WorksheetLow code sensitivityLast reviewed June 7, 2026
Electrical reference chart
RC Time Constant Chart
Use this RC time constant chart after the calculator result to document resistance, capacitance, tau, charge or discharge milestone, and the measurement point being checked.
Quick reference table
The RC time constant is tau = R x C. This circuit worksheet reference shows that 10 kOhm x 47 uF gives tau = 0.47 seconds, and five tau is about 2.35 seconds. A charging capacitor reaches about 63% after one tau; use the chart for charge, discharge, debounce, or delay checks while still verifying the real threshold and voltage.
RC milestone reference
| Elapsed time | Charging level | Discharge remaining | Use in a worksheet |
|---|---|---|---|
| 1 tau | 63% | 37% | First response estimate |
| 2 tau | 86% | 14% | Fast delay approximation |
| 3 tau | 95% | 5% | Near-settled timing note |
| 4 tau | 98% | 2% | Tighter settling estimate |
| 5 tau | 99%+ | <1% | Practical full-charge or discharge checkpoint |
Common RC result contexts
| Circuit task | What tau tells you | What to verify next |
|---|---|---|
| Input debounce | Approximate filtering delay | Switch bounce and logic threshold |
| Relay or transistor delay | Ramp timing at a control node | Trigger voltage, leakage, and tolerance |
| Bleeder discharge | Time estimate to reduce stored voltage | Actual measured voltage before handling |
| Low-pass filter estimate | Response speed before frequency review | Use reactance or filter calculator for AC response |
RC chart to calculator handoff
| Search or worksheet need | Use this chart for | Open the calculator when |
|---|---|---|
| RC time constant chart | Choosing one, three, or five tau as a quick milestone | Resistance, capacitance, and elapsed time need a repeatable result |
| Capacitor charge time | Estimating percent charged at common milestones | A specific voltage threshold or elapsed time must be calculated |
| Capacitor discharge time | Documenting discharge milestones before a measurement note | Stored-voltage timing has to be checked against actual R and C values |
| RC delay or debounce | Separating tau from the real switching threshold | A relay, input, transistor, or comparator threshold decides the delay |
Formula basis
tau = R x C. Charging fraction = 1 - e^(-t / tau). Discharging fraction = e^(-t / tau).
- tau is the RC time constant in seconds.
- R is resistance in ohms.
- C is capacitance in farads.
- t is elapsed time in seconds.
- Charge and discharge curves are exponential, not straight-line ramps.
Worked examples
Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.
- The chart assumes a simple first-order RC circuit with one dominant resistance and capacitance.
- Real timing shifts with capacitor tolerance, leakage, dielectric behavior, resistor tolerance, source impedance, and trigger threshold.
- Elapsed time is not proof of zero voltage or safe handling; measurement is still required where stored energy matters.
Code and standard notes. Planning limits that should be checked before final equipment selection.
- This chart is a timing worksheet and does not replace equipment instructions, component ratings, lockout procedures, or voltage verification.
How to use this chart
Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.
- Record R and CWrite the actual resistance path and capacitor value, including tolerance or measured value when timing matters.
- Calculate tau and milestonesDocument one, three, and five time constants so the result can be compared with the circuit behavior.
- Verify the real nodeMeasure or document the control node, threshold, or capacitor terminal voltage that proves the timing estimate matches the circuit.
Common mistakes to avoid. Review these before turning chart current into an equipment decision.
- Multiplying kOhms by uF without converting units and then reading the time in the wrong scale.
- Assuming one time constant means the capacitor is fully charged or fully discharged.
- Ignoring the actual trigger threshold, leakage path, or connected input that changes the apparent delay.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.
Why does five tau matter?
Five time constants is a practical estimate for near-complete charge or discharge in a simple RC circuit, although the curve never becomes mathematically exact.
Does tau equal the delay time?
Not always. Tau is the curve constant; the actual switching delay depends on the threshold voltage of the input, relay, transistor, or comparator.
Is RC timing the same as capacitive reactance?
No. RC timing describes voltage change over time. Capacitive reactance describes AC opposition at a specific frequency.
When should I use the RC calculator instead of the chart?
Use the calculator when the exact resistance, capacitance, elapsed time, or threshold voltage must be entered and saved as a project-specific timing result.