Understanding Series Inductance
When inductors sit in series, current has only one path through the circuit—it must pass through each coil in sequence. This differs fundamentally from parallel arrangements, where current can split across multiple branches.
The voltage drop across each inductor depends on its individual inductance and the rate of current change. A 10 H inductor will produce four times the voltage drop of a 2.5 H inductor under identical current conditions. Total voltage across the series string equals the sum of all individual voltage drops.
Series inductances work identically to series resistances in their addition rule: values simply sum. A 5 H inductor in series with a 10 H inductor behaves electrically like a 15 H inductor when AC current flows through them.
Calculating Total Series Inductance
The equivalent inductance of inductors in series equals the arithmetic sum of all individual inductances. Apply Faraday's law to each coil separately, then add the induced voltages.
Leq = L₁ + L₂ + L₃ + … + Ln
L<sub>eq</sub>— Equivalent or total inductance of the series circuitL₁, L₂, L₃, …, L<sub>n</sub>— Individual inductance values of each coil in the circuit
Series vs. Parallel Inductors
Series configuration: Inductors connect end-to-end. Current remains uniform through all components. Total inductance increases.
Parallel configuration: All inductor starts connect at one node; all ends connect at another. Current splits among branches. Equivalent inductance decreases (reciprocal sum rule applies).
Choose series connections when you need higher total inductance from available components. Choose parallel when you need to reduce inductance or allow separate current paths. Most filter and tuning circuits use series inductances because they're easier to analyze and provide predictable frequency response.
Common Pitfalls with Series Inductors
Avoid these mistakes when designing or analyzing series inductor circuits.
- Ignoring wire resistance and core losses — Real inductors have small series resistances and non-ideal cores. A 100 H inductor might have 0.5 Ω of wire resistance, which affects circuit behavior at high currents. Always check component datasheets for these parasitic values—they grow more important at higher frequencies.
- Confusing series and parallel rules — Series inductances add directly (like resistors in series). Parallel inductances follow the reciprocal rule. Reversing these formulas produces dangerously wrong calculations. A simple way to remember: series is additive; parallel requires division.
- Overlooking mutual inductance — When inductors sit physically close, their magnetic fields interact. This mutual inductance can either increase or decrease the total depending on winding direction. Most calculators assume zero coupling; measure your actual circuit if coils are tightly wound or intertwined.
- Exceeding current ratings — Series inductors share the same current. If your circuit's peak current exceeds the component rating of any single inductor, core saturation occurs and inductance collapses. Always verify that every inductor in the string can handle your full operating current.
Practical Calculator Usage
Enter individual inductance values for each component. The calculator accepts inductances in henries (H), millihenries (mH), microhenries (μH), or nanohenries (nH)—choose the unit most convenient for your values.
By default, the tool calculates equivalent inductance from your component values. Alternatively, switch to solve for unknown mode to determine a missing inductance: specify the desired total inductance and all other component values, then the calculator returns the unknown inductor needed.
You can add up to ten separate inductors. For circuits with more components, calculate the equivalent of early groups, then treat each group as a single inductor and calculate again.