziz

[Carl-NC]

Carl the two receive coils are rectangular in shape with a ratio of 3 to 1,they are wound opposite to one another and connected in series and placed in a 18" transmit loop,the transmit loop has an inductance of less than 1 u/h and a resistance of less than 10 milli ohm [its half inch copper tubing] .The two receive coils are adjusted within the transmit loop so as to cancle out any effects that result from any coupling with the transmit loop,this can be done with a scope,this results in a flat line during a transmit.As targets are placed at or near a receive coil a signal can clearly be seen.The coil current rises to maximium of 45 amps within 10 u/s,this high slew rate is higher than the slew rate at switch off of many pi detectors.

I think that explains the strong target signal during TX-ON!!

[Aziz]

Hi Zed,

Aziz i couldnt find any archived pics of any use,this anomoly that you speak of, it sounds like the flyback voltage being clipped by the diode in the mosfet,the timing looks about right,what do you think,maybe try a mosfet without a diode.

Zed

Finally, I found the reason for this anomaly:
Coils effective capacitance!

To test this, I have excluded the switching electronics (MOSFET and Diode) and replaced this with an ideal voltage controlled switch (Ron = 1e-12 Ohm, Roff = 1e+12 Ohm). Then I reduced the normal MONO coils capacitance from 300 pF to 30 pF. I have lower anomalies and faster decay then.

The cascaded coil damping method is in this ideal model even better.

Interesting result is the Voltage-time-product (see pictures above):
The integral U(t)*dt (t->infinite for one pulse) for both damping voltages seems to be equal, in which it represents probably the exposed magnetic energy per ampere.

The anomaly effect should be observable in real world. The more the coils capacitance, the better the observations should be.

Aziz

[ZED]

Carl the two receive coils are rectangular in shape with a ratio of 3 to 1,they are wound opposite to one another and connected in series and placed in a 18" transmit loop,the transmit loop has an inductance of less than 1 u/h and a resistance of less than 10 milli ohm [its half inch copper tubing] .The two receive coils are adjusted within the transmit loop so as to cancle out any effects that result from any coupling with the transmit loop,this can be done with a scope,this results in a flat line during a transmit.As targets are placed at or near a receive coil a signal can clearly be seen.The coil current rises to maximium of 45 amps within 10 u/s,this high slew rate is higher than the slew rate at switch off of many pi detectors.
All this was purely experimental,i wasnt attemting to design or build a detector,i was after imformation and data.

Zed

[ZED]

Aziz i couldnt find any archived pics of any use,this anomoly that you speak of, it sounds like the flyback voltage being clipped by the diode in the mosfet,the timing looks about right,what do you think,maybe try a mosfet without a diode.

Zed

[Aziz]

I think, the more coil splits with his each individual coil damping resistors are made, the more coil voltage we will have and a faster damping time. The MOSFET will breakdown anyway and will add some delays as a consequence of this.

It is very remarkable, that despite of the same transmit pulse energy (same coil current, same configuration except the coils) the flyback voltage increases with the splitted coil.

Aziz