Use an explicitly defined RC charging model to separate its time constant from a chosen completion criterion.
Define the electrical model
For an initially uncharged ideal capacitor charging through resistance R from a constant source, its voltage fraction is 1−exp (−t/RC). The time constant is RC. Primary reference: RC charging.
The model includes its initial condition and connection. A capacitor already carrying charge, an active drive or another current path can require a different expression. A time constant is a parameter of the selected model, not a universal declaration that charging has finished.
A hypothetical level comparison
Take fictional R = 10 kΩ and C = 10 µF, giving RC = 0.10 s. The modeled voltage reaches about 63.2% of its final value after 0.10 s. Reaching 90% requires −0.10 ln (0.10), approximately 0.2303 s. Reaching 99% requires approximately 0.4605 s.
The same physical model produces different reported times because the requested fractions differ. These values do not set a PLC filter, a relay release time or a machine-ready condition. A threshold-triggered circuit needs its own actual threshold and tolerance evidence.
Prepare a model worksheet
Record resistance, capacitance, initial voltage, source behavior and the target fraction. Calculate the time under those assumptions and retain the input units.
Ask whether parasitic paths and component variation matter to the intended conclusion. Keep a modeled capacitor voltage distinct from a measured output response. That separation helps identify whether an offered timing statement concerns a circuit parameter or an actual application requirement.
Customer Questions
Is one RC the end of charging?
The ideal model reaches only about 63.2% at that time.
Does 99% take the same time as 90%?
The stated fractional criterion changes the calculated time.
Does this prescribe a control delay?
It is a circuit-model comparison, not an approved setting.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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