Some tests on mono and bifilar coils

[conradelektro]

Coil formers are ready, see attached photo.

The aim is to wind ~ 200 mH coils.

In order to be able to count the windings (to have the same number on both coils) I need paper between each layer (which gives me a smooth surface to lay the next layer upon, also makes it easier to see if the wires are close together). The whole point is to have identical coils (besides monofilar and bifilar).

I guess this will increase self capacitance (increased space between wires)?

Greetings, Conrad

[gyulasun]

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I guess this will increase self capacitance (increased space between wires)?
...

Hi Conrad,

Normally, the increased space i.e. distance between two conductors reduces capacitance. However, in case the dielectric constant of the insulating layer (you wish to use to fill the space) is significantly higher than that of the air, then it can compensate for the decrease or even increase capacitance.  Somewhere you can find a tabelle on the net for dielectric constants of insulators like paper etc.

Gyula

[conradelektro]

@Gyula:

This table says that paper has dielctric constant of 3 (air 1) http://members.gcronline.com/cbrauda/0007.htm.

The plastic sheets I have are similar.

It is probably hard to say whether the paper between layers will distroy some effect one would want to detect? It sure destroys the purpose of the test if the two coils are largely dissimilar. Therefore I like the paper because it insures "same number of turns".

I wound quite a lot of coils and paper (or plastic sheet) between layers are a big help. Without a separator between layers one needs thick wire not to mess up. But with thick wire the inductance goes down at the same size.

I got the idea of a seperator sheet between layers of windings when taking wire off old transformers to get the core. Although the wire was quite thick, there were seperator sheets. May be to absorb heat but I thought it was to keep the windings neatly spaced.

Greetings, Conrad

[Farmhand]

I usually use home made bees wax paper between layers. Just getting the wires next to each other and lined up in neat layers will get a good result, you could do the same with the mono filar coil. I melt the wax in a non melt-able container then just soak a sheet or two at a time of normal typing paper in the hot wax till the paper soaks it up then the sheets will be totally saturated with wax. Just pick up with tweezers or similar and and let drip till it solidifies. Then I cut the wax paper so that it fits neat in the spool and makes one complete turn with a bit of overlap, each layer will need to be a bit longer so some planned cutting of the sheets into the longest possible strips, two pieces can be overlapped to complete a layer of paper as well. Might help save paper. Using home made paper means the wax paper will stick to itself very well and will be a superior insulator than store bought wax paper. Good home made bees wax paper also will hold the wire for you a fair bit by sticking it to the paper. You can also make the wax thick or thin as you desire. Bees wax is great for experimenting. a lump of bees wax will hold wires in place for some time rather than tape ect.

If you get the paper too hot it can make it a bit brittle, but normal melted wax (not real hot) usually produces a good result.

It should not make too much difference really as the paper gets molded into a wavy shape and the winding still fit together kind of thing. But as Gyula says it may because of the reasons Gyula gave.

Here's another table, paper alone is 1 to 3 depending on things and bees wax is 2.4 to 2.8. much more consistent. Fully saturated bees wax paper I would say 3. So your assessment would be spot on Conrad. 
http://www.csgnetwork.com/dieconstantstable.html

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Cheers

P.S. I actually made a video to show how I use the paper, and wind the coils with a battery drill.    http://www.youtube.com/watch?v=OCHdzdP5_28

If doing a bifilar just use two spools to feed and the fingers keep them next to each other. A rev counter on the drill would be handy. That can be done in several ways.

Just counting or calculating the average turns per layer (by measurement) and also the number of layers gives a close figure on turns.

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[MileHigh]

Conrad:

Let me make an ascii schematic of the coil.  I am going to assume that you are going to use a wire pair to wrap the bifilar coil - i.e. the source wire has two conductors.  So the way you will make the series bifilar coil is to connect the end of the 'left' wire to the beginning of the 'right' wire in the pair.

So we have the start, middle and end contacts.  Let's call them TP1, TP2, and TP3.

Then if you make a loop that is half way, you have two more contacts, let's call them LP1 and LP2.

Let's pretend the wire (pair) is 100 turns.

        <Turn1>                                   <Turn 100>
        <inner spool>      <middle>    <outer spool>
TP1--WWWWWWWWWW(LP1)WWWWWWWWWW--TP2
TP2--WWWWWWWWWW(LP2)WWWWWWWWWW--TP3
        <Turn 101>                              <Turn 200>


So, you notice that the TP1-TP2 voltage and the TP2-TP3 voltages already have 100 turns of the bifilar coil "distance" between each other.  So in theory you don't have to have LP1 and LP2 to see the "full bifilar separation."  The addition of the loop has limited value because there are other test points that do the same thing.  However, this is an exploratory mission, and they can't hurt, so why not add them?  (Note you only need one hole per spool.)

LP1-LP2 (turn 50 and turn 150) also have 100 turns "distance" but I am doubtful that you will see much difference between the loop sensing points and TP1-TP2 or TP2-TP3.

I suppose the question is will there be anything interesting when you look at the potential difference between the various test points.

Now let's assume that you are going to make a monofilar coil of 200 turns with the equivalent one-conductor wire.

So you have this:

        <Turn 1>                                  <Turn 200>
TP1--WWWWWWWWWW(TP2)WWWWWWWWWW--TP3

So you can see that you have a monofilar equivalent here.  There are 100 turns of "distance" between TP1-TP2 and also TP2-TP3.

In most cases you probably will see almost the same waveforms for both coils when you make voltage differential measurements that are 100 turns apart from each other.  It doesn't mean that there isn't any extra energy stored in the "capacitance" of the bifilar.  I used quotations just as a reminder that this capacitance is normally not active or visible at lower frequency ranges.  A capacitor with what can become a dead short across it's "plates" after a short amount of time is a strange breed of capacitor.

It's very important to mention a familiar theme:  If you are playing with the coils in their self-resonant mode, there may be observable differences but the whole operation at that frequency is out of the practical usable frequency range for the coil.  Is there any practical application for a bifilar (or monofilar) coil in self resonance?  Can you do anything with it?  I know it's the question that some people hate, but it's a perfectly valid question.

MileHigh