AC dimmer: Soft iron core in coil dims lamps.

AC dimmer: Soft iron core in coil dims lamps. This is a series LR circuit (as was D+20+2). The lamps are the resistance R in this case. Either 120 VDC or 120 VAC can be applied by throwing the knife-switch, lighting the lamps. When DC voltage is selected, inserting the laminated iron core will cause no variation in the brightness of the lamps. However, if 60 Hz AC voltage is selected, inserting the core will cause the lamps to dim. Completely inserting the core will cause the lamps to completely turn off. For the 120 VDC case, the resistance of the lamps (in parallel) is about 30 ohms, and the current flowing is about 4 amps; plenty of current to light the lamps. There is no inductive impedance; no induced emf. But in the 120 VAC case, there is an inductive impedance; and a rather large induced emf, especially when the core is inserted. When the core is inserted, the impedance of the inductor XL= 2 pi f L = 2x3.14x(60 Hz)x(.390 H) = 147 ohms, which means the current flowing in the circuit will be at least 80% reduced, and not enough to light the lamps.