# Time constant relevance and definition for an RC or any 1st order RC/RL or a 2nd order RLC circuit

Can someone clearly explain what it is meant by the time constant of an RC circuit. Because from what I have been told it is the time taken to charge the capacitor. However, when I search on google it mentions that it is the time taken to charge approx. 63% of the initial voltage. What if the initial voltage is 0V?

Would this definition apply to RL and RLc circuits as well?

I have probably missed something trivial.

Any help would be much appreciated. Thanks!

$$V(t) = V_0(1-e^{-t/\tau})$$
When $$t=\tau$$, $$V(\tau)=V_0(1-\frac{1}{e}) = 0.63V_0$$
So the time constant, $$\tau$$, in this case is the time in which the voltage, $$V(t)$$, equals 63% of the supply voltage, $$V_0$$. For an RC circuit, $$\tau=RC$$.