RC time-constant designer

Assumptions to check

• One ideal resistance and one ideal capacitance.
• A constant source or discharge path.
• No leakage, source impedance, load, dielectric absorption, or capacitor voltage coefficient.

Timing is exponential

An RC network does not move through a voltage interval at a constant rate. Its time constant τ equals R × C, and the remaining gap to the final voltage decays exponentially. That matters when a reset threshold, a comparator threshold, or an analog timing window is close to the practical boundary.

This engine gives the state at a selected time and a five-time-constant reference. It keeps the result tied to the declared first-order model so a real source impedance, load, capacitor leakage, or device threshold is not mistaken for a small detail.

Worked example

With 10 kΩ and 1 µF, τ is 10 ms. Charging from 0 V toward 5 V reaches about 3.16 V after one time constant. Five time constants is 50 ms, a useful near-settled reference rather than an exact endpoint. A threshold-driven circuit should calculate the actual threshold crossing instead of assuming every event happens at one τ.

Limits and next checks

The ideal response is not a component tolerance result. Electrolytic leakage, ceramic DC-bias loss, switch resistance, the following stage’s input resistance, and threshold hysteresis can dominate. Use measured or data-sheet values when timing has a tight limit, then check the actual threshold with worst-case R and C values.

Common mistakes

• Treating five time constants as an exact endpoint.
• Ignoring the input resistance or leakage that changes the effective resistance.
• Using a nominal capacitor value where DC bias or tolerance changes capacitance substantially.

Model limit and handoff

Use component tolerance, leakage, source resistance, load resistance, and the receiving threshold limits for a corner analysis or measured timing validation.

FAQs