ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
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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