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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%.

• Capacitors can be used, with a resistor, for timing.
• The capacitor acts a bit like a bucket of water.
• The resistor controls how fast the capacitor (bucket) fills or empties.
• The 555 timer uses RC combinations for timing.
• In this simulation, click the switch to the charging or discharging position.

View the RC timer in the Lushprojects Simulator

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

• T is the time in seconds.
• R is the resistor value in Ohms.
• C is the capacitor value in Farads.

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

Discharging

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.

• R = 47kΩ
• C = 2200 Microfarads.
• Charge for 10 minutes. Connect the resistor to the 5 Volt supply.
• Discharge for 10 minutes. Connect the resistor to the 0 Volt supply.
• Don't turn off the stopwatch at half time!

Tasks

• Calculate the time constant T = RC
• Note that capacitors are not usually manufactured with great accuracy and DC leakage may be noticeable when you take your measurements.

Plot and Label the Graph

Charging ...

• Mark the graph at the 50% mark after 0.69RC seconds
• Mark the graph at the 63% mark after RC seconds
• Mark the graph at the 66.6% mark after 1.1RC seconds
• Mark the graph at the 100% mark after 5RC seconds

Discharging

• Mark the graph at the 50% mark after 0.69RC seconds
• Mark the graph at the 37% mark after RC seconds
• Mark the graph at the 0% mark after 5RC seconds

Leakage

• Estimate the leakage resistance of the capacitor. (This may well not be constant.)

Picoscope Traces for a Faster Timing Circuit

• R = 1 kΩ
• C = 0.47 µF