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GCSE     Capacitor     Coupling     Decoupling     Explosion     More Uses     Parallel     >RC Timing<     Series     Smoothing    

Capacitor RC Timing


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  GCSE    Capacitors  0 of 22    Question 1307    Audio Amplifiers 
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Use a resistor and a capacitor to make a timing circuit.
Time Constant:   T   =   R C   =   The time for the capacitor to charge or discharge by 63%.

View the RC timer in the Lushprojects Simulator

The time constant calculations below are needed for designing timing circuits.

In this circuit, when the switch is flipped, the voltage across the capacitor changes quite fast and the current flowing is larger. As the capacitor approaches full charge (or discharge) the voltage changes more slowly and the current is smaller. The current is proportional to the rate of change of voltage.

Here is a Flash animation of a water analogy. The container represents the capacitor. It takes time to fill or empty. When the tank is nearly full or empty, the level changes much more slowly. Capacitors behave the same way.

RC Timing Calculations

Charging

Capacitor Charging Graph
  • R = 1000000 Ohms.
  • C = 1 Microfarad.
  • R C = 1 Second.
0% T = 0 Seconds
50% T = 0.69 R C Seconds
63% T = R C Seconds
66.6% T = 1.1 R C Seconds (555 Monostable)
100% T = 5 R C Seconds

 

Discharging

Capacitor Discharging Graph
  • R = 1000000 Ohms.
  • C = 1 Microfarad.
  • R C = 1 Second.
100% T = 0 Seconds
50% T = 0.69 R C Seconds
37% T = R C Seconds
0% T = 5 R C Seconds

 

Experiment

The aim is to measure the voltage across a charging capacitor at regular time intervals. Once the capacitor is fully charged, repeat the measurements while it is discharging. These results should be plotted on a graph. The graph should then be marked up with the key positions listed above.

Capacitor Charging Experiment

Capacitor-Charging-Experiment-Layout.gif

Tasks

Plot and Label the Graph

Charging ...

Discharging

Leakage

Picoscope Traces for a Faster Timing Circuit

Capacitor Charging Picoscope Measurements

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