# Given an RC time constant

Discussion in 'General Electronics Chat' started by Martino Chiro, May 8, 2015.

1. ### Martino Chiro Thread Starter Member

May 1, 2015
56
0
Given an RC time constant,
how can i choice the best values of R and C between standard values available on the market ?

2. ### AnalogKid Distinguished Member

Aug 1, 2013
4,686
1,297
It depends on all of the other circuit factors. Do you need an exceptionally low or high impedance, voltage, current, power level, accuracy, low cost, component type (through hole vs. SMT), frequency characteristics (non-inductive resistors), favorite vendor, availability? Lotsa things to consider. Most of the time, most of these things don't matter, or matter very little, and you wind up using parts you're familiar with. There are almost 600 standard 1% resistor values, but way fewer standard capacitor values. Think about the circuit impedance and time constant, calculate the capacitor, adjust the cap value to the nearest standard value you have or can get, calculate the resistor based on that cap, and then adjust things. For example, if you end up with a high resistor value in what you know will be a noisy environment, scale down the resistor and up the cap, and then consider if your circuit can properly drive the lower impedance. If not, adjust again. There rarely is one perfect or obvious set of values.

ak

OBW0549, Reloadron and atferrari like this.
3. ### dl324 Distinguished Member

Mar 30, 2015
3,376
651
Pick a capacitance that has an appropriate tolerance, and other significant parameters, then determine required resistance. You can trim resistance with a potentiometer, or use only a pot.

4. ### wayneh Expert

Sep 9, 2010
12,378
3,231
+1
For example, consider the RC timing components for a 555. The solution to get a specific time (a specific RC time constant) is not presented as a single point, but rather as a line of R vs C. Any point on the line gives the same RC. Your choice then depends on other factors.

5. ### crutschow Expert

Mar 14, 2008
13,479
3,367
Usually the resistance value needs to fall within certain limits to operate properly in the specific circuit you have. So generally you select a resistor value within that range and then select the capacitor to get the desired time-constant.
Since capacitors generally come with fewer choices for values, you may have to then tweak the resistor value to get the desired RC value (assuming the common 1% resistor values and 5% capacitor values for example).

6. ### dl324 Distinguished Member

Mar 30, 2015
3,376
651
Six of one, half dozen of the other. I usually pick a cap and then resistor. If I can't get the time constant I want, I pick a different C. It's worked for me for a few decades...

It has been mentioned a couple times, there are fewer choices for C.

The problem is made even more difficult because the OP is looking for the "best" values. Sometimes you just go with what you have in your parts bin. One of my former instructors worked on a satellite and one of his circuits has a 1W resistor it didn't need...

7. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Depends on what your metric for "best" is.