LTspice tapped inductor

[Clive Arthur]

I have an inductor wound on some 22mm plastic pipe, so essentially
air-cored. It's over 120 turns, 700mm long and uses resistance wire.
It's about 12uH.

There are 30 capacitors connected evenly along the coil commoned to a
copper pipe busbar. It simulates a long, peculiar transmission line.

I want to LTspice it. OK, lots of small inductors with some resistance
and the capacitors.

But these small inductors are coupled by virtue of being co-axial and
adjacent and being part of a single larger inductor. A tapped inductor
is surely a transformer, so how would I enumerate the coupling
coefficients, or is this something which can be ignored?

I know I can use an LTRA, but that doesn't simulate the discrete nature
of the capacitance, and I really want to simulate the simulated line.

--
Cheers
Clive

[Anthony William Sloman]

On Monday, August 15, 2022 at 7:57:33 PM UTC+10, Clive Arthur wrote:
> I have an inductor wound on some 22mm plastic pipe, so essentially
> air-cored. It's over 120 turns, 700mm long and uses resistance wire.
> It's about 12uH.
>
> There are 30 capacitors connected evenly along the coil commoned to a
> copper pipe busbar. It simulates a long, peculiar transmission line.
>
> I want to LTspice it. OK, lots of small inductors with some resistance
> and the capacitors.
>
> But these small inductors are coupled by virtue of being co-axial and
> adjacent and being part of a single larger inductor. A tapped inductor
> is surely a transformer, so how would I enumerate the coupling
> coefficients, or is this something which can be ignored?

K L1 L2 ... Ln 0.2

lets you set up a single coupling coefficient (here 0.2) for a collection of inductors. Obviously more remote winding are less closely coupled.

I don't suppose that there's anything stop you doing a series of coupled inductors, say

K1 L1 L2 0.2 K2 L2 L3 0.2 K3 L3 L4 0.2

which wouldn't be entirely right either

> I know I can use an LTRA, but that doesn't simulate the discrete nature
> of the capacitance, and I really want to simulate the simulated line.

My guess would be that the discrete nature of the capacitors won't make a lot of difference for frequencies where the wavelength is longer than a couple of sections.

--
Bill Sloman, Sydney

[jla...@highlandsniptechnology.com]

Why resistance wire? With enough resistance (namely many ns tau per
stage) it becomes a string of RCs, about as ugly a txline as possible.

What's total r ? How big are the caps?

Have you built one? What's the step response like?

What's the application?

[Anthony William Sloman]

On Tuesday, August 16, 2022 at 12:27:37 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Mon, 15 Aug 2022 10:57:25 +0100, Clive Arthur
> <cl...@nowaytoday.co.uk> wrote:
>
> >I have an inductor wound on some 22mm plastic pipe, so essentially
> >air-cored. It's over 120 turns, 700mm long and uses resistance wire.
> >It's about 12uH.
> >
> >There are 30 capacitors connected evenly along the coil commoned to a
> >copper pipe busbar. It simulates a long, peculiar transmission line.
> >
> >I want to LTspice it. OK, lots of small inductors with some resistance
> >and the capacitors.
> >
> >But these small inductors are coupled by virtue of being co-axial and
> >adjacent and being part of a single larger inductor. A tapped inductor
> >is surely a transformer, so how would I enumerate the coupling
> >coefficients, or is this something which can be ignored?
> >
> >I know I can use an LTRA, but that doesn't simulate the discrete nature
> >of the capacitance, and I really want to simulate the simulated line.
>
> Why resistance wire?

That might just be driven by application.

> With enough resistance (namely many ns tau per
> stage) it becomes a string of RCs, about as ugly a txline as possible.

He hasn't specified the resistance, or the capacitances, so the nature of the transmission line is obscure.

> What's total r ? How big are the caps?
>
> Have you built one? What's the step response like?

He says he has got one - maybe he built it. Clearly, measuring the actual step response is difficult for some reason or other so he wants to simulate it

> What's the application?

Always a good question. Clive Arthur has posted here often enough that he should have known that he'd get asked it. He's not clueless newbie.

--
Bill Sloman, Sydney

[Clive Arthur]

Trouble is, as ever, NDAs. I have built a simulator (maybe emulator is
a better word) as described. The R is representative of the real R, as
is the C - totalling 10R and 160uF, The L (12uH) is guestimated from a
reasonable assumption of propagation velocity and length. Yes, it's very
low impedance. It was quite a juggling act to get all the parameters
about right.

It's simply not possible at this stage to test with the Real Thing, so
my emulator will have to do, but I'd also like to Spice the emulator to
speed up a few things. The Real Thing cannot be changed.

So the question is, how to Spice it? Is the mutual inductance between
sections of a long air-cored inductor at all significant? Top signal
frequency 100kHz.

This sort of thing is a weakness of mine, though less so than it was,
which is why I ask.

--
Cheers
Clive

[jla...@highlandsniptechnology.com]

On Mon, 15 Aug 2022 17:10:48 +0100, Clive Arthur

10r and 160 uF is a time constant of 1.6 milliseconds. L/R is around a
microsecond. It's an RC network.

Really 160 uF?

>
>It's simply not possible at this stage to test with the Real Thing, so
>my emulator will have to do, but I'd also like to Spice the emulator to
>speed up a few things. The Real Thing cannot be changed.
>
>So the question is, how to Spice it? Is the mutual inductance between
>sections of a long air-cored inductor at all significant? Top signal
>frequency 100kHz.

Not with a 1.6 ms time constant.

>
>This sort of thing is a weakness of mine, though less so than it was,
>which is why I ask.

You could build a short section and measure it. Fiddle with Spice to
match the measurement. Then you can add sections in Spice.

Is this a high voltage delay line?

[bitrex]

On 8/15/2022 6:43 AM, Anthony William Sloman wrote:
> On Monday, August 15, 2022 at 7:57:33 PM UTC+10, Clive Arthur wrote:
>> I have an inductor wound on some 22mm plastic pipe, so essentially
>> air-cored. It's over 120 turns, 700mm long and uses resistance wire.
>> It's about 12uH.
>>
>> There are 30 capacitors connected evenly along the coil commoned to a
>> copper pipe busbar. It simulates a long, peculiar transmission line.
>>
>> I want to LTspice it. OK, lots of small inductors with some resistance
>> and the capacitors.
>>
>> But these small inductors are coupled by virtue of being co-axial and
>> adjacent and being part of a single larger inductor. A tapped inductor
>> is surely a transformer, so how would I enumerate the coupling
>> coefficients, or is this something which can be ignored?
>
> K L1 L2 ... Ln 0.2
>
> lets you set up a single coupling coefficient (here 0.2) for a collection of inductors. Obviously more remote winding are less closely coupled.
>
> I don't suppose that there's anything stop you doing a series of coupled inductors, say
>
> K1 L1 L2 0.2 K2 L2 L3 0.2 K3 L3 L4 0.2
>
> which wouldn't be entirely right either

Unfortunately LTSpice balks at doing the second and considers that a
"non-physical winding possibility" and wants you to just do it the first way

[bitrex]

Huh, that's weird. Actually it seems to only complain about non-physical
winding for certain values of coupling coefficient when you set it up
that way, if you set it like 0.2 it seems ok but if you try to do say
0.9 it balks

[Anthony William Sloman]

On Tuesday, August 16, 2022 at 2:11:20 AM UTC+10, Clive Arthur wrote:
> On 15/08/2022 16:12, Anthony William Sloman wrote:
> > On Tuesday, August 16, 2022 at 12:27:37 AM UTC+10, jla...@highlandsniptechnology.com wrote:
> >> On Mon, 15 Aug 2022 10:57:25 +0100, Clive Arthur
> >> <cl...@nowaytoday.co.uk> wrote:

<snip>

> >> What's the application?
> >
> > Always a good question. Clive Arthur has posted here often enough that he should have known that he'd get asked it. He's not clueless newbie.
> >
> Trouble is, as ever, NDAs. I have built a simulator (maybe emulator is
> a better word) as described. The R is representative of the real R, as
> is the C - totalling 10R and 160uF, The L (12uH) is guestimated from a
> reasonable assumption of propagation velocity and length. Yes, it's very
> low impedance. It was quite a juggling act to get all the parameters
> about right.
>
> It's simply not possible at this stage to test with the Real Thing, so
> my emulator will have to do, but I'd also like to Spice the emulator to
> speed up a few things. The Real Thing cannot be changed.
>
> So the question is, how to Spice it? Is the mutual inductance between
> sections of a long air-cored inductor at all significant? Top signal
> frequency 100kHz.

https://www.amazon.com/Inductance-Calculations-Dover-Electrical-Engineering/dp/0486474402

should let you work it out . I've even got a copy.

Chapter 16 - single layer coils on cylindrical winding forms - seems to be what you want. It goes from page 142 to page 162.
I could scan them and e-mail you the images. Making sense of the content isn't easy.

> This sort of thing is a weakness of mine, though less so than it was, which is why I ask.

Resistance is futile, but at least it is calculable.

--
Bill Sloman, Sydney

[Clive Arthur]

I wonder if that's because, say, L8 has 0.9 coupling to L7 which has 0.9
to L6 etc, so L8 has 0.9 to L7 plus 0.9 x 0.9 to L6 (etc) which is >1 ?
In which case, 0.5 would be the absolute max for a large number of
inductors?

So I tried it (LTspice) with 5 inductors and 4 couplings, all equal.
K = 0.58 fails, K = 0.57 works, and that's what passes for solid proof
round these parts. I think "Clive's Constant" has a certain ring to it.

That could be a clue, but like I said, not really my area.

--
Cheers
Clive

[bitrex]

Ya I thought the same thing at first but also found the > 1 hypothesis
wasn't the reason.

"Clive's Constant" works for me! 0.57 is probably large enough to
accommodate adjacent tapped windings on an air coil

[bitrex]

Er excuse me, I misunderstood your post at first. I had originally
thought they had to straight sum to 1 but you've done the math correctly
here, and 0.5 is the max in the _limit_ of infinite taps.

[Clive Arthur]

On 16/08/2022 06:01, Anthony William Sloman wrote:

<snip>

> https://www.amazon.com/Inductance-Calculations-Dover-Electrical-Engineering/dp/0486474402
>
> should let you work it out . I've even got a copy.
>
> Chapter 16 - single layer coils on cylindrical winding forms - seems to be what you want. It goes from page 142 to page 162.
> I could scan them and e-mail you the images. Making sense of the content isn't easy.
>
>> This sort of thing is a weakness of mine, though less so than it was, which is why I ask.
>
> Resistance is futile, but at least it is calculable.
>

Thanks, Bill.

I think with your original suggestion of multiple two-part K factors
using a common parameterised K coupled with Bitrex's observation about
how these interact and John's pushing for more information I stand a
good chance of getting somewhere. With luck, I should be able to adjust
K to make the LTspice response look like my emulator.

If it works it'll save a lot of time. However, if it eventually turns
out that the Real Thing is substantially different from the emulator,
well, back to the drawing board.

And John, yes it is a delay line, though that's not its purpose.
However, I do need to replicate the delay.

--
Cheers
Clive

[Jeroen Belleman]

Larry Benko measured coupling coefficients for a number of configurations.
See his web page <http://www.w0qe.com/Technical_Topics/coupling_between_coils.html>.

Jeroen Belleman

[Jeroen Belleman]

It's a pulse forming network? Radar? Lasers? Electrical weaponry?

Jeroen Belleman

[Clive Arthur]

Pragmatic approach...

Originally I used a web based air-cored coil calculator to design my
coil, and it measured pretty close IIRC.

Just now, I used the same calculator to see what inductance half of my
coil would be, that is, half the length and half the number of turns.
It turns out that half the coil is only a couple of percent under half
the inductance of the full coil, in other words, bugger all coupling.

(Of course, with perfect coupling, twice the turns would give 4 x the
inductance.)

So assuming the calculator is right, I probably don't need to bother
with coupling for my LTspice model, discrete inductors will do. That
saves a lot of typing, or copying and editing.

--
Cheers
Clive

[legg]

Link?

RL

[Clive Arthur]

On 17/08/2022 16:24, legg wrote:
> On Tue, 16 Aug 2022 17:35:39 +0100, Clive Arthur

<snipped>

>>
>> Pragmatic approach...
>>
>> Originally I used a web based air-cored coil calculator to design my
>> coil, and it measured pretty close IIRC.
>>
>> Just now, I used the same calculator to see what inductance half of my
>> coil would be, that is, half the length and half the number of turns.
>> It turns out that half the coil is only a couple of percent under half
>> the inductance of the full coil, in other words, bugger all coupling.
>>
>> (Of course, with perfect coupling, twice the turns would give 4 x the
>> inductance.)
>>
>> So assuming the calculator is right, I probably don't need to bother
>> with coupling for my LTspice model, discrete inductors will do. That
>> saves a lot of typing, or copying and editing.
>
> Link?
>
> RL

https://m0ukd.com/calculators/air-cored-inductor-calculator/

As I said, it seemed to give the right result when I measured the
original coil, and thinking about it, these radio amateur guys have been
doing this sort of thing for a good while.

--
Cheers
Clive

[Chris Jones]

If you have the patience to create a 3d model of the inductors, you can
simulate the coupling coefficients using FastHenry. It is open source.
There is a model viewer and updated versions of fasthenry at
fastfieldsolvers.com