20 gauge copper wire is rated for 5 amps. Suppose someone use 18 gauge copper wire and wraps it 60 times around one side of a 1 inch iron ring, with the two ends then connected to a plug. A separate piece of copper wire wraps around the other side 20 times, and goes to a 13.3 Ohm resister. So now this transformer has reduced voltage from 120 volts to 40 volts, and the 13.3 Ohm resister regulates the current to 3 amps. Suppose the resister has a good heat sink on it and thus won't melt. How hot will will the iron right and copper windings get? Would using electrical tape to guarantee electrical insulation result in burnt electrical tape? What do fully enclosed (no fan) 9 volt or 12 volt AC DC converters use to dissipate their heat? I'm guessing they use less than 3 amps, which dodges my real question. I notice computer power supplies, which use more current, have fans.