 Charging time constants of an RC circuit. A capacitor consists of two plates on which electrostatic charges develop. The charges cause a voltage to develop across the plates. The diagram below and the corresponding graph show the effect of a series RC circuit from the moment a voltage is applied until five time constants later when the capacitor is fully charged. During the five time constants, the minimum current throughout the RC circuit will be zero amps and the maximum current will be 10 milliamps. The speed at which the capacitor charges is determined by the value of the capacitor and the value of the resistor. With the switch in position B, there is no current flow and the voltage across the resistor and capacitor is zero volts, as shown below. The instant the switch is moved to position A and the source is applied, it is referred to as time constant zero. The following circuit conditions occur. The capacitor acts as a short and no voltage is dropped across it. With the capacitor acting as a short, the entire 10 volts from the supply drop across the resistor. The capacitor begins to charge at the fastest possible rate. With the only opposition to current flow coming from the resistor, it is at its maximum value of 10 milliamps. During the first time constant, several circuit values have changed by 63%. The following conditions occur. The voltage across the capacitor increases from zero to 6.3 volts. The remaining voltage of 3.7 volts from the 10 volt supply drops across the resistor. The current decreases from 10 milliamps to 3.7 milliamps. During each of the time constant intervals between 1 to 2, 2 to 3, and 3 to 4, the capacitor voltage, resistor voltage, and circuit current values change by 63% as shown below. Let's see time constant 2. Here is time constant 3 and time constant 4. After 5 time constants, all of the circuit values are changed 100% as shown below. The following circuit conditions occur. The capacitor is fully charged and acts like an open. The entire power supply potential of 10 volts is across the capacitor. The voltage across the resistor is 0 volts. Since the capacitor is no longer charging and acts like an open, the circuit amperage is 0 amps. You have completed this activity, charging time constants of an RC circuit.