i. *RC Time Constant*. The time required to charge a capacitor

to 63% (actually 63.2%) of maximum voltage, or to discharge it

to 37% (actually 36.8%) of its final voltage is known as the

TIME CONSTANT (TC) of the circuit. The charge and discharge

curves of a capacitor are shown in figure 36. Note that the

charge curve is like the curve in figure 34, graph 1), on page

53, and the discharge curve like the curve in figure 34, graph B.

FIGURE 36. RC TIME CONSTANT.

The value of the time constant in seconds is equal to the

product of the circuit resistance in Ohms and the circuit.

capacitance in farads. The value of one time constant is

expressed mathematically as t=RC. Some forms of this formula

used in calculating RC time constants are:

t(in seconds)

= R (in Ohms) x C(in farads)

t(in seconds)

= R (in megohms) x C(in microfarads)

t(in microseconds)

= R (in Ohms) x C(in microfarads)

t(in microseconds)

= R (in megohms) x C(in picofarads)

j. *Universal Time Constant Chart*. Because the impressed

voltage and the values of R and C or R and L, in a circuit are

usually known, a UNIVERSAL, TIME CONSTANT CHART (figure 37 on

the following page)can be used to find the time constant of the

Integrated Publishing, Inc. |