Charging Up

The RC time constant

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A charging capacitor follows $V = V_0(1 - e^{-t/RC})$.
• The time constant $\tau = RC$; after $1\tau$ it's at 63%, after $5\tau$ over 99%.
• Larger R or C means slower charging.

Steps

1. Filling a capacitor
2. Set the timing
3. Almost full
4. The first instant
5. After one time constant
6. Ready to flash