Therefore, the voltage across the coil at the end of the first alternation will be practically zero. These values are shown

on the curves in figure 1-24B. The voltage across the coil is so near zero, you can assume that the current has reached

maximum and is no longer increasing, therefore, the voltage across the resistor is maximum.

At the end of the first alternation, the input signal voltage drops to zero very suddenly. If you assume that the signal

source has no internal resistance, then the circuit at this time may be represented by the circuit in figure 1-25.

With no applied voltage to sustain the current, which is at maximum, the current would attempt to stop flowing. The

magnetic field, due to this current, would begin to collapse. The collapsing of the magnetic field induces a voltage

across the coil of such a polarity that it tends to maintain the current flow in the same direction. The coil is now acting

as a generator with the polarity shown in figure 1-25.

The instant the field begins to collapse, the greatest current will flow through the circuit, producing a high voltage drop

across R. With no losses in the circuit, the voltage across both the resistor and the coil would have an amplitude of

about 20 V. As the induced voltage falls off, the current decreases, as does the voltage, across the resistor. At any time

during current decay, the voltage across the coil and the resistor will be equal in amplitude since the two are effectively

in parallel.

Notice that the wave shape across the coil is AC, although the input voltage is pulsating DC (figure 1-24B). It is a

peaked wave, as was the voltage across the resistor in the RC circuits previously studied. The voltage across the

resistor in the RL circuits is a triangular wave. This compares to the capacitor voltage in the RC circuits.

Time constants affect the wave shapes in RL circuits, just as they do in RC circuits. If the current has more than

enough time to reach maximum (the time constant is short compared to the time for the shortest alternation), the wave

shape across the coil will be a sharply peaked wave. As with RC circuits, in order for the time constant to be short, the

time of the shortest al

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