When a capacitor is charging or discharging through a resistor, the voltage changes rapidly at first slows down over time. We will just look at the charging curve on this page.

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Diagram

Video below shows how to build this circuit on a breadboard.

Time constant is usually indicated by it’s initials (TC) or the Greek letter tau ( τ ).

RC time constant is determined by the capacitance of the capacitor in farads times the resistance in ohms. τ = RC

First time constant changes the starting voltage to about 63% of the final voltage. Each time constant after that changes about 63% of the remaining voltage. After 5 time constants, the voltage is almost completely changed and therefore the capacitor is usually declared as being fully charged or discharged after 5 time constants.





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Video

Quick RC time constant of charging capacitor schematic to breadboard build and oscilloscope



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