Ferroresonant transformers really really want sine waves as input. Many UPS's put out square waves, or slightly shaped square waves. There are many high-frequency harmonics in a square wave, and these disrupt the careful LC-resonance-at-saturation regulation mechanism. Not only do ferroresonant transformers want sine waves, they want sine waves of a specific frequency. I've never tried running a 60Hz unit at 50Hz, but I do know from hard experience what happens at 400 Hz :-(. There are UPS's that do put out nice sine waves, but these aren't all that commonly found. DEC computers from 1970 onwards typically used switching supplies for all high-current supplies, though there are usually some low-current supplies with linear regulators. (There are some exceptions, such as the "large" 8/E using a linear supply while the "small" 8/F used a switching supply)... DEC did sell ferroresonant line conditioners (some of them quite large; one that I passed by took 440VAC 3-phase in and had about 70 15 amp circuits out), but these came before the computer. -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology Voice: 301-767-5917 7328 Bradley Blvd Fax: 301-767-5927 Bethesda, MD, USA 20817