Seattle, WA
Quoted from Pin_Guy:Ohms law I (current) = V/R. Since the resistance of the transformers primary windings is constant and the voltage from the grid is constant; Ohms law says the current pulled by the transformer is unchanging...even if you disconnect all the wires from the secondaries.
Granted, transformers primaries have some magic interaction with the secondaries that I don't understand and cant be treated the same way a a heater coil of the same resistance.
The load, or impedance, on the primary side of the transformer (neglecting losses, as mentioned already) is reflected from the secondary side. The voltage on the primary side stays the same, as it’s direct from your home outlet. But the current draw changes because the load on the secondary side of the transformer is variable and that is all reflected back thru coupling in the transformer from your induced secondary current. Ohm’s law will always hold but ‘r’ (really z, in this case) on the primary side will be calculated based on the secondary load at any given moment and the windings on the xfmr. If the current draw never changed, you’d be magically getting power from somewhere.
