MM0704, Lesson 1
ternation must be at last ten times as long as the time constant of the RL circuit. That is,
In the preceding formula, R is the resistance in ohms; L is the inductance in henrys; and t is the time for the shortest
alternation in seconds.
If the length of the TC for the circuit is very long, compared to the time for an alternation, the current does not have
time to increase much. The voltage across the inductor produces a distorted square wave, while the voltage wave shape
across the resistor is triangular.
In order for the output to be a good reproduction of the input the TC of the RL circuit must be long with respect to the
time of the longest alternation. If the time of the alternation is only one-tenth (or less) of the time for one TC, the
circuit is said to have a long TC. That is,
Summation of RC and RL Circuits. RC and RL series and combinations are used primarily in radar circuits as wave
shaping or timing devices. A time constant may be established for each RC and RL circuit. This TC represents a
period equivalent to the time required for the circuit to complete approximately 63.2 percent of its transient action.
That is, the capacitor in the RC circuit can charge to 63.2 percent of the applied voltage in 1 TC and the current in the
RL circuit can increase to 63.2 percent of the maximum current for the circuit in 1 TC. These transitory periods last for
about 5 TCs.
In any series RC circuit, 1 TC is the product of the resistance and the capacitance. In any series RL circuit, 1 TC is the
quotient of the inductance and the resistance L/R.
The number of time constants in a specified time can be determined by dividing the specified time by the value of one
time constant. In RC circuits,
In RL circuits,
Summary. The charging of the capacitor in a series RC circuit and the current rise in a series RL circuit follow
exponential curves. The Universal Time Constant Chart can be used for both RC and RL circuits in order to determine
the percent of transient action completed in any specified number of TCs.
A circuit is said to have a long time constant when the time required for transient action is long compared to the time
allowed to charge. A circuit has a short time constant when the transient time is short compared to the time allowed to
charge. Thus by definition,
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