Coil question Du Jour

[K7SGJ]

I have spent a fair amount of time trying to find an answer to this question. It is regarding the relationship between the diameter of wire, and the inductance of a coil. I know it is a design consideration in the calculation of the inductance of some types of coils and transformers; and I know how to compute the value of L, the Q, and the other parameters of an air wound coil. There are also formulas (formulae) for calculating the inductance of a coil wound on a coil form with an adjustable slug iron core; as long as the permeability is known. All that stuff was basic theory needed to pass 1st phone, Advanced, 8th grade science, etc.

Here is my quandary. Say I have a multi aperture toroidal core. The type used as a wide band RF output or driver transformer, for example. Lets make the core a two holer of say, type 43 material, with three turns of silver coated Teflon wire.

The core chemistry will remain the same, the turns count will remain at three, the test frequency will remain at 20 MHz, and no parameter will change, except for the wire size.

What effect would changing from say a 16 AWG silver plated Teflon wire to an 18 AWG wire, of the same type and style, make on the inductance at the test frequency? It is my contention, that the change in inductance would be negligible. I'm sure it would change slightly, but not enough to greatly change the output level at the test frequency.

I figure, that due to the characteristics of a toroidal core, and the way it affects inductance, which is why they work so well in wide band circuits, that about the only thing that would make a radical change in said inductance or output of the transformer, would be the number of turns. The main consideration for wire size here would be the amount of current that must be safely passed.

About the only thing I can locate, is data on the actual ID, OD, and composition of the multi aperture transformer form itself, and the maximum number of turns one can make with a given size wire, for the given ID. Obviously, if the toroid form was thicker, thinner, of have more or less iron in it, a change of inductance would be expected, just as would more or less turns. But how about changing the wire diameter from 1.0 mm to 1.3 mm?

It appears that in a straight air wound coil, or even one wound straight on a variable slug coil form, the diameter of the wire would have a greater impact on the overall inductance then that of the toroid.

Of course, I could be full of shit, too.

[NQ6U]

My understanding is that, all other variables staying the same, wire gauge does not make a difference in the inductance value. Bigger wire handles more current, of course.

[kb2vxa]

Besides the fact that everyone is full of shit unless cleaned out prior to colonoscopy...

I don't hold a BSEE, but years of working with one taught me that the only characteristic of an inductor that changes with wire diameter is DCR, so the larger the cross sectional area the lower the DCR and the greater the current carrying capacity. At RF the skin effect takes over so we're looking at surface area here. My best guess is changing from say a 16 AWG silver plated Teflon wire to an 18 AWG wire would have little effect, also taking into consideration that silver has higher conductivity than copper. Since conductivity is measured in Siemens, silver has a greater tendency to impregnate the core.

[K7SGJ]

Well, I guess since there are only two players, I'll have to make the questions harder next time, or maybe easier.

You are both right. The diameter of the wire, especially in a toroidal coil, is of negligible consideration when considering inductance value.

The major factors in coil design are the area of the coil, length, permeability of the core, and number of turns. The formula for a standard coil isn't too bad to work, but the properties of a toroidal core make the formula quite complex. Thankfully, there are programs for the more popular toroid cores that will do the math for you. Some of the better calculator programs can be found at Amidon's website. Toroids, because of their properties, are an excellent choice for wideband applications in solid state equipment.

And, as mentioned, there is the skin effect, which becomes more of a concern at higher frequencies. That's why at VHF and UHF frequencies, tubing or very heavy gauge wire was often used prior to stripline design. Higher currents could be run with a lower DC resistance to overcome.

The reason this has become important to me, is that some of the radio equipment I am restoring is solid state, and the original driver and output devices are no longer available. As a result, I have to use available devices that have different characteristics. As a result, I have found that output power is lower and more frequency sensitive. In a rig that should produce 100 watts from 80-10 meters (actually slightly less on 10), the newer devices (in an unmodified rig) will only produce 50-60 watts at around 14 MHz and down to about 20 watts as you approach 80 or 10 meters. Although the newer devices are capable of 150+ watts, they just can't do it without changes to the tuned circuits, drive circuit, and bias supply.

Details to follow, flem at ten.

[kb2vxa]

On that job I mentioned I learned there are a LOT more major design factors than meet the eye, but we're not working with magnetic components for military applications here like I was.

To be frank about it, you're complicating things WAY beyond the real world where those solid state devices are concerned. It just comes down to cross referencing replacements from charts and you'd be surprised that those general replacements cover such a wide range of brands and part numbers. It all comes down to how for example transistors are graded, numbered and branded. They're supplied raw from the factory in 55 gallon fiber drums to a grading house where they pass through a series of grading stations each with a machine that can be operated by a trained monkey (set up by the engineer). At each stage characteristics are measured pass/fail and the pass goes into another drum and the fail goes down the line. When they get to the last one the pass goes into a drum, the fail goes into another and the fails are ground to powder and recycled. Each drum contains transistors with characteristics that meet many different brand name's requirements including general replacement and are branded and numbered accordingly. The bottom line here is where each drum is concerned it doesn't matter what the brands and numbers are, they're all the same. As always I write from knowledge and experience, I was the engineer's aid who calibrated those machines.

You said when you replaced those no longer available devices power outpoot went down by half so obviously you used the wrong replacements. One of my experiences was quite the opposite. One time I made the caress mistake of adjusting a balanced tuner coupling a Drake TR-7 to a 160M sloping V fed with open wire ladder line at full transmitter outpoot forgetting to drop it to minimum and blew the finals. Exact replacements being unavailable I used generics and the outpoot went UP by 20%.

[koØm]

My understanding is that, all other variables staying the same, wire gauge does not make a difference in the inductance value. Bigger wire handles more current, of course.

At one time, I was doing Appliance Repair at a CB shop and, the owner and I just could not agree on Tank Coils in RF amplifiers.

I am left-handed so, I grab the wire in my left hand and wind the coil in a clockwise direction until I had enough turns; my boss said that this was wrong and that I should hold the wire in my right hand and form the coil in a counter-clockwise direction.

I could only used right handed CCW wound coils in amplifier tank circuits that I repaired (of course, I tried coils turned in either direction and it didn't affect anything that I could see); I chalked it up to "Superstition" on his part.

.

[W9JEF]

At one time, I was doing Appliance Repair at a CB shop and, the owner and I just could not agree on Tank Coils in RF amplifiers.

I am left-handed so, I grab the wire in my left hand and wind the coil in a clockwise direction until I had enough turns; my boss said that this was wrong and that I should hold the wire in my right hand and form the coil in a counter-clockwise direction.

I could only used right handed CCW wound coils in amplifier tank circuits that I repaired (of course, I tried coils turned in either direction and it didn't affect anything that I could see); I chalked it up to "Superstition" on his part.

.

The only time the direction in which a coil is wound becomes relevant
is where polarity between two windings is important, like an oscillator coil
where positive feedback is taken off a secondary winding.
A neutralizing link (negative feedback) is another example.

And the larger the conductor diameter, the smaller the inductance.
Here's a calculator: http://ampbooks.com/home/amplifier-calculators/wire-inductance/

[kb2vxa]

"I chalked it up to "Superstition" on his part."
I'd chalk it up to stupidstition, he didn't understand the difference between AC and DC thinking they're together in a band making screeching noises some people confuse with music. With DC the direction of the winding determines the polarity of the magnetic field by the right hand or left hand rule. You might call it the rule of thumb, north or south thumb depends on which hand it's on. As far as phasing the windings is concerned it doesn't matter as long as you don't get the start and finish mixed up. To avoid confusion, many times transformer windings are multifilar.

These are things I'll not forget any time soon, I wound more transformers than I could count, screwed up quite a few too. I mentioned that I worked as a QA/QC test technician and engineering aid, now I'll tell you how it all started. When I applied for the job that's the position they advertized for but instead they stuck me in the winding department. They were about to fire me for fouling things up but since I had nothing to lose I confronted the big boss with the fact I signed on for a test department position, so give me a try at something I understand and FYI am pretty good at. That was the turning point, not only were there no more foul-ups, I streamlined a few things speeding up the operation, ran the department freeing the foreman to disappear most of the day (;->) and developed some tests to detect errors thus saving the company tons of money. Got a couple of fat raises out of it too. OK, enough blowing my own fuse... horn, I refer you to my avatar and shut my gob.