# 005 RC time constant-Capacitor-Resistor

RC time constant refers to the voltage curve from charging or discharging a capacitor through a resistor.

While charging, the voltage shoots up quickly at first, about 2/3 of the voltage change. This is called the 1st time constant. After that, the voltage rise keeps slowing down until it reaches it’s final voltage.

Brief capacitor charging RC time constant demonstration circuit

This creates a distinct curve when graphed. It takes 5 times as long as the initial 2/3 charge to reach 99.99% of the final voltage. So we say a capacitor is fully charged through a resistor after 5 time constants.

## Video:

### Diagram for video below

voltage ramps are covered in lesson 006. Link provided above.

### RC time constant basic properties

• A capacitor’s voltage changes based on it’s capacitance and how much current is flowing in or out of it. Voltage rises as current is forced in (charges), and voltage falls as current naturally leaves (discharges). The voltage is due to an imbalance of charges on 2 separated conductive “plates”. More imbalance = more voltage across them.
• A higher voltage difference across a resistor means that more current passes through it. The voltage across an RC resistor depends on the supply voltage (final voltage) and the capacitor voltage. As the capacitor charges, there is less voltage difference across the resistor, and thus, less current flows.
• Charging/discharging a capacitor through a resistor means that the voltage will change rapidly at first and very slowly as it gets closer to the final voltage.
• One time constant can be calculated by multiplying the capacitor’s capacitance in Farads by the resistors resistance in Ohms. Remember to convert, millifarads (mF), microfarads (µF), nanofards (nF), picofarads (pF), to farads before making the calculation.
• One time constant is how long it takes to change/discharge almost 2/3 of the remaining voltage difference.