RC time constant: watch a capacitor charge

The circuit

Connect a DC source, resistor and capacitor in series. Measure the voltage across the capacitor with the oscilloscope. At the instant power is applied, an uncharged capacitor behaves like a low impedance load. As it charges, current falls and the capacitor voltage rises.

The time constant τ is measured in seconds when resistance is in ohms and capacitance is in farads. After one time constant, the capacitor reaches about 63% of the final voltage. After five time constants, it is effectively charged for most practical purposes.

Try it in resistor.dev

1. Open the RC example using the button above.
2. Run the simulation and select the capacitor as an oscilloscope channel.
3. Set the resistor to 10 kΩ and the capacitor to 100 µF. The time constant is one second.
4. Increase the resistor or capacitor by ten times, reset the simulation and compare the curve.
5. Reduce either value by ten times and observe how the charging time changes.

What the graph means

The voltage is not a straight line because the current depends on the difference between source voltage and capacitor voltage. At the beginning that difference is greatest, so current is greatest. Near the final voltage the difference is small, so the rate of change becomes small.

A real capacitor can have leakage, equivalent series resistance and voltage limits. Select Engineering accuracy mode and a capacitor model when those effects matter; use component documentation for a real design.