391 Charge Cycle A voltage divider containing resistance and capacitance is

391 charge cycle a voltage divider containing

This preview shows page 103 - 106 out of 317 pages.

3.9.1 Charge Cycle A voltage divider containing resistance and capacitance is connected in a circuit by means of a switch, as shown at the top of figure 3-9. Such a series arrangement is called an RC series circuit. In explaining the charge and discharge cycles of an RC series circuit, the time interval from time t 0 (time zero, when the switch is first closed) to time t 1 (time one, when the capacitor reaches full charge or discharge potential) will be used. (Note that switches S1 and S2 move at the same time and can never both be closed at the same time.)
Image of page 103
NEETS MODULE 2-Alternating Current and Transformers UNCLASSIFIED 3-17 UNCLASSIFIED Figure 3-9 Charge of an RC series circuit
Image of page 104
NEETS MODULE 2-Alternating Current and Transformers UNCLASSIFIED 3-18 UNCLASSIFIED When switch S1 of the circuit in figure 3-9 is closed at t 0 , the source voltage (E S ) is instantly felt across the entire circuit. Graph (A) of the figure shows an instantaneous rise at time t 0 from zero to source voltage (E S = 6 volts). The total voltage can be measured across the circuit between points 1 and 2. Now look at graph (B) which represents the charging current in the capacitor (i c ). At time t 0 , charging current is MAXIMUM. As time elapses toward time t1, there is a continuous decrease in current flowing into the capacitor. The decreasing flow is caused by the voltage buildup across the capacitor. At time t 1 , current flowing in the capacitor stops. At this time, the capacitor has reached full charge and has stored maximum energy in its electrostatic field. Graph (C) represents the voltage drop (e) across the resistor (R). The value of e r is determined by the amount of current flowing through the resistor on its way to the capacitor. At time t 0 the current flowing to the capacitor is maximum. Thus, the voltage drop across the resistor is maximum (E = IR). As time progresses toward time t 1 , the current flowing to the capacitor steadily decreases and causes the voltage developed across the resistor (R) to steadily decrease. When time t 1 is reached, current flowing to the capacitor is stopped and the voltage developed across the resistor has decreased to zero. You should remember that capacitance opposes a change in voltage. This is shown by comparing graph (A) to graph (D). In graph (A) the voltage changed instantly from 0 volts to 6 volts across the circuit while the voltage developed across the capacitor in graph (D) took the entire time interval from time t 0 to time t 1 to reach 6 volts. The reason for this is that in the first instant at time t 0 , maximum current flows through R and the entire circuit voltage is dropped across the resistor. The voltage impressed across the capacitor at t 0 is zero volts. As time progresses toward t 1 , the decreasing current causes progressively less voltage to be dropped across the resistor (R), and more voltage builds up across the capacitor (C). At time t 1 , the voltage felt across the capacitor is equal to the source voltage (6 volts), and the voltage dropped across the resistor (R) is equal to zero.
Image of page 105
Image of page 106

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture