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I have been thinking of various modes of transforming electricity . At the higher end we have coil transformers like power & distribution type. One is step up while the latter is step down .They follow Faradays principle and Lenz's law .  You can also step up using a series of diodes for lower voltages .  Further inductance and capacitance can be used in buck and boost transformers . Also I understand for a DC to DC transformation you can first rectify the AC to DC , chop it and then transform using either the isolated or non-isolated type.  An op amp multiplies DC with the help of AC .  It would be nice to develop a set of rules to figure out which criteria would be the best for which use.  Also to know the finest transformer companies in India and the world and their latest developments .  I can think of Texas Instruments ,Infineon , Alsthom, Vishay and the like .  Invite you to contribute to this discussion