Figure 19. Series RL circuit

c. After 70 usec, 7 time constants have elapsed. EL is then

equal to zero, and ER and It are equal to 100 percent of their

maximum value or 1 volt and 1 mA, respectively. The current and

voltages of any series RL circuit can be determined by substituting

the appropriate values of E, R, and L.

8.

STEPBYSTEP PROCEDURE FOR DETERMINING TRANSIENT RESPONSE.

a. When a universal time constant chart is not available, the

response curve can be approximated by using the equation for finding

the rate of change of current through an inductor. This equation

states that the rate of current change is equal to the instantaneous

voltage EL, divided by the inductance L. The following stepbystep

procedure can be used to provide an approximate response of an RL

circuit to any waveform.

b. In this procedure, it is assumed the current does not increase

continuously, but in small steps. At the instant that the voltage in

Figure 19 is applied to the circuit, the current is zero and the

applied voltage appears across L. The initial rate of current

increase is equal to EL/L or 1/10 x 103, which is equal to 100

amperes per second, or 0.1 mA per usec. After 1 usec, the current is

considered to have increased from zero to 0.1 mA. With a current of

0.1 mA flowing in the circuit, ER = 0.1 x 103 x 103 or 0.1 volt. EL

then equals 0.9 volts and the rate of current change is reduced to

0.9/10 x 103 or 0.09 mA per usec. The current still is increasing,

and at the end of 2 usec, it becomes 0.1 + 0.09 = 0.19 mA.

Figure 19. Series RL circuit

41