How to calculate time constant and charge time of RRC circuit

analogcapacitordelayresistors

This circuit is part of my time delay circuit. I want to calculate exact value of delay time. Please help me.

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

There are two time constants here. One is when the input is driven and the other when it is open.

Probably the input is driven high, then left to float low. When the input is driven, you assume it has 0 impedance to ground. Note that this impedance is parallel to R1. What is R1//0? When the input is open-circuit, then it has infinite impdance to ground. The effective resistance seen at the top of C1 to ground should be obvious in that case.