Capacitors & RC Timing
Charge, discharge, the time constant τ = RC, and how an RC circuit filters signals.
Lesson progress
t = 0 s · slow motion: 1 s on screen = 1τ = 1 s
- Capacitor Vc
- 0 V
- of 9.0 V
- Current I
- 0 A
- into C
- Time constant τ
- 1 s
- 5τ = 5 s
- Stored energy E
- 0 J
- Q = 0 C
The universal RC curve
Every RC circuit, measured in τLive scope
last 6τAs a filter: frequency response
fc = 1 / (2πRC) = 159 mHzBelow fc the capacitor has time to follow the input, so the signal passes at full strength (0 dB).
Above it, each 10× rise in frequency cuts the output by another 20 dB (a factor of 10). This is a first-order low-pass filter.
Switch the input to Square wave to place your signal on this curve.
A tiny rechargeable bucket of charge
A capacitor is two metal plates separated by an insulator. Push current in and charge piles up on the plates, building a voltage across them. Unlike a battery, a capacitor stores energy in an electric field. It charges and discharges in milliseconds and can do so millions of times.
Put a resistor in front of it and the resistor limits how fast the charge can flow, so the capacitor fills gradually: quickly at first, then slower and slower as it nears full. The time scale is set by a single number, the time constant τ = R × C. That makes the RC circuit the basis of timers, flashing lights, switch debouncing, filters and smoothing in power supplies.
Try this