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Is it reasonable to use a ferrite transformer from a big SMPS (LCD TV power supply) as a mains transformer, assuming I split the transformer in half and rewound the coils to provide the proper impedance? Ferrite transformers are generally operated at high frequency, but I don't know enough about transformers to know what happens if they are operated at low frequency.

I just want to know if it would have a reasonable efficiency, I'm not planning to do anything with it myself.

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    \$\begingroup\$ Normally ferrite Bmax is restricted by dissipation, however at mains frequency, you could push it all the way to saturation, maybe 0.4T depending on material. Easy to do an experiment to find out. Transformer VA will be lower than for same size iron by ratio of Bmax, so perhaps only 20% of iron. ur tends to be lower than iron, but for some low frequency materials not by much, so you could expect Imag to be higher, perhaps twice to 10x higher than for iron of same size. \$\endgroup\$ – Neil_UK Sep 17 '17 at 8:58
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It is fully possible, but you will need a lot of turns to keep the peak flux density below saturation. Because of this, you will quickly run out of winding area. This in turn will call for angel-hair thin wire to make it fit. Thus high resistance and losses. Use \$ U_{rms}=4.44 \cdot f \cdot N \cdot A \cdot B \$. With ferrite, B would be in the 0.3 T range. A normal laminated iron transformer would go to about 1.2 T and give you a better core-voltage-turn-to-winding-area-ratio for 50-60 Hz.

Also, think about Al for the core and your number of primary turns. I'm sure it's possible to supply the magnetizing inductance, but that current times your primary resistance will determine your static loss for the transformer.

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    \$\begingroup\$ Thanks @WarrenHill English isn't my first language but I should get around to learning this math formatting language. \$\endgroup\$ – winny Sep 17 '17 at 13:45

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