Capacitors can be used for timing. The time constant calculations below are needed for designing timing circuits.

T = RC

• T is the time in seconds. This is the time it takes to charge a capacitor to 0.63 or 63% of the supply voltage.
• R is the resistor value in Ohms.
• C is the capacitor value in Farads.

 

Key Facts

V_s is the supply voltage and V is the voltage across the capacitor.

• After one time constant, V = 0.63V_s for a charging capacitor
• After one time constant, V = 0.37V_s for a discharging capacitor
• V = 0.5V_s after time 0.69RC
• V = V_s after time 5RC for a charging capacitor
• V = 0 after 5RC for a discharging capacitor

 

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 below.

R = 68k and C = 4700 Microfarads

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.

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 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.)

 

Time for about 10 minutes changing and another 10 minutes discharging. Your graph should be approximately this shape.

Here is an example using 1k and 0.47µF