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I'm looking to use a (surplus, NOS) Sola (catalog number 22212) ferroresonant voltage regulating transformer (120V primary and secondary) as a low-power isolation transformer (4A) in case I need to float some project. Will this work? What safety issues would I run into vs. using a designed-for-purpose isolation transformer?

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It will work, and there shouldn't be any safety issues.

Just be aware:

  • Ferroresonant transformers are horribly inefficient, so the input power will be considerably higher than the output power. (i.e., they get hot!)
  • The output waveform is more like a square wave than a sine wave.
  • The input power factor is horrendous.

These characteristics occur because the core is deliberately driven into saturation by the resonant winding in order to get the voltage-regulating effect.

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  • \$\begingroup\$ The PF I'm not concerned about as long as the load it presents to the power network is roughly linear (simple leading or lagging PF is A-OK, piles of harmonics not so much). Heat I know how to deal with (forced air, if nothing else!), and the squared output waveform won't bug modern gear one bit (SMPSen, AIUI, don't give a hoot about input waveforms). \$\endgroup\$ – ThreePhaseEel Jan 8 '15 at 4:49
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    \$\begingroup\$ It isn't a reactive power factor, it's a lot of harmonic distortion. \$\endgroup\$ – Dave Tweed Jan 8 '15 at 5:08

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