what you want to google for is "ferroresonant power supplies"
http://www.mcitransformer.com/i_notes.html
for example
A ferroresonant power supply is very similar to an unregulated power supply except for the characteristics of the ferroresonant transformer.
The ferroresonant transformer will supply a constant output voltage over a wide variation of the transformer input voltage. The problems with using a ferroresonant power supply include that it is very sensitive to slight changes in line frequency and would not be switchable from 50 Hz to 60 Hz, and that the transformers dissipate more heat than conventional transformers. These power supplies are heavier and will have more audible noise from the transformer resonance than regulated linear power supplies.
Received on Sat Nov 13 2004 - 23:36:35 GMT