And so began the search for a transformer. I had thought that this would be easy. Specify the turns ratio, inductance of the primary and secondary and power handling capabilities and presto. Sadly, turns ratio seems to be about the last thing you can search on. Unlike inductors, which can generally be searched for like resistors and capacitors, transformers seem to be made with very specific applications in mind, and 'Nixie clock power supply' isn't one of them. When I did find transformers with the right kind of inductance they almost invariably turned out to have a 1:1 turns ratio - isolating transformers in other words. I vented on the nixie clock google group, which is full of exceedingly smart and tolerant people. The kind who would think nothing of winding a couple of transformers before breakfast. I strongly recommend it (the group, that is).

So what was I looking for? This:

• Turns ratio of about 1:10. From what I had read so far, this would be ideal.
• Primary inductance of a few uH. Again, from what I had read.
• Small size. Because.
• High saturation current. This is important. If you try and force too much current through an inductor, it stops being an inductor. The net effect is that it acts as a resistor - usually quite a low-value resistor - effectively shorting your inputs. Plus high current capability helps to stop the transformer melting.
• Low resistance. This lowers losses due to resistance (duh!).
• High voltage isolation - you don't want 200V to break down the internal insulation.

Of course, some of these are trade-offs: Small size, high power? Sure.

In the end I scoured the Wurth Electronics and the Coilcraft web sites. Both companies produce a range of transformers that can be wired in multiple configurations - they contain six inductors wound on the same core. I found these on the Wurth Electronics web site. They seemed to be about right, and Wurth were kind enough to send me several samples of each of them. Wurth also have LTSpice models of their transformers - a major benefit that I will get into later.