LR time constant: Square wave drives series LR on oscilloscope.

LR time constant: Square wave drives series LR on oscilloscope. A signal generator places a 2.2 kHz square wave across a coil of 507 turns and a series resistor (470 ohms). A laminated core is slowly inserted into the coil. When the voltage in the square wave goes suddenly positive, a current starts to flow in the inductor. This current is opposed by the induced emf in the inductor. However, as the current starts flowing, there is also a voltage drop across the resistor. Thus the voltage drop across the inductance is reduced, and there is less impedance to the current flow from the inductance. The current through the LR circuit rises exponentially until it reaches the value V/R, with a characteristic time constant L/R. When the square wave is suddenly zero, the current decays exponentially to 0, with the same time constant. When the square wave goes negative, similar arguments apply. When the core is fully inserted, L/R is large, and the scope signal is no longer a 'square' wave, but a series of scalloped rises and falls.