A resistor feeding a capacitor makes the simplest timing circuit there is. The capacitor's voltage does not jump; it rises along a curve whose speed is set by the product of R and C — the time constant. Laptop boards use RC pairs to delay enable signals, to create reset pulses, to slow down rail start-up and to filter noise. This calculator gives the time constant, the time to settle and the filter cutoff frequency.

How to use it

  1. Enter the resistance and the capacitance, with their units.
  2. Read the time constant τ, the 5τ settling time and the cutoff frequency.
  3. Use the table to see how far the capacitor has charged or discharged after each time constant.

The formulas

  • τ = R × C — in seconds when R is in ohms and C in farads.
  • After 1τ the capacitor reaches about 63 % of its final voltage; after 5τ, more than 99 %.
  • As a low-pass filter, the cutoff frequency is fc = 1 ÷ (2π × R × C).

Worked example: an enable delay

A regulator's enable pin is pulled up through 100 kΩ and has 0.1 µF to ground. τ = 100,000 × 0.000 000 1 = 10 ms. If the enable threshold is around 60 % of the pull-up voltage, the regulator starts roughly one time constant after power appears — about 10 ms later than the rail feeding the pull-up. That is a deliberate step in the power sequence. A capacitor that has gone leaky, or the wrong value fitted during repair, changes that delay, and in rare cases breaks the sequence. Sequencing is covered in Power Sequence Pages in Laptop Schematics Explained.

Where it matters on a laptop board

  • Delays and soft start on enable and reset lines.
  • Filters on sense inputs, thermistor lines and audio paths.
  • Discharge of capacitors through bleed resistors after power is removed — see Capacitor Discharge Calculator.
  • Readings that drift on your meter: when you measure resistance on a big rail, the meter is charging an RC network and the value climbs.

The underlying idea is described in the glossary under capacitance.

FAQ

Is the capacitor fully charged after 5τ?

For practical purposes, yes: it is within 1 % of the final voltage. Mathematically it never quite gets there.

Why does my RC delay not match the calculation?

Ceramic capacitors lose capacitance under DC voltage, and chip inputs may have their own pull-up or leakage. Expect the real delay to be somewhat shorter or longer.

What is the cutoff frequency used for?

For filters: signals well below it pass, signals well above it are attenuated. Noise filters on sense lines use it.