New PI technology proposals.

[Andy_F]

I am sorry. Correctly to model so (inductance in the "improved" variant should be in 4 times less, than in classical). But also so anything good is not present.

[Unregistered]

Are you guys including the inevitable capacitance when modeling this? The exercise is rather futile if you aren't.

A capacitance free coil, if it actually existed, wouldn't need a damping resistor. The energy would simply dissipate through it's own resistance.

[Carl-NC]

Andy,

An inductor with half the turns has 1/4 the inductance. Therefore your individual coils should be 75uH. Then, to account for their interaction, you need to make them mutually coupled. Aziz has his schematic set up correctly.

- Carl

[Carl-NC]

Segmented PI Coils

I am breaking this out separately, please see the post in the Coils section.

- Carl

[Aziz]

Hi Andy,

I am sorry. Correctly to model so (inductance in the "improved" variant should be in 4 times less, than in classical). But also so anything good is not present.

the L2 and L3 need its own critical damping. If not done, the anomaly coil current will still be there on the upper coil half (L2). This will make the coil slower.

Aziz

[Stefan]

Aziz,


I have been playing around with your LT Spice simulation program PI Mono4SPL and modified the standard coil arrangement to include the characteristics of a mono coil for a Minelab coil. So I added 0.4 ohms in series with the coil and in parallel a 200pF shield capacitance. Also adjusted RDSYS1 to 0.5 ohms. The results confirmed my assertion that by allowing the coil to achieve max flyback you increase the target response. When I increased the flyback from 160 v to 390 the simulation showed approx 3 fold increase in target response. Clamping was at 390 V. Interesting indeed but it makes sense as you are putting more energy into the coil and the resultant is a higher flyback .The SD2000 by the way clamps at 200V.

A differential input using an instrument amplifier will yield improvements. Although you get rid of the coax cable the general arrangement is a twisted pair shielded instrument cable. This will help with CMR. However an instrument cable will still introduce capacitance effects, about 100pF/m shield to twisted pair. There is a way around this and it is to use a driven shield. Easy to accomplish using an instrument amplifier. The driven shield will minimise capacitance effects from the shield.

Stefan

[Andy_F]

An inductor with half the turns has 1/4 the inductance. Therefore your individual coils should be 75uH.

Indeed on the second scheme.

Then, to account for their interaction, you need to make them mutually coupled.

If we have mono coil, K (coupling) it is approximately equal 1, that is present at my model (K = 1).

the L2 and L3 need its own critical damping. If not done, the anomaly coil current will still be there on the upper coil half (L2). This will make the coil slower.

At K, approximate to 1, both half of coil damping identical, and models are equivalent. "Abnormal currents" and "slow coils" is not from physics area .

[Aziz]

Hi Andy,

just compare two ideal MONO coil decay curve simulations (without any mutual couplings and ideal inductors):
Same conditions, except the system 2 (right side) has only 50 pF coil capacitance instead of 100 pF (left side).

The principle is lowering the effective coil capacitance to increase the effective damping resistor. This will lead to higher flyback voltage.

Remove the .txt from the spice model, to load this into LTspice.

Aziz

[Andy_F]

just compare two ideal MONO coil decay curve simulations (without any mutual couplings and ideal inductors):
Same conditions, except the system 2 (right side) has only 50 pF coil capacitance instead of 100 pF (left side).

It is obvious, that at reduction of parasitic capacity, speed of attenuation increases. It is old, as the world. But at what here "New PI technology" ?

[Aziz]

It is obvious, that at reduction of parasitic capacity, speed of attenuation increases. It is old, as the world. But at what here "New PI technology" ?

The result is obvious: taking benefits of higher flyback voltages without clipping them. Then a higher magnetic field energy can be dissipated into heat. So the difficulty is the breakdown voltage of the MOSFET:
either increasing this by selecting a higher voltage fet and diode or decoupling the higher flyback voltage from these parts.

Aziz

[Stefan]

The result is obvious: taking benefits of higher flyback voltages without clipping them. Then a higher magnetic field energy can be dissipated into heat. So the difficulty is the breakdown voltage of the MOSFET:
either increasing this by selecting a higher voltage fet and diode or decoupling the higher flyback voltage from these parts.

Aziz

Aziz,


Concernining your statement :

"Then a higher magnetic field energy can be dissipated into heat".

All that you are doing is counter to this claim. You don't want to put the flyback energy back into the coil, this will slow down the coil. The ideal arrangement would be to have the damping resistor isolated for the majority of the flyback and then connected at a predetermined time prior to the flyback voltage going to zero to damp the coil. By doing this you generate high flyback voltage potentals but no current through the coil. This will give a very fast collapsing magnetic field to excite the target and earlier sampling.

Regards,

Stefan

[Aziz]

Hi Stefan,

you forget, we have two energy conserving elements: coils inductance (resulting in magnetic field energy) and coils total capacitance (+parasitic, resulting in charge voltage).

A current must flow through these energy storing elements. There is no need for switching the Rd and it minimizes additional noises. Switching on a high voltage isn't so trivial.

Important is: the stored energy should be converted in resistive elements to heat: damping resistor, coils resistor, ..

It is more efficient, if you can achieve a higher current through these resistive elements during the flyback period (P=I*I*R). As the damping resistor can not be changed arbitrary (for critical damping), one approach is to increase the voltage on these resistive elements (P = U*U/R). The latter one is more efficient. All the idea is based on increasing the flyback voltage.

Aziz

[Stefan]

Hi Aziz,

Good discusion but would like to point out a few points getting back to basics.

The voltage time constant for a simple RC circuit is τ=RC. Increase the resistance increase the time constant.

The current time constant for a simple LR circuit is τ=L/R. Increase the resistance reduce the time constant.

So ignoring for the moment the interwinding capacitance effects of the coil and shield capacitance a shorter time constant is achieved by having a high loading resistance. This limits the amount of flyback current flowing through the coil. Try not to allow the MOSFET to clamp as this is forcing more flyback current through the coil and extending time constant.

Once we introduce interwinding capacitance and shield capacitance the model becomes an LCR circuit and will tend to oscillate. So need a dampening resistor to give an overdamped response. The key is to minimise the interwinding and shield capacitance and this will allow a higher damping resistor and a lower time constant because we are limiting the flyback current through the coil. To maximise power conversion in the coil we need low current high voltage.

Regards,

Stefan

[1843]

Hi Aziz,

I don't think the drain diode must be fast... a 1N400X can be used since never a reverse current flows through it.


Regards,
1843

[Qiaozhi]

Aziz,

I've built & tested the CT coil, plus an otherwise identical mono coil. Specs are:

Mono:

• 20 turns of 24awg
• 290mm diameter
• 299uH / 4 ohms

CT:

• 10 + 10 turns of 24awg
• 290mm diameter
• 77uH / 1.1 ohms and 76uH / 1.1 ohms

Hi Carl,

There appears to be a small error here.

The lower inductance values are understandable, as the relationship between inductance and number of turns is not linear, but the resistance values must be wrong.

In the first example the 20 turns of 24AWG wire had a total resistance of 4 ohms, but the second example (which is essentially the same, just with a center-tap) only totals 2.2 ohms.