Abstract: The steady-state response of the first-order dynamic circuits is causal periodic signal in the causal periodic square wave signal stimulus.The maximum value of square-wave voltage is u0,and the maximum and minimum values of the capacitor voltage are uCmax and uCmin,they obey a relation： u0-uCmax=uCmin.The waveform of capacitor voltage on the time-axes has special symmetry.By using the steady-state reponse of RC circuit voltage of symmetry,we can export the circuit time constant τ,and the formula for capacitance measurements provides a fast and reliable way.

Key words: first-order dynamic circuits, time constant, steady-state response, symmetry