RC Circuits

Time Constant

The time constant of an RC circuit is the product of its resistance and capacitance. For R in ohms and C in farads, the time constant t is in seconds.

t = RC

Capacitor charges to 63% in one RC

Capacitor discharges to 37% in one RC

5RC Rule of Thumb:  A capacitor charges or decays to within 1% of its final value in 5 time constants.

Low Pass Filter / Integrator

Low Pass: f <= 1/(2PiRC); Integrator: f >> 1/(2PIRC)

Gain vs. Freq                Integrator Vout

High Pass Filter / Differentiator

High Pass: f >= 1/(2PiRC); Differentiator: f << 1/(2PIRC)

Gain vs. Freq                Differentiator Vout


f3dB = 1/2PifRC = cutoff frequency; Vout = (2PifRC/(SQRT(1+(2PifRC)^2)))Vout; dB = 20log10(Vout/Vin)

Rise Time:  Time required to go from 10% to 90% of Vout.

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Page author: Dawn Rorvik (rorvikd@evergreen.edu)
Last modified: 05/20/2003