Usually there is some space to add some test turns of a wire over a transformer or choke bobbin. Let's say one or ten or whatever. Now you apply a known small AC test voltage on a winding you like to count and measure what you get at a test winding with a good voltmeter. Voltage rate will give you turns rate. If you expect a significant current in a measured winding, you can account for it, too by measuring the current and resistance of the winding that you are measuring.

For laminated iron cored transformers test winding can be placed somewhere nearby the main bobbin on a core kern too, you likely will still get tight enough magnetic coupling between the windings. Some care should be taken if there are air gaps in the core.

Thse same experiment goes for a winding with some taps if you want to know turns ratios, of course.
Sure with hard epoxy potted magnetics you are usually out of luck, until you are motivated enough to attempt to decap.

Yep!

Aww that sucks!

Non destructive turns counting? Tell me more!

I’ve seen this mentioned before on LG, quite a few times and heard of people wanting to make their own ballast’s!
But this type of electrical engineering here seems to be steeped in secrecy and mystery! 🤔, people in Britain that can do it tend to not tell you much about it, i.e, the grade of wire, the wind numbers, the math that goes into it!

Imagine if you had the formula to hand to be able to make your own 200 watt SLI/H autoleak transformer, (something that is so rare in the UK, people have to use alternative gear)!
Maybe Roman (RRK) can shed some more light on this as he did above?😎

There is no secret or mystery in calculation of a transformer, including a leak transformer. All the math is already 150 years old A tricky part may be there is no direct recipe to make a transformer or an inductor, you need get a balance of cost, weight, size  and efficiency. All these parameters are mutually dependent!  Add also a necessary 'fun' measure to a hobby project. Designing a balanced ballast may be an ok job professional, but too mundane to complete as a hobby.

First thing you have to respect in designing a magnetic part is saturation induction of a core, called Bmax. As Bmax defines a maximum of counter-EMF in a winding that works against an applied voltage, generally it will dictate a number of turns per volt for a given core cross section at a given line frequency. Okay. Next you have a turns ratio between primary and secondary approximately as a ratio of line voltage and OCV required. Note that in practical leak  transformers secondary winding is usually fed from a tap of primary, so the ballast is in fact somewhat in between of a choke fed by an autotransformer and true leak transformer. Next you add a magnetic shunt. Calculating magnetic resistance is complex, one may even tune magnetic shunt gap experimentally.

And so on.

As for (re)making a SLI/H ballast. As usual, before you start to make anything, the first question to ask is 'why'. Depending on the answer you can go different ways. Let's  say the answer is 'I just want to light up a rare lamp correctly'. Well then you have to remember that any leak transformer is electrically equivalent to a combo of an (auto)transformer and a choke. Making this is significantly easier than making a custom leak transformer. You can reuse some scrap transformer for an OCV required and then add a group of chokes calculated to proper impedance for prescribed lamp current.

Or the answer may be 'I want to have a historically correct replica of the ballast!" Okay then, the best you can do is to find a surviving original and take all the measurements. Turns can be measured non-destructively. Modern magnetic materials are likely slightly better than historical ones, so you may measure the core dimensions and re-create it with modern laminations.

Or you can answer 'I just want some fun)' Well that's legitimate for sure. Making a transformer/choke is relatively trivial. I did some at 50Hz and HF too. A winding machine is not complex and not very expensive, generally it is a bobbin holder with a counter and a hand or motor crank. Enamelled wire is easy to buy, even some modern varieties available, even off Aliexpress. You can cannibalize some scrap transformers for magnetic materials and coil formers.

Welp, July 2024. The pole is down and the luminaires are gone. As of this year the pole is also gone

I live in the US, so all of our ballasts and ignitors and capacitors etc. have wire leads instead of clamp screw terminals like in Europe and other places. Anybody know why?

Wires can break off, then you are poop-outa-luck a lot of the time. Terminals are practically infinitely reusable, which is nice for enthusiasts. I have used, used, and reused many components, and the strands of the stranded wire starts to fall off. Then you have to cut it shorter and re-strip the wire. I wouldn’t have to do that if it had terminals. For a lot of my gear now, I put screw terminals blocks on them so I can connect to them without wearing out the wires.

Why does the US have to be difficult and cheap? Y’all don’t know how easy you have it.

I have heard of some around California in the past, I don’t think it is a regularly scheduled thing. I might do some meet ups in the future with other people when I move out, it sounds like it would be an interesting experience.

Do you guys ever do mass meet ups?, UK lighting forum members usually do 2, one at Dave’s, (Dave the lighting guy on here), who is a street lighting engineer, and usually one at Kev’s down in Cornwall, (though I have missed the last two at Kev’s), Kev is an Electrician here and both Dave and Kev have a vast collection of British lamps and street lighting equipment👍 it’s good to all meet up when you can guarantee that everyone there will have exactly the same interests as you 😎

@AngryHorse
Yea, pretty much the same thing here in the US in terms of male interests, but instead of “football”, men are generally interested in baseball, american footbalthe occasional soccer or something else (idk, golf?). Cars and sports are definitely not my thing.
Even more so, girls aren’t my thing either (if you know what I mean… hehe …), but I am glad to know that things get better as I get older. I am looking forward to interacting with engineering-y people in college that are hopefully interested in semi similar things.