In a steady-state DC circuit, the voltage drop over an inductor is purely resistive and calculated by Ohm's Law using its DC Resistance (DCR): V = I × DCR. In AC circuits or switching transients, the voltage drop is governed by inductive reactance (XL = 2πfL) and Faraday's law of induction (V = L(di/dt)). If you are sizing an inductor for a buck converter or an AC line filter, ignoring the AC losses and focusing only on the datasheet DCR will result in undersized components and thermal failure.
The Core Physics: Why Voltage Drops Across an Inductor
To accurately predict the voltage drop over an inductor, you must separate the DC losses from the AC losses. Every real-world inductor is a non-ideal component consisting of an ideal inductor in series with a resistor (the copper wire) and a parallel parasitic capacitor.
1. Steady-State DC Voltage Drop
When DC current flows, the magnetic field is static. The inductor acts as a simple wire. The voltage drop is strictly I × DCR. For example, a 10µH power inductor with a 20mΩ DCR carrying 5A of continuous DC current will drop exactly 0.1V. The power lost as heat is I²R (25 × 0.020 = 0.5W).
2. Transient and AC Voltage Drop
When current changes, the inductor opposes that change. In a switching regulator operating at 500kHz, the current is a triangular ripple wave. The voltage drop here includes the DCR loss plus core losses (hysteresis and eddy currents). According to Analog Devices application notes on inductor selection, core losses can easily equal or exceed copper DCR losses at high frequencies, meaning your actual measured voltage drop and thermal dissipation will be significantly higher than the DC math suggests.
Inductor Types and Selection Criteria
Choosing the right inductor dictates how it handles voltage drop, saturation, and thermal load. Here is how the core construction impacts performance and which type fits your specific job.
| Core Material | Construction Style | Typical Tolerance | Tempco (ppm/°C) | Best Application |
|---|---|---|---|---|
| Air Core | Self-supporting coil | ±2% to ±5% | ~+50 (Copper only) | RF tuning, high-frequency filters (>10MHz) where core saturation must be avoided entirely. |
| Shielded Ferrite | Molded drum/bobbin | ±10% to ±20% | -1000 to -3000 | DC-DC buck/boost converters. Tight magnetic field prevents EMI coupling to nearby traces. |
| Powdered Iron | Distributed air gap | ±10% to ±15% | +50 to +350 | AC line filtering, high-current chokes. The distributed gap prevents hard saturation at high DC bias. |
| Toroidal (Ferrite/Iron) | Ring core, wound | ±10% to ±20% | Varies by mix | Audio crossovers, mains EMI suppression. Lowest external magnetic leakage and high efficiency. |
Which type for which job? If you are building a 5V/3A buck converter, use a shielded ferrite inductor to keep switching noise off your analog rails. If you are building a 120V AC EMI filter for a motor drive, use a powdered iron or toroidal core; ferrite will saturate instantly under the 60Hz AC peak current, causing a massive, uncontrolled voltage drop and tripping your upstream breaker.
Decoding Inductor Markings and Safe Substitution
When you are scavenging parts or replacing a failed component, reading the physical markings is critical to predicting the voltage drop and current handling.
Reading SMD and Through-Hole Codes
Surface mount inductors typically use a 3-digit code similar to resistors, but the base unit is microhenries (µH), not ohms.
- 100: 10 × 100 = 10µH
- 471: 47 × 101 = 470µH
- 4R7: The 'R' acts as a decimal point = 4.7µH
Through-hole axial inductors often use a 4-band color code (identical to resistors) but the values are in microhenries. A Brown-Black-Brown-Silver band translates to 1-0-×10 with 10% tolerance, yielding 100µH. For precise decoding, the Coilcraft Inductor Finder and datasheet cross-references are mandatory, as some manufacturers use proprietary lot codes instead of value codes on tiny 0402 packages.
How to Substitute Safely
If the exact BOM part is out of stock, you can substitute, but you must respect three electrical boundaries to maintain the expected voltage drop and prevent failure:
- Match or Exceed Isat and Irms: Isat (saturation current) is where inductance drops by 20-30%. Irms is the thermal limit where DCR heating raises the part by 40°C. Never substitute a part with a lower Isat; the inductor will turn into a short circuit during peak load.
- Check the DCR: If your circuit relies on a low voltage drop over the inductor for efficiency, a substitute with higher DCR will waste power as heat. Keep substitute DCR within ±15% of the original.
- Inductance Tolerance: For power supply filtering, a 4.7µH part can be replaced by a 6.8µH part (shifting the LC pole slightly). For tuned RF circuits or precise timing filters, you must match the exact value and tolerance.
Failure Modes and Visual Diagnostics
Inductors rarely fail silently. Because they handle high current and store magnetic energy, their failure modes are usually thermal or mechanical. Here is what to look for on the bench.
- Open Circuit (Burnt Winding): Visual Symptom: Discoloration of the potting compound or a visible scorch mark on the PCB pads. The enamel insulation on the copper wire melts due to excessive I²R heating, fusing the wire. Multimeter Test: Reads OL (infinite resistance).
- Shorted Turns (Insulation Breakdown): Visual Symptom: The component looks physically fine, but the circuit runs hot and the switching frequency shifts. Multimeter Test: DCR reads significantly lower than the datasheet spec. This happens when voltage spikes break down the thin enamel between adjacent windings, effectively reducing the number of turns and the inductance.
- Cracked Ferrite Core: Visual Symptom: Visible hairline fracture on the drum or shielded casing, often accompanied by an audible high-pitched whine (coil whine) during operation. Caused by mechanical shock or thermal cycling. This introduces an unintended air gap, dropping the inductance and increasing the AC voltage drop due to localized saturation.
- Core Saturation (Functional Failure): Visual Symptom: No physical damage to the inductor, but the switching MOSFET in the circuit is blown. The inductor was subjected to a current beyond its Isat rating, causing the magnetic core to saturate. Once saturated, the inductor acts as a dead short, spiking the current and destroying the drive transistor.
Frequently Asked Questions
Why is my measured voltage drop over an inductor higher than the datasheet DCR suggests?
If you measure the voltage drop across an inductor in an active AC or switching circuit and it exceeds the calculated I × DCR value, you are measuring the combined impedance, not just resistance. The total voltage drop includes the reactive drop (I × XL) and core losses (hysteresis and eddy currents). To isolate the pure resistive drop, you must measure the component out-of-circuit with a DC milliohm meter, or measure the phase angle with an oscilloscope to separate the real power (heat) from the reactive power.
Does the voltage drop over an inductor change with ambient temperature?
Yes. The DC resistive voltage drop increases as temperature rises. Copper wire has a positive temperature coefficient of roughly +0.39% per °C. If an inductor has a 20mΩ DCR at 20°C, and it heats up to 100°C under load, the DCR will increase to approximately 26mΩ. This means your voltage drop and thermal losses will increase by 30% at operating temperature compared to the cold datasheet specification. Always calculate thermal runaway margins using the hot DCR, not the room-temperature DCR.
How do I measure the true AC voltage drop over an inductor in a switching regulator?
Do not use a standard digital multimeter; the high-frequency PWM ripple will confuse the ADC and yield garbage readings. Use an oscilloscope with a differential voltage probe or two matched passive probes using the math function (Ch1 - Ch2) to measure across the inductor terminals. This rejects the common-mode switching noise. According to All About Circuits AC theory guidelines, measuring the inductor voltage directly reveals the switching duty cycle and the exact di/dt slope, allowing you to calculate the true dynamic inductance under load.
Can I wire two inductors in parallel to reduce the overall voltage drop?
Technically, wiring two identical inductors in parallel halves the effective DCR, which reduces the DC voltage drop and shares the thermal load. However, the total inductance is also halved (L/2), which will double your ripple current in a power supply. Furthermore, unless the two inductors are perfectly matched in DCR and physically thermally coupled, one will hog the current, overheat, and fail, shifting the entire load to the second one in a cascading failure. It is almost always better to select a single, larger inductor with a lower DCR and higher current rating than to parallel two smaller ones.
