To add inductors in series, connect the output lead of the first inductor directly to the input lead of the second. The total inductance is the simple sum of the individual values ($L_{total} = L_1 + L_2$), provided the components are placed orthogonally or are magnetically shielded to prevent mutual coupling. While the inductance adds up, the maximum current handling capacity of the series string is strictly limited by the inductor with the lowest current rating in the chain.
The Series Inductor Topology: Node Labels and Core Behavior
When designing a series inductor network, we define the circuit using three primary nodes to track voltage drops and current flow:
- Node A (Input/Source): The entry point where current from the voltage source or preceding stage enters the first inductor ($L_1$).
- Node B (Junction): The physical and electrical connection point between $L_1$ and $L_2$. In a purely theoretical DC circuit, this node is just a wire. In high-frequency AC or switching circuits, Node B has parasitic capacitance to ground that can create unintended resonant poles.
- Node C (Output/Load): The exit point of the second inductor ($L_2$), feeding the load or the next stage of the filter.
The governing equation for series inductors (assuming zero mutual inductance, $M=0$) is:
$L_{total} = L_1 + L_2 + ... + L_n$
However, real-world inductors are not ideal. They possess parasitic DC Resistance (DCR). When you wire them in series, their resistances also add linearly:
$DCR_{total} = DCR_1 + DCR_2 + ... + DCR_n$
Why Series Over Parallel? And What Breaks at the Extremes
Why choose a series topology instead of parallel? You use series inductors when you need a higher total inductance value than what is physically available, cost-effective, or physically fits on your PCB in a single package. Parallel inductors are used when you need to increase current handling capacity while maintaining a specific inductance value. Series increases inductance but keeps the current bottlenecked; parallel keeps inductance lower but splits the current.
Understanding failure modes is critical for power supply and filter design. Here is how the series topology behaves under extreme fault conditions compared to a parallel configuration.
| Fault Condition | Series Topology Behavior | Parallel Topology Behavior |
|---|---|---|
| One Inductor Opens | Circuit breaks completely. Current drops to zero. Node B floats to source voltage. System fails safe (no power to load). | Total inductance drops. The remaining inductor is forced to carry 100% of the load current, likely leading to thermal overload or saturation. |
| One Inductor Shorts | Total inductance drops to the value of the remaining inductor. DCR drops. Current may spike if the control loop cannot compensate, risking downstream overvoltage. | Catastrophic failure. The shorted inductor creates a near-zero impedance path across the source, tripping breakers or destroying the driving MOSFET/IC. |
| One Inductor Saturates | The saturated inductor's value collapses to its air-core equivalent (near zero). Total inductance drops sharply, allowing high-frequency noise to pass through the filter. | The saturated inductor drops in impedance, hogging the majority of the ripple current. This accelerates its own thermal runaway while the other inductor remains underutilized. |
For a deeper look into how these fault conditions affect DC-DC converter stability, refer to the design guidelines in the Bourns 78F Series Radial Inductor Datasheet, which details saturation current ($I_{sat}$) versus RMS current ($I_{rms}$) limits.
Design Walkthrough: Building a 10mH Low-Pass Filter
Let us walk through a practical design scenario. You are building an audio crossover or a high-order LC low-pass filter and need exactly 10mH of inductance. You check your inventory and find you do not have a single 10mH inductor with a low enough DCR, but you have plenty of mid-range radial chokes.
Selected Components:
- L1: Bourns 78FR56K (5.6mH nominal, 1.2A max RMS current, 2.5Ω max DCR)
- L2: Bourns 78FR47K (4.7mH nominal, 1.4A max RMS current, 2.1Ω max DCR)
Step 1: Calculate Total Inductance
$L_{total} = 5.6mH + 4.7mH = 10.3mH$. This is within a standard 5% tolerance of our 10mH target.
Step 2: Determine the Current Bottleneck
Because the same current flows through Node A, Node B, and Node C, the maximum continuous RMS current of the string is limited by the weakest link. L1 is rated for 1.2A, and L2 is rated for 1.4A. The series string is strictly limited to 1.2A. Pushing 1.3A will cause L1 to overheat, even though L2 could theoretically handle it.
Step 3: Calculate Total DCR and Voltage Drop
$DCR_{total} = 2.5\Omega + 2.1\Omega = 4.6\Omega$.
At our maximum current of 1.2A, the DC voltage drop across the series pair will be $V = I \times R = 1.2A \times 4.6\Omega = 5.52V$. If this is a 12V system, losing 5.5V to inductor DCR is unacceptable. This highlights a major trade-off of series inductors: you achieve high inductance, but you accumulate parasitic resistance. For high-current applications, you must select physically larger inductors with thicker wire gauges to keep the combined DCR low.
Step 4: Manage Mutual Inductance ($M$)
If you place L1 and L2 side-by-side on the breadboard with their axes parallel, their magnetic fields will interact. The total inductance becomes $L_{total} = L_1 + L_2 \pm 2M$. Depending on the winding direction, your 10.3mH target could accidentally become 12mH or 8mH. To eliminate this, mount the inductors at a 90-degree angle (orthogonally) to one another, or keep them spaced at least two component-diameters apart. For more on magnetic coupling, see this All About Circuits guide on series and parallel inductors.
Step-by-Step Breadboard Testing Protocol
Do not trust the math until you verify it on the bench. Here is how to prototype and test your series inductor string using an LCR meter (like a Uni-Trend UT612 or similar bench meter).
- Zero and Calibrate: Short the probes of your LCR meter and zero out the lead resistance and parasitic inductance. Set the test frequency to 1kHz (standard for power/audio inductors) or 100kHz if you are designing an RF filter.
- Measure Individual Baselines: Measure L1 and L2 independently. Record both the inductance ($L$) and the equivalent series resistance ($ESR$ or $DCR$). Verify they are within the manufacturer's stated tolerance (usually ±10% for radial chokes).
- Wire the Series Topology: Insert L1 and L2 into the breadboard. Use a short jumper wire to connect the output lead of L1 to the input lead of L2 (creating Node B). Leave the input of L1 and output of L2 exposed for probing.
- Measure Parallel Alignment (The Error Check): With both inductors standing straight up (axes parallel), probe Node A and Node C. Note the reading. It will likely deviate from your calculated sum due to mutual coupling.
- Rotate to Orthogonal: Bend or reposition L2 so its physical body is at a 90-degree angle to L1. Probe Node A and Node C again. The reading should now snap closely to your calculated $L_{total}$ (e.g., ~10.3mH).
- Verify DCR: Switch your multimeter to the lowest Ohms range (or use a 4-wire milliohm meter if available). Measure the resistance across Node A and Node C. It should match the sum of the individual DCRs (e.g., ~4.6Ω). If it is significantly higher, you have a poor breadboard contact at Node B causing contact resistance.
Frequently Asked Questions
How to add inductors in series with different current ratings?
When wiring inductors with mismatched current ratings in series, the entire circuit is bottlenecked by the component with the lowest RMS current rating. For example, if you place a 5A inductor in series with a 1A inductor, the maximum safe continuous current for the entire string is 1A. Exceeding this will cause the 1A inductor's winding to overheat, potentially melting the enamel insulation and creating an internal short circuit. If you need higher current handling, you must wire inductors in parallel, not series.
Does adding inductors in series increase the maximum current capacity?
No. Wiring inductors in series increases the total inductance and the total DC resistance, but it does absolutely nothing to increase the current handling capacity. The same exact current flows through every component in a series loop. To increase current capacity, you must wire identical inductors in parallel, which splits the load current between them while reducing the overall inductance value. For a comprehensive breakdown of series vs parallel magnetic components, review the Electronics Tutorials guide on inductors in series.
What happens to the resonant frequency when inductors are added in series?
Adding inductors in series increases the total inductance ($L$). Because the self-resonant frequency (SRF) of an LC circuit is inversely proportional to the square root of inductance ($f_r = 1 / (2\pi\sqrt{LC})$), increasing $L$ will force the resonant frequency to drop. In filter design, this means a series inductor string will attenuate lower frequencies more effectively than a single smaller inductor, but it will also hit its self-resonant peak earlier, potentially allowing very high-frequency noise to bypass the filter through the inductors' parasitic parallel capacitance.
How does mutual inductance affect series inductor calculations?
If two series inductors are placed physically close to one another with their magnetic axes aligned, their magnetic flux lines will intersect. This creates mutual inductance ($M$). If their fields aid each other, the total inductance becomes $L_1 + L_2 + 2M$. If their fields oppose each other, it becomes $L_1 + L_2 - 2M$. This makes the circuit highly unpredictable and sensitive to physical layout. To prevent this, always place series inductors orthogonally (at 90-degree angles) or use magnetically shielded inductor types (like toroidal or shielded drum cores) which confine the flux path internally.
