This is the very heart of the coil and one of the most prominent visual aspects of it, so it's important to have it looking good, as well as being functional.




The setup that I used for winding and varnishing my first coil.


You can rotate the coil by hand and wind at the same time but it can be quiet tedious and at times difficult. This is because you need to apply [light to moderate] tension as you wind and also to ensure that each turn of wire is snug to its previous turn, as shown in the picture further down this page. The reason being is that this ensures that the electromagnetic lines of force from the primary will cut a maximum number of turns of wire in the secondary.

The magnet wire that I used was AWG 27, 0.355 mm diameter or 14 thousandths of an inch, which is about as small as I personally would wish to go. I found the handling fiddley at this size, and I needed a magnifier to make sure the windings were staying close together, but some people are happy to go smaller than this if the coil demands it.

I am lucky enough to have a lathe that has a variable frequency drive, allowing any speed I wish. So I engaged the lathe's back-gear and wound it at about 15 rpm. I also made up a foot operated switch which meant I had both hands free to tension and guide the wire into place.
Those without a lathe often make up drives using gears (belts and pulleys) and a variable speed drill. Either way it is easier if there are two of you present, as tangles can occur etc.
For my second coil further down the page, I used the lathe to both turn the secondary and also guide the wire, leaving me to just having to apply some light tension by hand as it wound on.


After winding, the varnish can be applied fairly thick so long as the coil is then left turning whilst it dries. A fairly slow speed (~30 rpm) is all that is needed just to stop the varnish running to the bottom while it is still wet.
The large driving wheel on the jig was from a washing machine drum. It's always worth scrounging these sort of things, because you never know when they will be useful!

The wire's diameter is determined by what size coil form you wish to end up with, which in turn is decided by how much power your HV source can deliver. The steps therefore in deciding all this are:-

1: Determine the output power of your HV source (what ever it is you are using, NST,Obit, MOV, PIG, etc)

2: This will decide the coil form diameter (this is not the wire diameter - AWG )

3: Based also on your HV source's power you can decide on the aspect ratio of the coil form. (The height of the coil form divided by the diameter. A 20 inch tall. 4 inch diameter form would have a 5:1 aspect ratio)

4: Your HV source's power level will give you an idea of the number of turn needed. At one time people said the total length of secondary wire used, should be equal to a quarter of the coils wavelength, but this is now known to be total nonsense. The minimum number of turns will be around 900 to 1000 for medium and high power coils of 2Kva to 3 Kva and upwards. While up to around 1500 turns for the lowest power of under 0.5 Kva. These are very approximate guidelines that people have found to work.

5: It is now simply a case of getting your calculator out. Using wire tables find a suitable gauge that will give you the correct number of turns. The figures given in all the tables are for bare wire, so add around 10% to allow for insulation. Also allow for around 5 inches (2.5 inches each end) to be left unwound on the coil former.





If you fail to count the number of turns as you wind the coil it is pretty easy to calculate the number afterwards. Most cameras nowadays have a macro lens which you can use to photograph a section of the coil alongside a ruler. The image above has been reduced to one quarter of its original size for display on this webpage. The original full size image makes it very easy to count the number of turns per inch that you have wound.




My second secondary, 6 inch diameter.

This was again wound on the lathe but this time the gearbox was engaged and the wire fed into place automatically as the saddle travelled down the bed. As the coil was longer than the bed of the lathe this had to be done in three sections.




The coil was longer than the lathe.

The lathe's chuck is driving another chuck mounted higher up. This is to allow the 6 inch coil to fit on a 4.5 inch centre height lathe. The spirt level you can see is clamped to the cross-slide so it moves along with the saddle and feeds the wire on. It's a very 'Heath Robinson' approach - but it worked!




Almost finished. The wire passes through a hole in the plastic which acts as a guide.

This contraption is clamped to the spirt level which is slowly advancing the length of the coil. In the photo above the last photo, it can just be seen at the right hand end of the coil. By luck the gearbox feed per revolution of the chuck, was exactly the same as the wire diameter. The end of the secondary tube is supported by a makeshift tailstock that is out of view. This method produced a much neater winding than my first coil shown at the top of the page.