Capacitors in DC Circuit

Capacitors in DC Circuit (Charging)

Charging and discharging are the main effects of capacitors. A capacitor charges when it is connected to a DC voltage source through a resistor. Initially, when the switch is open, there is no voltage flowing across the capacitor's plate. When the switch is closed, the current jumps to its maximum value, and the capacitor begins to charge. As the charging process continues, the current decreases, and the voltage across the capacitor increases until it equals the source voltage. When the capacitor is fully charged, there is no current. A capacitor blocks constant DC. A capacitor charges following a nonlinear curve as shown above.

Capacitors in DC Circuit (Discharging)

 

The capacitor discharges when a conducting path is provided across the plates without any applied voltage. A capacitor can serve as a voltage source, temporarily, by producing a discharge current. When the switch is closed, the capacitor begins to discharge. Initially, the current jumps to a maximum. The direction of the current during discharge is opposite to that of the charging current. During discharging, the current and capacitor voltages decrease. When the capacitor has fully discharged, the current and the capacitor voltage are zero. The discharging curve is shown in the illustration above.  

RC Time Constant

A certain time is required for the capacitor to fully charge or discharge. The rate at which the capacitor charges or discharges is determined by the RC time constant of the circuit. It is symbolized by Greek letter τ (Tau), where τ = RC. When R is increased, the charging current is reduced, thus increasing the charging time of the capacitor. When C is increased, the amount of charge increases, thus, more time is required to charge capacitor for the same current. In one time constant, the capacitor voltage changes approximately 63%. It reaches its final value at approximately 5τ.