Capacitor Time Constant
Capacitor Time Constant
- A discharging capacitor loses charge quickly at first and then more slowly. We need one repeatable point for describing how fast this happens.
- One time constant is a checkpoint on the curve. It is not the time taken for the capacitor to become completely empty.
Find one time constant on the curve
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- At one time constant, 37% of the starting charge, p.d. or current remains.
- After another time constant, 37% of the previous value remains. This is about 14% of the starting value.
Symbols
- = time constant (s)
- = discharge resistance (Ω)
- = capacitance (F)
- Larger R or C makes the discharge slower, so the time constant is larger.
- For a network, use the combined capacitance.
Worked example
Read one time constant from a graph
The initial current is 0.13 mA. Find the current after one time constant.
Answer
Move from 0.048 mA to the curve, then down to the time axis. A recent graph gave 4.3 s (9702/43/M/J/24 Q6(b)(ii)).
Your turnquick check
A 220 μF capacitor discharges through 15 kΩ. Find the time constant.
Show worked answer
Answer