Recipe

Size an RC for a delay or a filter

Use when: you need a capacitor voltage to take a chosen time to rise, fall, or settle

Goal: an R and C from your drawer whose time constant gives the delay you want

Taught in RC time constants

1. 1

   Write down the time you need and whether it is to about 63% (one τ) or to fully settled (five τ).

   Example

   A debounce filter that should ignore about 1 ms of bounce: aim for \tau \approx 1 ms.

   Why: one τ gets about two-thirds of the way and five τ gets over 99%, so the target sets τ.

2. 2

   Choose a common capacitor value: 100 nF, 1 μF, 10 μF.

   Example

   C = 100 nF, a ceramic from the kit.

   Why: capacitors come in fewer handy values than resistors, so fix C first.

3. 3

   R = \tau / C, in ohms and farads.

   Example

   R = 0.001\ \text{s} / (100 \times 10^{-9}\ \text{F}) = 10\,000\ \Omega.

   Why: τ = RC rearranged for the one unknown.

4. 4

   Use the nearest E12 value and recompute.

   • R comes out under about 1 kΩ — pick a bigger C, so the resistor doesn't draw a large current.
   • R comes out over about 1 MΩ — pick a smaller C, so tiny leakage currents don't upset the timing.

   Why: real capacitors are often ±10–20% off, so a standard R is always close enough, but extreme values cause new problems.