An RC circuit is to be designed in which a capacitor (C) must

charge to 20% (0.20) of the maximum charging voltage in 100

microseconds (0.0001 second). Because of other considerations,

the resistor (R) must have a value of 20,000 Ohms. What value

of capacitance is needed?

Given: Percent of charge = 20% (0.20)

t

= 100 microseconds

R

= 20,000 Ohms

Find: The capacitance of capacitor C.

Solution: Because the only values given are in units of time

and resistance, a variation of the formula to find RC time is

used:

RC = R x C

where: one RC time constant = R x C and R is known.

Transpose the formula to:

R C

C = _

R

Find the value of RC by referring to the universal time constant

chart in figure 37, on the previous page, and proceed as follows:

(1) Locate the 20 point on the vertical scale at the left side

of the chart (percentage).

(2) Follow the horizontal line from this point to intersect

curve A.

(3) Follow an imaginary vertical line from the point of

intersection on curve A downward to cross the RC scale at the

bottom of the chart.

Note that the vertical line crosses the horizontal scale at

about .22 RC as illustrated below (figure 38 on the following

page).

The value selected from the graph means that a capacitor

(including the one we are solving for) will reach twenty percent

of full charge in twentytwo one hundredths (.22) of a single RC

time constant. Remember that it takes 100 microseconds for the

capacitor to reach 20% of full charge. Since 100 microseconds

is equal to .22 RC (twenty

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