MM0704, Lesson 1
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.
Figure 1-25. RL Circuit at End of First Alternation.
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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