Completly passive precise AC-mains voltage regulation.

[BrianHG]

Ok, I always figured this was possible, but not with this precision...

[Tomorokoshi]

That was bloody interesting!

[bob91343]

I have a couple of those, built into other gear.  However, these days the power lines are much better regulated and have made constant voltage transformers unnecessary.  The main vendor of them was Sola.

[james_s]

I used to encounter those Sola regulating transformers now and then but I haven't seen one in a while now. They do work very well at regulating, although as I recall the output waveform is not very nice and they also run hot so waste a lot of power. I don't really see them as being necessary in most cases now, power supplies are pretty robust and computers and such are not particularly sensitive at all to voltage fluctuations.

[TimFox]

Solas are very useful devices, a bit heavy, with a required resonating capacitor.  Some comments:
1.  They are/were available either as approximately square wave outputs (great for rectifiers) or "harmonic neutralized" with reasonably sinusoidal outputs.
2.  They are good at rejecting changes in input voltage and load current, but the output varies directly with frequency, so they must be purchased for 50 or 60 Hz use.
3.  Back before 1970, the electronics shop at Argonne National Laboratories built most of their vacuum-tube circuits using Sola transformers that combined plate supply and heater supply windings to simplify voltage regulation requirements.

[BrianHG]

Here is part 2 with scope shots.  Yes, the signal turns into a square wave at high input voltages.