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,

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,

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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