Engineering Principles V11

Clive W. Humphris

CAPACITANCE: Capacitor Charging.  

Connecting a capacitor in series with a resistor across a steady DC supply causes the capacitor to eventually become charged to a voltage equal to the battery. The time it takes is determined by the resistor and capacitor values.

Normally the capacitor is considered to be charged when the voltage reaches 63% of the supply voltage. This takes a period of time equal to C × R in seconds.

The reason for the exponential waveform shape is that when the capacitor is uncharged it is effectively a short circuit and a relatively high current flows, determined mostly by the value of R. As the capacitor charges a voltage is developed across it which opposes or subtracts from the applied voltage (V).

As V - VC decreases and VC approaches V, the charging current tends towards zero. It takes a period of approximately five time constants for C to become fully charged (or discharged).

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