# Time constants, resistance and RC circuits

1. Apr 30, 2007

### t_n_p

I just need some help understanding some of the theory behind these..

Can anybody help explain the relevance of a time constant in an RC circuit and why the time constant increases as resistance increases?

2. Apr 30, 2007

### denverdoc

Well, generally time constants will tell you how fast an inductor or capacitor can be energized or de-energized. If we talk about just capacitors, its how fast they can be charged or discharged. Since resistance tends to choke off current like a small hose vs a big one when filling or emptying a watertank, things get slower with increasing resistance generally. The same principles apply to alternating currents as well as to direct current, but there we usually talk about frequencies instead of a time constant.

3. Apr 30, 2007

### Metaleer

t_n_p, the time constant in an RC circuit is usually R*C... so it is proportional to the first power of the resistance and the capacitance: if you were to put in a 2 Ohm resistor instead of a 1 ohm resistor, the time constant would be doubled. Usually, 4.6 * R*C means a capacitor which is 99% charged, so with the time constant you can more or less figure out how long a capacitor will take to charge to a certain percentage.