What is the meaning of the time constant

Time constant

The time constant, TimeConstant, is an electronic parameter that results when a capacitance or inductance is charged via a resistor. The time constant is the product of the resistance value and the capacitance of the capacitor (RxC) and, in the case of an inductance, the quotient of the inductance and the ohmic resistance (L / R).


The time constant is the time that a capacitor needs to be charged to 63.2% (e-function) of the applied voltage via the resistor. The same applies to LR links. The time constant is designated by the Greek letter tau and is given in seconds (s). With an exponentially increasing process, the potential increases after a time constant of 1 tau (RxC) to 63.2% of the final value, after 2 tau it is 86.5% and after 3 tau 95.0%. In the case of a falling process, the value after 1 tau is reduced by 36.8% of the initial value, after 2 tau by 13.5%, etc.

The time constant is used wherever RC combinations or LR combinations are available or used, such as differentiators and integrators, filter circuits and filters, when determining the frequency of multivibrators and snubber elements, when raising and lowering frequencies such as emphasis of the RC combination, as in certain cases it affects the bandwidth of measuring and transmission equipment, as well as the transient behavior of electronic circuits. The cut-off frequency of quadrupoles can be determined from the time constant via the angular frequency - Greek small omega. This applies, for example, to analog filters in which the following reciprocal relationship exists between the time constant and the cutoff frequency. Then the time constant tau corresponds to the quotient 1 / 2Pi x limit frequency.