# time constant

The time constant, Time Constant, is an electronic characteristic value that results when a capacitance or inductance is charged through 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 inductor, the quotient of the inductance and the ohmic resistance (L/R). The time constant is the time required for a capacitor to be charged to 63.2 % ( e-function) of the applied voltage via the resistor. The same applies to LR elements. The time constant is denoted by the Greek letter tau and is given in seconds (s). In an exponentially increasing process

, the potential increases to 63.2 % of the final value after a time constant of 1 tau (RxC), after 2 tau it is 86.5 % and after 3 tau 95.0 %. In the case of a decreasing process, the value ach 1 tau is reduced by 36.8 % from the initial value, after 2 tau by 13.5 % and so on.

Wherever RC combinations or LR combinations are present or used, as in differentiators and integrators, in filter circuits and filters, in the frequency determination of multivibrators and snubber elements, in the raising and lowering of frequencies such as the emphasis, the time constant of the RC combination must be taken into account, since in certain cases it affects the bandwidth of measuring and transmission equipment, as well as the transient response ofelectronic circuits.

The cut-offfrequency ofquadripoles 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. According to this, the time constant tau corresponds to the quotient 1/2Pi x cut-off frequency.