# RC frequency calculation

#### togbott

Joined Nov 20, 2011
8
Im struggling to build a formula to accept a frequency and capacitance as input, then return a value of resistors I should use.

I realize that TC = RC.

I think I understand that:
1/frequency = resonant frequency

However, Im stuck from this point out on how to compute the needed resistors for 100hz frequency with a 1uF capacitor.

#### togbott

Joined Nov 20, 2011
8
Just for clarification, I am hunting for a formula, not any specific answer.

#### wmodavis

Joined Oct 23, 2010
739
Not enough information.
Formula T=RC is a formula for time constant not frequency.

"I think I understand that:
1/frequency = resonant frequency" What? That does not make sense. Rather 1/f = t.

You seem to be talking about making an oscillator that will produce a frequency of 100 Hz. If so, an oscillator requires more than a resistor and a capacitor. So the rest of the story depends on what you are using for additional circuitry, and you haven't revealed that important even critical piece ofinformation to allow one to provide any substantive help.

#### togbott

Joined Nov 20, 2011
8
My apologies.

I think you may be right. I could be swimming between two different concepts... though I don't know for sure.

My understanding was I should be attempting to build a simple RC filter. If I wanted to try to dip power at x frequency/sec, I needed to build a low-pass filter. End goal: make LEDs dim and brighten using the unique charge/discharge (graph) characteristic of capacitors.

While I realize there are time-honored "timer" circuits, I was excited at the simple nature of RC circuits.

#### crutschow

Joined Mar 14, 2008
26,024
The formula for the "corner" frequency (-3dB point) of a simple RC filter is RC = 1/(2$$\pi$$f).

There is no "resonant" frequency of an RC circuit.