Magnetism by induction is the process where a changing magnetic field through a conductor generates an electromotive force (voltage) across that conductor. In a real circuit, this principle dictates your energy storage limits, ripple current, and the physical footprint of any switching power supply, transformer, or wireless charging coil you build. Makers frequently confuse it with electrostatic induction (charging an object via electric field proximity without magnetic fields) or assume it only applies to massive AC generators rather than the microscopic SMD components sitting next to their microcontrollers.
The Core Physics (and the One Analogy You Need)
At the bench, we rely on Faraday’s Law of Induction, which states that the induced voltage is proportional to the rate of change of the magnetic flux. For an inductor, we compress this into the practical circuit equation:
V = L × (di / dt)
Where V is the voltage across the inductor, L is the inductance in Henries, and di/dt is the rate of change of current over time. This equation tells us that an inductor resists changes in current. The faster you try to change the current, the higher the voltage spike the inductor generates to fight that change.
Think of an inductor as a heavy, mechanical water wheel placed inside a plumbing pipe. When you first open the valve, the water (current) cannot flow instantly because the heavy wheel (magnetic field) takes time to spin up. Once the wheel is spinning at full speed, it maintains a steady flow. If you suddenly slam the valve shut, the momentum of the heavy wheel keeps pushing water forward, creating a massive pressure spike (voltage spike) that can burst the pipe if you don't have a relief valve (a flyback diode or snubber).
Worked Numeric Example: Sizing a Buck Converter Inductor
Let’s apply magnetism by induction to a real design problem. You need to step down a 12V battery to 5V to power an ESP32 and a few sensors, drawing a maximum of 2A. You select a buck converter IC switching at 500 kHz.
First, we decide on our ripple current (ΔIL). A standard industry rule of thumb is to set the peak-to-peak ripple at 30% of the maximum output current.
Now, we use the standard buck converter inductance formula derived directly from Faraday's Law:
L = [ Vout × (Vin - Vout) ] / [ Vin × fsw × ΔIL ]
Plugging in our real values:
- Vout = 5V
- Vin = 12V
- fsw = 500,000 Hz
- ΔIL = 0.6A
L = [ 5 × (12 - 5) ] / [ 12 × 500,000 × 0.6 ]
L = 35 / 3,600,000
L = 0.00000972 Henries, or 9.72 µH
We round to the nearest standard E12 series value: 10 µH. But inductance is only half the battle. Because of magnetism by induction, the physical core material can only hold so much magnetic flux before it saturates. You must calculate the peak current (Ipeak):
Ipeak = Iout + (ΔIL / 2) = 2A + 0.3A = 2.3A
Your chosen 10 µH inductor must have a saturation current (Isat) rating strictly greater than 2.3A, or the core will saturate, inductance will plummet to near zero, and your switching IC will short out and burn.
Where You Meet This in Practice
While the math above applies to power supplies, the physical phenomenon of magnetism by induction shows up across several distinct DIY and prosumer domains:
- Switching Power Supplies (Buck/Boost/Flyback): Storing energy in a magnetic field during the MOSFET's 'on' time and releasing it to the load during the 'off' time.
- Wireless Charging (Qi Standard): A transmitter coil generates an alternating magnetic field at 110–205 kHz. A receiver coil in the phone experiences a changing flux, inducing a voltage to charge the lithium cell.
- BLDC Motor Regenerative Braking: When you short the phases of a spinning brushless motor, the rotating permanent magnets induce a reverse voltage that creates a braking torque, dumping kinetic energy back into the battery via the ESC.
- Inductive Proximity Sensors: Metal detectors and CNC tool-height probes use an LC oscillator. When metal enters the magnetic field, eddy currents are induced in the metal, draining energy from the coil and shifting the oscillation frequency.
Decision Tree: Picking the Right Inductive Component
Do not just buy the cheapest 10 µH inductor on Amazon. The core material and shielding dictate whether your circuit works or fails. Use this decision matrix to terminate your part selection.
| Application Scenario | Core Material | Shielding Requirement | Concrete Part Pick |
|---|---|---|---|
| High-Current Buck (2A-10A) ESP32 main rail, motor drivers |
Ferrite or Carbonyl Iron | Shielded (Molded) Prevents EMI from resetting nearby logic |
Bourns SRP1265A-100M (10µH, 11A Isat, shielded SMD) |
| Low-Current IoT Buck (<1A) Battery-powered sensor nodes |
Ferrite | Semi-Shielded or Unshielded Acceptable if no sensitive RF traces nearby |
Wurth 74404054100 (10µH, 1.6A Isat, compact SMD) |
| RF Filtering / Matching Antenna matching, high-Q LC tanks |
Air Core or Ceramic | Unshielded Low core loss at MHz frequencies is critical |
Coilcraft 0805CS-100X (10nH, high Q, wirewound chip) |
| Boost Converter (High Voltage) Nixie tubes, LED drivers |
Powdered Iron | Shielded Handles high DC bias without early saturation |
Coilcraft DO3316P-103 (10µH, high voltage isolation) |
Common Failure Modes When You Ignore Induction Limits
I once watched a student fry a $15 TI TPS54308 buck controller because they used an unshielded 10µH drum core inductor that saturated at 1.2A. When the core saturated, the magnetic field collapsed, inductance dropped to near zero, and the component effectively became a piece of wire. This shorted the 12V input rail directly to ground through the high-side MOSFET, instantly vaporizing the silicon.
Here are the three ways magnetism by induction will bite you if you ignore the datasheet:
- Core Saturation (The Silent Killer): Every magnetic core has a limit defined by its B-H curve. Once you hit the 'knee' of the curve, adding more current no longer increases the magnetic flux. Fix: Always check the Isat (saturation current) rating, not just the Irms (thermal heating) rating. Isat must exceed your peak ripple current.
- EMI Radiation (The WiFi Killer): Unshielded inductors leak magnetic flux into the surrounding air. If you place an unshielded drum-core inductor within 15mm of an ESP32 or a 2.4GHz antenna trace, the 500 kHz switching harmonics will desensitize your receiver, dropping your WiFi range from 50 feet to 5 feet. Fix: Pay the $0.40 premium for molded, fully shielded inductors for any board with wireless radios.
- Parasitic Ringing (The Voltage Spike): The physical wire wrapped around the core has parasitic capacitance. When the MOSFET switches off, the inductor's stored magnetic energy resonates with this parasitic capacitance, creating high-frequency voltage spikes that can exceed the breakdown voltage of your diode. Fix: Add an RC snubber network across the switching node and ground, or select an inductor with a high self-resonant frequency (SRF).
Frequently Asked Questions
Can I put two 5µH inductors in series to get 10µH?
Yes, inductors in series add linearly (Ltotal = L1 + L2), provided they are physically placed far apart so their magnetic fields do not interact. If you place them close together, mutual induction will alter the total value depending on their physical orientation.
Why do some inductors have a gap in the core?
An air gap deliberately introduces a region of high magnetic reluctance. This prevents the core from saturating at high DC currents. Gapped ferrite cores are standard in flyback transformers and high-current buck inductors, trading a slight drop in overall inductance for a massive increase in saturation current handling.
Does the physical orientation of the inductor on the PCB matter?
Absolutely. For shielded inductors, it matters less, but for unshielded or semi-shielded parts, you must orient the coil's magnetic axis perpendicular to any sensitive analog traces or antenna feeds to minimize induced noise. Check the manufacturer's datasheet for the specific magnetic flux leakage diagram.
For 90% of DIY and prosumer DC-DC builds, the decision is straightforward: calculate your required microhenries, verify your peak current, and select a shielded ferrite SMD inductor with a 30% safety margin on the saturation current rating. Do not compromise on shielding if your board includes wireless communication.
