A Revised Inductance Calculator

it

the

==================================

Gary wrote -

=================================

Have you ever used a 12" ruler to measure the 'length' of a coil?

Wind a single layer solenoid on a cylindrical former about 1.5" in diameter.

Wind on exactly 10 spaced turns of 0.1" diameter wire.

This is a very common sort of coil used in HF radio transmitters.

Place the measuring rule along the surface of the coil in the direction of the coil's axis.

Between which two points near the ends of the coil do you measure its length?

Rotate the coil 180 degrees on its axis and you will find you can measure a different length.

Or you can define the length of the coil as being the number of turns times the winding pitch.

Which of these lengths do you insert in your inductance calculating program?

They can give you 3 inductance values differing by several percent.

As the number of turns decreases the differences increase.

As someone else has pointed out, when there is only one turn the answer can be silly.

So length must be precisely defined. Furthermore the definition must apply to any number of turns, perhaps excluding a single turn which can be a special case. Single-turn coils are frequently used as radio antennas.

The above problems are solved in program SOLNOID3 which can be downloaded in a few seconds from website below and run immediately. Or it can be run without downloading.

SOLNOID3, besides inductance, calculates several other useful properties of single layer solenoids from AF to UHF.

--
=======================
Regards from Reg, G4FGQ
 Click to see the full signature
Reply to
Reg Edwards
Loading thread data ...

A lot of folk define it as the winding pitch times the number of turns. Some have even defined it as the winding pitch times (the number of turns plus one). It's not so different from the diameter problem (for a single-layer uniform solenoid coil): is it inside diameter, diameter to wire centers, or outside diameter? I don't much care how someone defines the dimensions, so long as they are clear about it.

Cheers, Tom

Reply to
Tom Bruhns

==============================

Indeed, if you define it as such, then just as simply stated - if you insert coil length = wire diameter in your program you will obtain the wrong answer.

The question arises - how will you know you have the wrong answer? And is it too big or too small?

Sorry to be so pessimistic. It is suggested you investigate the proportions of coil length, coil diameter,wire diameter and wire spacing such that the formulae you use in your program become inaccurate.

On the other hand - unless you are designing coils intended for International Standards of Inductance, an absolute accuracy of a few percent is quite adequate for ordinary engineering equipment construction purposes.

But by far the easiest and fastest way of constructing most inductors is to wind some turns on a some sort of former and measure it. If it's too big then just look up at the ceiling for a few seconds and do a little unwinding.

Incidentally, standards of reactance are defined by precision coaxial capacitors whose values can be calculated as accurately as their few dimensions can be measured. Parts in a million. Standards are maintained (or used to be) in temperature-controlled, de-humidified enclosures. Frequency is involved but that's a trivial matter of counting against a clock.

--
Reg.
Reply to
Reg Edwards

i agree with you 100%. you have to define length. and 1 turn coils? ...

i have an email to the OP in queue. i WAS going to ask if he had any pointers on calculating mutual inductance from known core permeability and coil geometry (some Neumann double line integral application that i can't find on the web), but since he doesn't know how hard it would be to calculate Q, and doesn't understand why I asked for a length definition ...

regards, mike

Reply to
Active8

Tom sez -

=============================== Tom, I couldn't agree more.

It is our great misfortune the trouble with lengths of coils is that the Grand Master Grover himself, failed to make himself clear. He doesn't even mention the problem. Neither do Nagaoka, Wheeler, et al. But I confess to have read only a small fraction of their works.

In my opinion, after trying to fathom out how their minds worked, they did this quite deliberately.

That excellent plagiarist Terman, whose early edition I have read, undoubtably noticed their omissions but had nothing to add.

The end-effects of a coil and a fractional number of turns, to them, although significant when there are only a very few turns, were incalculable, and still are. As a consequence they were unable to define coil length. If coil length is indeterminate then so is inductance and none of their formulae can be said to be 'exact'.

In my program SOLNOID3 I restrict myself to whole numbers of turns. The inductance of one isolated complete turn is known exactly. So also is the inductance of the small number of turns above which the end effects become insignificant. It is all to do with how the mutual inductance between non-adjacent turns affects overall inductance.

So by mathematical sleight-of-hand I merged the two 'exact' formulae into one formula which handles any number of turns from one upwards. The resulting corrections are significant only for 2, 3 or perhaps 4 turns. But the ONLY reason I merged the formulae into one was to make life easy for program users. So far as they are aware there is just one magic all-embracing formula which works for any number of turns.

Coils of two or three turns are likely to be used only by radio amateurs who insist on making magloops out of them because they mistakenly imagine they must be more efficient than the usual single turn versions using the same amount of copper.

The whole of this discussion is a storm in a tea cup. Everybody ignores the lengths of a coil's connecting wires and their mutual inductive coupling with the coil itself. It's equivalent to a fractional increase in the number of turns. We should all be grateful for the existence of variable tuning capacitors.

SOLNOID3 also calculates the inductance of long coils of widely-spaced turns, better described as coarse helicals, which if stretched out enough become straight lengths of wire. You will notice the calculated self-resonant frequency of a coil of wire streched out straight is the same as that of a half-wave dipole of the same wire length. Clever program, eh? Free to USA citizens. ;o)

---- ======================= Regards from Reg, G4FGQ For Free Radio Design Software go to

formatting link
=======================

Reply to
Reg Edwards

i agree with you 100%. you have to define length. and 1 turn coils? ...

i have an email to the OP in queue. i WAS going to ask if he had any pointers on calculating mutual inductance from known core permeability and coil geometry (some Neumann double line integral application that i can't find on the web), but since he doesn't know how hard it would be to calculate Q, and doesn't understand why I asked for a length definition ...

maybe twocoils will do. i emailed some P haitch D re MANDK which does it for Tesla coils. his doc says he won't wrap in in a GUI because a winders F77 compiler cost $500. i offered to wrap it up in a GUI in exchange for help using the Neumann integral, etc. we'll see.

regards, mike

Reply to
Active8

good for you. why mention it if you can't give it away. the fact adds nothing to this discussion.

Reply to
Active8
[snip]

my understanding is that the length and diameter of Hi-Q coils are equal. my interest in the one turn coil stems from a program from distinti.com. the documentation shows a Faraday triple integral and their double integral (not Neumann's - it's from their IEL or Inertial Electrical Law), the answers of which (for M) disagree slightly, but their method works out the same as the M calculation in MANDK which i mentioned in my other reply to you.

anyway, the distinti program calculates M for a single turn coil which can then be scaled by multiplying by N1 and N2 for comparison to measured data from multi-turn coils (don't ask why they didn't just write that in to the program), or so they'd have me beleive. i haven't had the time to evaluate all this and i have no measurements of real coils available.

i'm currently digging though my old physics text to try to put it all together and may be close to a double integral solution - not sure. a double integral would require less computing time than the Fareaday method. I'm inscenced that the calculation of M isn't included in my physics text, just

v1 = M(di2/dt) ... you have to measure it.

i am willing to wrap GUIs around DOS apps in exchange for help. i have a nice platform independant GUI framework in C++.

regards, mike

Reply to
Active8

Um, perhaps not. It seems to no longer be offered. You could try the contact info on

formatting link
and see if he still sells it.

Probably neither...

Cheers, Tom

Reply to
Tom Bruhns

In article , snipped-for-privacy@hotmail.com mentioned...

From what I've read, maybe here, a one inch piece of wire has about 10 nanohenrys. So if it goes down to that little, just put a lower limit on the value that can be entered, and do a fair calculation of anything above it.

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS?   Check HERE First:###
 Click to see the full signature
Reply to
Watson A.Name - 'Watt Sun'

In article , snipped-for-privacy@hotmail.com mentioned...

The absolute maximum Q at a freq is the reactance of the coil at that freq divided by the DC resistance. But the Q will always be lower than that because the actual AC resistance will be higher because of skin effect, when much of the inner copper wire has no current. So you could do a simple calc based on reactance / DC res and say that the max Q is that value, but expect less due to skin effect.

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS?   Check HERE First:###
 Click to see the full signature
Reply to
Watson A.Name - 'Watt Sun'

Does it do flat spirals?

Mark L. Fergerson

Reply to
Mark Fergerson

I do know how hard it is to measue Q, I just didn't have a formula handy as I was writing.

As to a single turn coil with a pitch, what is the point of it? The purpose of Pitch is to reduce inter-wire capacitance and raise the Q. This is almost non-existant on a single turn coil with even no pitch. Wire leads and parts close to the coil will be more of a problem. And No Formula can account for those errors.

Length of a coil simply put is the parallel distance, end to end. (Coil only, no leads)

Between all the e-mails I have recieved, there isn't much time to reply here, and currently there is a Forest Fire, Guessing within about 2 miles of my town. Can't see it for the surrounding mountains and currently were down wind from it, but I have posted pictures on my site of the Hellicopters hauling water to fight the fire. There return time is about every 3 minutes.

So I'm busy with this.

Take care......Gary

Reply to
Gary Lecomte

I think this is the discussion thread he's referring to:

formatting link
*%2Binsubject:wheeler%26hl%3Den%26lr%3D%26ie%3DUTF-8%26as_drrb%3Db%26as_mind%3D12%26as_minm%3D5%26as_miny%3D1981%26as_maxd%3D1%26as_maxm%3D8%26as_maxy%3D2002%26selm%3D60sgmt%25241r6%2540fridge.shore.net%26rnum%3D2

(Cut & paste the pieces if the URL wraps)

Wow - that discussion was 6 years ago? Seems like only a year or two.

Reply to
pat.lawler

if you've got Xc, you have freq and can calculate skin depth. do a resistivity calc and you have ac resistance, no?

mike

Reply to
Active8

ACMI or MADNK do, IIRC. read my posts for the links.

mike

Reply to
Active8

Not quite. The current distribution in a solenoid coil is not uniform due to proximity effects. (Terman describes this well in Radio Engineering.) The current is forced to flow in a smaller cross sectional area than predicted by a simple skin depth calculation. Thus, the proximity effect increases the loss over that predicted by the simply calculating the increased resistance due to skin effect, as the standard skin effect formula assumes the conductor is in free space.

There are, of course, other loss factors that are usually considered to be part of a single "loss resistance" when one measures the Q of an inductor. For example, if the inductor is wound on a former, there will be some loss in the coil form. If the coil is not some distance from nearby conductors, there will be some induced eddy current loss.

The Radiotron Designer's Handbook (4th ed) has some graphical calculation procedures, as well as some emperical equations for estimating coil Q. I've found these to be reasonable estimations, in the +/- 10% range.

Jack

Reply to
Jack Smith

On Thu, 31 Jul 2003 19:42:32 +0100, Fred Abse Gave us:

No. You don't get that feeling unless you fart in public, and are noticed as the culprit. Silent and deadly doesn't count unless you are stupid and admit to it.

Reply to
DarkMatter

On Fri, 01 Aug 2003 03:18:51 GMT, Active8 Gave us:

It is simple. The ideal turn is one with a zero diameter, and inductance is at maximum for that turn. Such items are not really feasible.

A quarter inch diameter coil of any given size wire will yield more inductance than a 1/2 inch diameter coil of the same wire. Not because the wire length differs, but because the coil turn diameter differs. The only difference the wire length causes is the ESR value, which does affect other inductor parameters. Frequency is also a factor. An inductor will not read the same at 100kHz as it will when tested at 1kHz or 10kHz.

Reply to
DarkMatter

On Fri, 01 Aug 2003 03:27:47 GMT, Active8 Gave us:

Piss and moan a little, why doncha?

You were smart, up until that moment.

Sheesh, just get online at any wire manufacturer, and use their online calculators. They are much more comprehensive, and have compensations built in, I'm sure.

Reply to
DarkMatter

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.