The resistive-capacitive (RC) time constant is the time required to charge a capacitor to 63.2 percent of its maximum voltage.

Click on the arrows to select various values of resistance and capacitance. Note resulting changes in the RC time constant.

The amount time required to charge and discharge a capacitor is a very important factor in the design of circuits. Resistors are often used in combination with capacitors in order to control the charge and discharge time necessary for the intended application. Resistance directly affects the time required to charge a capacitor. As resistance increases, it takes more time to charge a capacitor. The amount of time for the capacitor to become fully charged in a RC circuit depends on the values of the capacitor and resistor in the circuit.

As you adjust the values of resistance and capacitance, note how the rate of charge greatly decreases over time. The latter part of the charging time is many times longer than the first part. In fact, a capacitor reaches 63.2% of its charge in one fifth of the time it takes to become fully charged. Because of this, capacitors in actual applications are generally not fully charged. Capacitors in circuits are generally charged to just 63.2% of full capacity. The time required for a capacitor to charge to 63.2% of its full capacity is referred as its RC time constant.