Table of Contents
Inductors in Series
When inductors are connected in series, the total inductance is the sum of all individual inductances, provided there is no mutual coupling between them. This is analogous to resistors in series. The same current flows through each inductor, and the total voltage across the series combination equals the sum of voltages across each inductor.
In practice, inductors in series are used to achieve inductance values not available from standard components, in filter circuits, and in RF tuning applications. Mutual inductance between closely placed inductors can increase or decrease the total inductance depending on the orientation of their magnetic fields.
Series Inductance Formula
The inductive reactance XL represents the opposition to AC current flow at a given frequency. The energy stored in the magnetic field is proportional to the square of the current.
Inductive Reactance
| Inductance | XL at 1 kHz | XL at 10 kHz | XL at 100 kHz |
|---|---|---|---|
| 1 mH | 6.28 Ω | 62.8 Ω | 628 Ω |
| 10 mH | 62.8 Ω | 628 Ω | 6,280 Ω |
| 100 mH | 628 Ω | 6,280 Ω | 62,800 Ω |
Series vs Parallel
- Series: Total inductance increases. Ltotal = L1 + L2 + ... Same current through all.
- Parallel: Total inductance decreases. 1/Ltotal = 1/L1 + 1/L2 + ... Same voltage across all.
- Series is used when higher inductance is needed; parallel when lower inductance or higher current capacity is needed.
- Mutual coupling must be considered when inductors are physically close to each other.
Frequently Asked Questions
What happens to inductance when inductors are in series?
The total inductance increases. It equals the sum of all individual inductances. For example, a 10 mH and a 22 mH inductor in series give 32 mH total. This assumes no magnetic coupling between inductors.
Does mutual inductance affect series inductors?
Yes, when inductors are physically close, their magnetic fields can couple. If the fields are aiding (same direction), total inductance is L1 + L2 + 2M, where M is the mutual inductance. If opposing, it is L1 + L2 - 2M. This calculator assumes no mutual coupling.
Why use series inductors instead of a single inductor?
Series combinations allow achieving non-standard inductance values from standard components. They also distribute voltage stress across multiple components and can provide adjustable inductance when one inductor is variable (trimmer).