Learning Objectives:
•
•
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To study the transient response in storing an electrical charge on a capacitor in an RC
circuit.
To study the transient decay of an initial charge on a capacitor through a resistor.
To determine the time constant in an RC circuit and how it can be changed.
Background/Introduction:
1. RC time constant circuits are used extensively in electronics for timing (setting
oscillator frequencies, adjusting delays, blinking lights, etc.). It is necessary to
understand how RC circuits behave to analyze and design timing circuits.
2. Circuit in Figure 1 (with the switch is opened) the capacitor is initially uncharged, and
had a voltage, VC equal to 0 volts. When the switch is closed, current begins to flow,
and the capacitor begins to accumulate stored charge.
3. Since Q = CV, as stored charge (Q) increases, the capacitor voltage, VC increase as
well. However, the growth of capacitor voltage is not linear, in fact, it is called an
exponential growth. The formula which gives the instantaneous voltage across the
capacitor as a function of time is:
𝑽𝑪 = 𝑽𝟎𝒆 −𝒕𝝉
4. This formula describes exponential growth, in which the capacitor voltage is initially 0
Volts and grows to a value of 𝑉𝑆 after enough time has gone by.
5. Time constant of a series RC circuit,  can be calculated to gain a perspective of how
the transient will take. The formula is:
 = RC
6. A capacitor will be mostly charged or discharged after five time constant, 5. This also
called as transient period.
Components/Equipment:
No
Component/Equipment
1
Multimeter
2
Function Generator (Voltage supply)
3
Stopwatch
4
Capacitor
5
Resistor
6
Switch
Value
12V
470µF
68kΩ, 100kΩ
Figure 1: RC circuit
Unit/group
1
1
1
2
1
1