New PI technology proposals.

[Aziz]

Hello friends,

can anyone check the self-capacitance on the half coil of the CT coil. I suspect, the humble simplification of Csplit=C/N isn't correct.
As the self-capacitance of the full coil doesn't disappear, it has a capacitive effect to the half driven coil. In this case, there is no free lunch.
Also the splitted coil critical damping wouldn't work in this case.

Aziz

[Aziz]

Hi Qiaozhi,

both systems doesn't expose same magnetic energy due to different current flows of each coils.
System 1:
I1 = U/(Ron+R) = 12/(20+2) = 0.545 A

System 2:
I2 = U/(Ron+R/2) = 12/(20+1) = 0.571 A

As the system 2 drives only the half of the coil, it should drive with 2 time more current to the system, to be compareable.
So I2 should be 0.545*2 A = U/(Ron/2 + R/2) = 12/(10+1) = 1.091 A.
The switch on system 2 needs Ron resistance of 10 Ohms.

Aziz

[Qiaozhi]

Hi Qiaozhi,



L, C, R are correct values for the totals coil parameters. If you want to compare two systems, these values must be equal. If you split the coil, care must be taken. Following formulas can be used for the split coil:

Lsplit = L / (N*N),
Csplit = C / N,
Rsplit = R / N,
where N = number of splits.

The damping resistor is calculated:
Rd = sqrt(L / (4*C) ),
where sqrt = square root, L=coils inductance, C=coils capacitance.
If you split the coil, each section has its own damping resistor Rdsplit. In this case, use the Lsplit and Csplit values. If you have additional capacitances seen by the coil (like the drain diode, Mosfets Coss, coax cable, coil shielding) then these values must be taken into account for the damping resistance.

If you split the coil, the Mosfets and diodes on-resistance have to also taken into account, to achieve same exposed magnetic field energy (E = 0.5*L*I*I) for comparing two systems. In this case, a drain resistance is easy to adjust two systems switch-on currents.


L, C, R may differ from model to model i have published but not in comparing two systems.

Aziz

Hi Aziz,

I've hacked together a SPICE netlist (without a schematic) as it was the quickest approach for this simple circuit. You will see the parameter statements are implementing the equations that you provided. However, I am not getting the same results you claim to be seeing. Please have a look at the netlist below, and see if you can spot any mistake. The split coil has a lower flyback voltage than the mono.

-------------------------------------------------------
* PI Coil Simulation

* Coil parameters
.param L=300u
.param C=100p
.param R=2
.param RD=SQRT(L/(4*C))
.param N=2
.param Lsplit=L/(N*N)
.param Csplit=C/N
.param Rsplit=R/N
.param RDsplit=SQRT(Lsplit/(4*Csplit))

* Mono coil
L1 VP12 1 L
RL1 1 2 R
C1 VP12 2 C
RD1 VP12 2 RD
YSW1 VSWITCH 1 0 3 0 param: RON=20 ROFF=100MEG

* Split Coil
L2 4 5 Lsplit
RL2 5 VP12 Rsplit
C2 4 VP12 Csplit
RD2 4 VP12 RDsplit
L3 VP12 6 Lsplit
RL3 6 7 Rsplit
C3 VP12 7 Csplit
RD3 VP12 7 RDsplit
YSW2 VSWITCH 7 0 8 0 param: RON=20 ROFF=100MEG

* Coil coupling
K23 L2 L3 1

* Voltage sources
VP12 VP12 0 DC 12
V1 3 0 PULSE 0 1 100n 100n 100n 100u 50m
V2 8 0 PULSE 0 1 100n 100n 100n 100u 50m

* Simulator commands
.TRAN 100u 100m
.PROBE ALL
.PLOT V(2,VP12) V(7,4)
.END
-------------------------------------------------------

[Aziz]

Hi Qiaozhi,

Hi Aziz,

The problem is that I'm not using LTspice, so I don't have the inductor model. Examining the asc file with a text editor, tells me that L=300uH, C=100pF and R=2 ohms. I will use those values, unless you tell me otherwise.

L, C, R are correct values for the totals coil parameters. If you want to compare two systems, these values must be equal. If you split the coil, care must be taken. Following formulas can be used for the split coil:

Lsplit = L / (N*N),
Csplit = C / N,
Rsplit = R / N,
where N = number of splits.

The damping resistor is calculated:
Rd = sqrt(L / (4*C) ),
where sqrt = square root, L=coils inductance, C=coils capacitance.
If you split the coil, each section has its own damping resistor Rdsplit. In this case, use the Lsplit and Csplit values. If you have additional capacitances seen by the coil (like the drain diode, Mosfets Coss, coax cable, coil shielding) then these values must be taken into account for the damping resistance.

If you split the coil, the Mosfets and diodes on-resistance have to also taken into account, to achieve same exposed magnetic field energy (E = 0.5*L*I*I) for comparing two systems. In this case, a drain resistance is easy to adjust two systems switch-on currents.


L, C, R may differ from model to model i have published but not in comparing two systems.

Aziz

[Qiaozhi]

Hi Qiaozhi,



the .zip file is a .rar file (rename necessary). It contains 5 files. One of them should have the name TargetResponsebbct4splits.asc. (Just loaded and checked - no missing files there).
I am using the standard inductor model of LTspice IV. Filling then the parameters in the Dialogbox: Inductance, Series Resistance, Parallel Capacitance. This must be a liner inductor model.

Aziz

Hi Aziz,

The problem is that I'm not using LTspice, so I don't have the inductor model. Examining the asc file with a text editor, tells me that L=300uH, C=100pF and R=2 ohms. I will use those values, unless you tell me otherwise.

[Aziz]

Hi friends,

1N4148 diodes seems to accept high voltages without going out of order. I have roughly tested up to 800-1000 V with a series resistor of 6.8k.
Now the magnet wire is not convenient for such applications and found very interesting loadspeaker cables: tin plated stranded wires with insulation. I didn't find a thinner one and will look for the next time. There were a 4 mm^2 and 6mm^2 wires available and of course a little too thick for coils.

Aziz

PS: My test was using a switch with a 12 V power supply firing the coil up to 5 A and switching the switch 100-200 times on and off for different periods. Until I have new coils, I will make tests going into several kVolts.

[Aziz]

Hi Qiaozhi,

Hi Aziz,

The rar file you attached only contains the BYV28 and IRF740 subcircuits. Do you have the subcircuit model for the coil?
You said earlier that you are not using the ideal inductor model. What model are you using?

the .zip file is a .rar file (rename necessary). It contains 5 files. One of them should have the name TargetResponsebbct4splits.asc. (Just loaded and checked - no missing files there).
I am using the standard inductor model of LTspice IV. Filling then the parameters in the Dialogbox: Inductance, Series Resistance, Parallel Capacitance. This must be a liner inductor model.

Aziz

[Qiaozhi]

Hi Qiaozhi,



here it comes.

To compare two different systems with same conditions (same exposed magnetic field energy), the coil current of the system 1 is 300 mA and of system 2 is 600 mA. It is the example of center-tapped coil configuration with splitted PI critical coil damping.
This example is not posted yet. So feel free to play with it.
The enclosed zip file is a .rar file. Remove .zip file extension before unpacking.

Aziz

Hi Aziz,

The rar file you attached only contains the BYV28 and IRF740 subcircuits. Do you have the subcircuit model for the coil?
You said earlier that you are not using the ideal inductor model. What model are you using?

[simonbaker]

analogy

I am wondering how center-tapped, half-driven TX coil is different from analogy with full transformer, like attached image (very simplified circuit)?

Forget about metal detectors -- could such a circuit achieve faster decay because of high voltage secondary? Or is there simply an impedance-matching effect where the secondary circuit resistor looks like R/100 to the primary circuit and circuit behaves same as if just primary with R/100 resistor?

-SB

[simonbaker]

breather summary

For those trying to follow this thread:

Is the following summary correct?

A) There are two main concepts being debated in this thread:

1. Whether distributed dissipation resistors (divide coil into sections and put parallel resistor across each section) in the TX coil cause faster decay of current and thus magnetic field during switch-off than a single resistor across entire coil.

2. Whether a center-tapped coil can be driven on one half but with damping resistors on both halves such that a higher total "flyback" voltage is achieved, which leads to faster damping of the coil current and magnetic field. Avoiding MOSFET breakdown is part of the goal (or maybe all of the goal).

3. A third side-issue is that center-tapped coil allows "differential" receive circuit which reduces noise and thus offers that benefit.

B) Carl did an experiment posted in the Coils section that "debunked" concept 1 -- dividing coil into four sections with separate resistors did not achieve faster decay. Therefore we are no longer discussing that design. Or is it still in play?

C) Aziz found errors in original assumptions about using center-tapped coil in concept 2, but it seems there is still some investigation that a center-tapped coil will allow higher "flyback" voltage and thus faster decay. There is a question as to whether effects of capacitance are being considered correctly.

Is this where we are? If not, where are we?

Regards,

-SB

[UWLocator]

3.6 ohms... but now I see why. I'm using a BK 878 RLC meter and the R setting apparently uses an AC signal as well, so inductance plays a role. It has settings for 120Hz and 1kHz, and when I switch to 120Hz the "resistance" drops to 1.5 ohms, which is about what my regular multimeter says.

- Carl

That is why I still like the old Simpson 260. I know it uses DC only for resistance. I find even the cheap digital meters have a pulsed signal.

It also is much easier to see current swings on the needle.

Rip

[Carl-NC]

Does it show 1.68 ohms across the full coil on the 4-tap?

3.6 ohms... but now I see why. I'm using a BK 878 RLC meter and the R setting apparently uses an AC signal as well, so inductance plays a role. It has settings for 120Hz and 1kHz, and when I switch to 120Hz the "resistance" drops to 1.5 ohms, which is about what my regular multimeter says.

- Carl

[Qiaozhi]

Does it show 1.68 ohms across the full coil on the 4-tap?

Rip

Good question.
I would expect it to measure the same as the untapped coil - same inductance, and same DC resistance.
Maybe Carl can recheck the measurements.

[UWLocator]

Yes, I noticed that. And my 4-tap coil has 5T coils with 0.42 ohms, for a total of 1.68 ohms. Don't know why.

- Carl

Does it show 1.68 ohms across the full coil on the 4-tap?

Rip

[Carl-NC]

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.

Yes, I noticed that. And my 4-tap coil has 5T coils with 0.42 ohms, for a total of 1.68 ohms. Don't know why.

- Carl