How do resistors affect capacitors?

1 Answer
Jul 8, 2018

The resistor slows the rate of charge (or discharge) by limiting the current that can flow into or out of the capacitor.


When capacitors and resistors are connected together the resistor resists the flow of current that can charge or discharge the capacitor.
The larger the resistor , the slower the charge/discharge rate.
The larger the capacitor , the slower the charge/discharge rate.

If a voltage is applied to a capacitor through a series resistor, the charging current will be highest when the cap has 0 Volts across it.
(i.e. when it is first connected the full voltage will be across the resistor).

The maximum charge current is limited to #I = V/R#
where #V# is the applied voltage and #R# is the series resistance.

The voltage on the capacitor changes as it charges or discharges.

As the capacitor charges the voltage across the resistor drops
(# V_R = V - V_"cap"#) so the current through it drops.

This results in a charge curve that starts off at it's maximum charge rate and tails off to a slower and slower charge rate as the capacitor nears its fully charged state.

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The same thing happens when discharging through a resistor, with maximum discharge at the beginning.

The time taken for a particular capacitor value to charge to 63.2% of full charge through a particular resistor value to is known as the "time constant" for the RC combination.

The time constant is always the same for the same RC values regardless of applied voltage.