What's inside a Villiers HT coil?
I firstly scored where I wanted to part the hard outer covering. Coil is dead so no worries about the outcome.
It took some punishment from a Stanley type knife to break through.
The outer part appears to have been a simple Tufnol or Bakelite tube, added after the end bobbin type pieces.
Wrapped up like king tut I think, with bandages and resin, it did a good job, I have found others in awful states where water has got in and HT windings are just a lump of blue copper oxide. The connection here is often where the trouble is, folk trying to save time and money by soldering onto this little plate often melt the HT wire away, be warned.
It's a small task to get to the actual wire, this one was well made.
On later models, a piece of very high insulative mica is used, like this inch square piece.
Finally got down to the first row of very fine wire.
Like a hair from your head, or even thinner, it's amazing that makers could wind thousands of turns and many layers of stuff like this without a break or shorting out anywhere.
First check that micrometer reads zero.
Then isert wire Then insert wire and gently tighten up jaws.
2.5 thous of an inch, I doubt I could manage to wind such frail stuff, even in my younger days when my eyes were good.
The HT connection, in all of its glory.
Took some time to get all the High Tension wiring and insulation off.
Which left just the Low Tension windings, much thicker wire and only three layers, with no insulation between, the wire is cotton and resin covered I think and that would appear to be enough. The solder is the end of the windings, the start is under the windings themselves.
Even the ends of the soft iron corepiece have been sealed off with resin.
The 60 degree chamfer to these end holes is where the corepiece was held in a lathe or coil winding machine.
Taking one end bobbin off, I could see there were only about three lots of LT windings layers.
I'd previously thought that he tiny slot machined here was for the later mounting of the coil into the magneto, but I'm now of the opinion that it was more likely to do with the wire winding process, maybe for locking it in place well.
Checking the thickness or gauge of the LT wire here.
Nearly 38 thous, there are many different wire gauges that used to vary from country to country, the USA uses its own, but I imagine countries with metric measures use that system instead.
Where the start of LT wire was soldered, it just came away, I think there's a drilled hole into the iron core here, but I doubt soldering iron to copper made a very good joint. Maybe better than simply laying the wire onto the iron though maybe, an electrolytic action might also be set up in damp conditions.
I doubt I'll ever use this now, I'm not very likely to try rewinding it all!
The end of the LT thick wire goes to the rod to make that a common earth for the capacitor, points and the sparking pug also, of course. Otherwise the system would not work. The coils, capacitor, points and spark plug are all wired in parallel to each other, something I didn't learn for quite some time, nobody seems to tell you these things, unless you're an electrician, you have to work it out for yourself, but now I've done it for you.
This is really (dare I say surprisingly) instructive, thanks for posting the images David.
I am sure many members will have mowers with engines using this type of coil. If they are like me they will probably treat is as a "black box" component that either works (in which case great) or doesn't (in which case, probably easier to replace than repair).
In all the years I have been collecting and restoring mowers I had never thought to take one of these apart. It really does help to understand the difference between the primary and secondary windings and there the various components connect internally and with other parts of the electrical system.
Thanks for taking the time to post the pictures and explanation of the strip down, I certainly found it interesting to see just what makes up the internals of one of these. It really is impressive that such fine wire can be wound to achieve the desired outcome!
I'm just happy that folk found it of interest.
What I omitted to mention is the hard to understand bit, as the flywheel turns each time, the magnets pass the 'pole pieces' or the curved metal parts that almost touch the inner flywheel, when the maximum magnetic flux occurs, the points SHOULD be just opening, creating an ac current at the points and capacitor, the iron core acts like a transformer, thus transforming the voltage in the LT windings to a huge spike in the very fine HT windings (it's called inductance, the transfer voltage is induced) it's something to do with ratios of turns and size of wires in both LT and HT windings, my electrician brother did try to explain it to me, but I couldn't take it in!
Thanks for taking the time and effort to post those photos.
The coils, capacitor, points and spark plug are all wired in parallel to each other
Not quite. The primary of the coil is in series with the points. The points act as a switch, so need to be connected in series with the coil. The condenser/capacitor is in parallel with the points. The secondary of the coil is in parallel with the spark plug.
it's something to do with ratios of turns and size of wires in both LT and HT windings
In transformers, there is a simple relationship between the turns ratio and voltage ratio. For example: 10 turns on the primary (LT) and 100 turns on the secondary (HT) gives a turns ratio of 10:100 (or 1:10), so 1 volt AC input to the primary will produce 10 volts AC on the secondary.
The diameter (or gauge) of the wire determines the maximum current that it can carry. The primary of a coil uses large gauge wire because it needs to carry a large current (in amps) albeit at a low voltage. For the secondary the opposite applies: it needs to produce a very high voltage but at a very low current (milliamps), thus the gauge can be much smaller.
In an ignition coil, the turns ratio may be 1:100 or more. In your photo of the primary, I count about 50 turns per layer, so with 3 layers there would be around 150 turns in total. If we use the 1:100 ratio, then the secondary would have 15,000 turns. With that many turns, the secondary wire diameter needs to be very small in order for the coil's overall diameter to remain practical for fitting within the flywheel casing.
GTC, thank you for clarifying a few points I wasn't very clear on, but to the average mower enthusiast I hope I got my shorthand type diagnosis over well enough. Thanks for your reply.