## Reference Designer Calculators

### RC Time Constant Questions

Here are some of the questions related to RC time constant that can be used for Academic and Engineering Applications

Question 1

Consider the following RC circuit used for Power ON reset generation. What is the time constant of this circuit. Since R = 100k and C = 0.1uF, the RC time constant according to the calculator is given by

T = 0.01 seconds or 10 milliseconds

Question 2

In the above circuit what should be the value of R for a time constant of 500 millisecods Answer

It will be 5 Mega Ohms. Obviously it is not a good design value ( a 5 Mega Ohm, dirts and moisture on PCB can change its value) to achieve 500 milliseconds delay. We may want to use a 1 Mega Ohm resistor with a 0.47 uF Cap to acheive a time constant of 470 milliseconds.

Question 3

An RC circuit is given a step voltage of V at t = 0 seconds. What is the value of voltage at t = 2RC. Answer

According to the equation

$v(t)= ( 1-e^{-\frac{t}{RC}}).V$

At t = 2RC, we get

$v(t)= ( 1-e^{-2}}).V$