RC / RL Time Constant
A first-order circuit charges to 63.2% of its final value in one time constant τ, and is within 1% after about 5τ. For RC, τ = R·C; for RL, τ = L/R.
RC time constant
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RC settled (5τ)
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RC cutoff
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RL time constant
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The RC cutoff is the −3 dB point of a simple low-pass filter.
A helper, not a substitute for engineering judgment. Verify against the governing standard.