Inductor timing tool
RL Time Constant Calculator
Calculate tau for an inductor-resistor circuit using inductance, resistance, and first-order circuit assumptions.
Calculate RL Time Constant
Solve tau from inductance and resistance, or back-calculate L or R for a target first-order response.
Result
Time constant
0.01 s
Result notes. Keep inputs, assumptions, and result together before using this value in project records.
Keep the entered values, assumptions, and result together when adding this calculation to job notes or submittal records. Final installation choices should align with the applicable code edition, equipment listing, manufacturer instructions, local amendments, and AHJ requirements.
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Related charts
Inductor Reactance Chart
Use this inductor reactance chart for XL = 2 pi f L: 100 mH at 60 Hz = 37.7 ohms, 10 mH at 1 kHz = 62.8 ohms, and coil checks.
Impedance Reactance Chart
Use this impedance reactance chart for XL = 2 pi f L, XC = 1/(2 pi f C), 100 mH at 60 Hz = 37.7 ohms, and 100 uF = 26.5 ohms.
Series Parallel Resistance Chart
Use this series parallel resistance chart: 100 ohm + 220 ohm = 320 ohm series, 330 ohm || 680 ohm = 222 ohm, then mixed total = 442 ohm.
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Related tools
Formula and field context
Calculate tau for an inductor-resistor circuit using inductance, resistance, and first-order circuit assumptions.
Formula context
Inductor Reactance Chart
Inductive reactance is XL = 2 pi f L: 100 mH at 60 Hz = 37.7 ohms, while 10 mH at 1 kHz = 62.8 ohms. Reactance rises as frequency or inductance increases. Use this chart after the calculator result to separate ideal reactance from real coil behavior such as winding resistance, saturation, current rating, and core material.
Formula
XL = 2 x pi x f x L. Ideal AC current estimate: I = V / XL.Variables to keep with the result
- XL is inductive reactance in ohms.
- f is frequency in hertz.
- L is inductance in henries.
- Winding resistance is separate from reactance and must be added in real impedance checks.
Formula and variables
The RL time constant is tau = L / R. Inductance is converted from millihenries to henries and resistance is entered in ohms. The result is seconds. A higher inductance increases the time constant, while a higher series resistance reduces it. This relationship describes the first-order current response of a simple inductor-resistor circuit.
U.S. field context and example
RL timing can support notes for relay coils, solenoids, chokes, control circuits, discharge paths, and troubleshooting where an inductive load does not respond instantly. For example, a 100 mH inductor with 10 ohms of series resistance has tau = 0.1 / 10, or 0.01 seconds. Keep the resistance basis clear: it may include winding resistance, an added resistor, source impedance, or the effective resistance of the circuit path.
Assumptions and limits
This is a simple first-order model. Real coils can include core saturation, back EMF, temperature-driven resistance changes, contact behavior, suppression devices, and driver limitations. Use the full inductor, impedance, or circuit calculator when AC reactance, stored energy, measured waveform behavior, or equipment-specific coil data is part of the decision.
Common mistakes
Common errors include using millihenries as henries, leaving winding resistance out of the resistance value, assuming the time constant is an exact operate or dropout time, and ignoring suppression diodes or MOVs across coils. Document whether the values are measured, nameplate, or assumed before using the result in job notes.