Brief Capacitor Discharge RC Time Constant Circuit

A capacitor with a parallel resistor and in series with a push button switch, will be fully charged whenever the button is pressed.

Simple discharging capacitor RC time constant circuit schematic diagram by electronzap electronzapdotcom
Simple discharging capacitor RC time constant circuit schematic diagram by electronzap electronzapdotcom

Releasing the button will rapidly discharge the capacitor through the resistor at first, but then discharge more slowly over time. Measuring the voltage with an oscilloscope shows the RC time constant voltage curve during discharge through a resistor.

One RC time constant is the time in seconds that it takes to change about 63% of the remaining voltage. Since it deals with remaining voltage, each following time constant changes the voltage less than the earlier ones did. The slope becomes flatter as it gets closer to the final voltage.

Discharging capacitor RC time constant pictorial schematic diagram by electronzap
Discharging capacitor RC time constant pictorial schematic diagram by electronzap

A 1,000µF (one thousand microfarad) capacitor being discharged by a 1,000Ω resistor will have an RC time constant of 0.001F x 1000Ω = 1 second.

It takes one second to discharge the first 63% of the voltage, then one second to discharge 63% of the remaining voltage, and so on.

After 5 time constants, about 99.9% of the capacitor’s starting voltage is gone. At that point it is considered fully discharged.

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