What are valid RC time constant are there general rules or capacitance to resistance ratios that are recommended?

Like a 1 Farad capacitor (which if existed would be huge) and a three ohm resistor makes a real bad choice for a three second delay.

 

You always try to keep the capacitor small and the resistance large because all (small) resistors are the same price but big capacitors cost big money.

The other people may now point out the exceptions.

 

I agree with #12.

An upper limit for the value of the resistor is given by the fact that in a real world circuit there are always parasitic resistances that are in parallel to the nominal resistance, their exact value is often not known.

So you want to keep the resistor much smaller than the parasitics, so that they don't have much influence.

 

It mostly depends on the impedance of the circuit you are tapping into the RC circuit with. Modern JFet op amps are extremely good.

There is also the issue of really large electrolytic caps being extremely leaky. This makes them unusable for many applications.

 

What are valid RC time constant are there general rules or capacitance to resistance ratios that are recommended?

Like a 1 Farad capacitor (which if existed would be huge) and a three ohm resistor makes a real bad choice for a three second delay.

One Farad capacitors do exist and they really aren't huge.

 

One Farad capacitors do exist and they really aren't huge.

Yes, but, they cannot be used anywhere. Like the more common ones.