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 |
How to use this chart
Convert capacitance first
Convert uF, nF, or pF into farads before multiplying by resistance so the calculator result is in seconds.
Pick the milestone
Use one tau for first response, three tau for near-settled checks, or five tau for practical full-charge and discharge notes.
Tie timing to a threshold
After the calculator result, record the voltage threshold or measurement point that decides whether the time is acceptable.
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
Panel input debounce estimate
With R = 10 kOhm and C = 47 uF, tau = 0.47 seconds. A five-tau settling note is about 2.35 seconds, before checking whether the input threshold trips earlier.
Bleeder discharge timing note
With R = 100 kOhm and C = 220 uF, tau = 22 seconds and five tau is about 110 seconds. The chart gives a timing estimate, but the voltage must still be measured at the actual terminals.
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Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.