The most common inductor values follow the E12 series (1.0, 1.5, 2.2, 3.3, 4.7, 6.8, 10, 15, 22, 33, 47, 68, 100 µH, etc.). These values are spaced logarithmically to cover a full decade with a 10% tolerance overlap, ensuring that no matter what exact inductance a circuit demands, a standard off-the-shelf part will be within 10% of the target. Whether you are designing a 500 kHz buck converter or an RF matching network, understanding how these values scale, how they are marked, and how to substitute them when your BOM is out of stock is fundamental bench knowledge.
The Standard Common Inductor Values (E-Series)
Inductors, like resistors and capacitors, are manufactured according to the IEC 60063 preferred number series. For general-purpose power and RF inductors, the E12 series (12 values per decade) is the baseline. Tighter tolerance RF inductors might use the E24 series, while bulk power chokes often stick to E6 (1.0, 1.5, 2.2, 3.3, 4.7, 6.8) because core saturation limits how precise the manufacturing can be anyway.
| Multiplier | E12 Base Values (nH) | E12 Base Values (µH) | E12 Base Values (mH) |
|---|---|---|---|
| 10^0 | 1.0, 1.5, 2.2, 3.3, 4.7, 6.8 | 1.0, 1.5, 2.2, 3.3, 4.7, 6.8 | 1.0, 1.5, 2.2, 3.3, 4.7, 6.8 |
| 10^1 | 10, 15, 22, 33, 47, 68 | 10, 15, 22, 33, 47, 68 | 10, 15, 22, 33, 47, 68 |
| 10^2 | 100, 150, 220, 330, 470, 680 | 100, 150, 220, 330, 470, 680 | 100, 150, 220, 330, 470, 680 |
Source: Standard preferred values derived from IEC 60063.
Inductor Types: Which Core for Which Job?
Selecting the right common inductor value is only half the battle; the core material dictates how that value behaves under load, temperature, and frequency. Here is the selection criteria for the four primary inductor constructions you will encounter on the bench.
| Core Type | Construction | Typical Tolerance | Tempco (ppm/°C) | Typical Use Case |
|---|---|---|---|---|
| Air Core | Wire wound on non-magnetic ceramic/plastic form | ±1% to ±5% | Near 0 (wire expansion only) | High-frequency RF, VHF/UHF matching, filters |
| Ferrite Core | Wire wound around MnZn or NiZn ferrite bobbin/drum | ±10% to ±20% | +1000 to +3000 (highly non-linear) | Switch-mode power supplies (SMPS), EMI chokes, broadband transformers |
| Iron Powder | Insulated iron particles compressed into a toroid or core | ±10% to ±15% | +50 to +300 (very stable) | High-current DC-DC converters, output filter chokes, high Q RF |
| Ceramic (SMD) | Multi-layer ceramic or thin-film wire on ceramic substrate | ±2% to ±10% | +100 to +500 | High-frequency SMD RF circuits, impedance matching in GHz range |
Decoding Physical Markings and SMD Codes
Unlike resistors, inductors don't always have clearly printed values, especially as package sizes shrink to 0402 and 0201. Here is how to read the markings on the physical parts.
Through-Hole Color Bands (4-Band System)
Radial and axial leaded inductors use a 4-band color code identical to resistors, but the unit of measure is microhenries (µH), not ohms.
- Band 1 & 2: Significant digits.
- Band 3: Multiplier (number of zeros).
- Band 4: Tolerance (Gold = ±5%, Silver = ±10%, No band = ±20%).
Example: A choke with Brown - Black - Brown - Silver bands translates to 1 - 0 - x10^1 µH ±10%, which equals 100 µH ±10%.
SMD Chip Inductor Codes
Surface mount inductors use a 3-digit or 4-digit alphanumeric code. The unit of measure is microhenries (µH) unless specified otherwise by the datasheet.
- 3-Digit Code: First two digits are significant, third is the multiplier (power of 10). Example: '101' = 10 x 10^1 = 100 µH.
- 'R' Decimal Indicator: The letter 'R' replaces the decimal point. Example: '4R7' = 4.7 µH. 'R10' = 0.10 µH (100 nH).
- 4-Digit Code (Precision): First three digits are significant, fourth is multiplier. Used for tight-tolerance RF inductors. Example: '1001' = 100 x 10^1 = 1000 nH (1 µH).
Safe Substitution When the Exact Part is Missing
Supply chain shortages happen. When you need to substitute a missing inductor, matching the nominal inductance value (e.g., swapping a 4.7 µH for another 4.7 µH) is not enough. You must evaluate five critical parameters to avoid destroying your switching node.
- Saturation Current ($I_{sat}$): The current at which inductance drops by 20% to 30%. If your substitute has a lower $I_{sat}$ than the original, the core will saturate during peak load, inductance will collapse to near zero, and your switching MOSFET will likely explode from overcurrent.
- RMS Current ($I_{rms}$): The continuous DC current the wire can handle before overheating. Tied directly to the DC Resistance (DCR).
- DC Resistance (DCR): A substitute with higher DCR will waste more power as heat ($I^2R$ losses) and reduce overall converter efficiency.
- Self-Resonant Frequency (SRF): The frequency where the inductor's parasitic parallel capacitance resonates with its inductance. Your substitute's SRF must be at least 10x higher than your circuit's operating frequency. If you put a 10 µH power choke (SRF ~5 MHz) into a 20 MHz RF filter, it will act like a capacitor.
- Shielding: I once swapped an unshielded drum-core inductor for a shielded composite-core part in a 2A buck converter simply because it was 'in stock'. The magnetic flux from the unshielded part radiated directly into a nearby ESP32's 2.4GHz PCB antenna, killing the Wi-Fi range by half. Always match the shielding topology in mixed-signal or RF environments.
For detailed derating curves and substitution tools, manufacturer resources like the Coilcraft Design Tools are invaluable for cross-referencing $I_{sat}$ and DCR tradeoffs.
Failure Modes and Visual Diagnostics
Inductors are generally robust, but they do fail, usually due to thermal or mechanical abuse. Here is what to look for when troubleshooting a dead board.
- Wire Burnout (Open Circuit): Caused by exceeding the $I_{rms}$ rating. The thin copper enamel burns away and the wire snaps. Visual Symptom: Look for a 'caramelized' or dark brown discoloration on the clear epoxy coating of radial inductors, or a melted shrink-wrap sleeve on toroids. A multimeter will read 'OL' (infinite resistance) instead of a low DCR.
- Insulation Breakdown (Short Circuit): High voltage spikes (like flyback from a relay coil) can arc through the thin wire enamel, shorting adjacent turns together. This drastically reduces the inductance. Visual Symptom: Often invisible to the naked eye. Requires an LCR meter to spot a sudden drop in µH and a drop in the Q-factor.
- Core Cracking (Mechanical/Thermal): Ferrite is essentially ceramic and is highly brittle. Dropping the board or subjecting it to rapid thermal cycling (like wave soldering) can crack the core. A cracked core introduces a physical air gap, which drastically alters the permeability and drops the inductance. Visual Symptom: Visible hairline fractures on the ferrite drum or toroid, often accompanied by a buzzing or 'singing' noise during operation due to magnetostriction vibrating the loose core halves.
Frequently Asked Questions
What are the most common inductor values for high-frequency buck converters?
For modern high-frequency buck converters (switching between 1 MHz and 3 MHz), the most common inductor values range from 1.0 µH to 4.7 µH. At these frequencies, the required inductance to maintain a specific ripple current percentage drops significantly. Using lower µH values allows manufacturers to use physically smaller chip inductors with higher saturation currents. For older or lower-frequency converters (100 kHz - 500 kHz), you will typically see 22 µH, 33 µH, and 47 µH as the standard choices.
Can I safely substitute a higher common inductor value in my power supply design?
Yes, but with strict caveats. Substituting a 4.7 µH inductor with a 10 µH inductor of the same physical size will reduce your output ripple current, which is generally good for noise. However, because the physical volume of the core hasn't changed, the 10 µH part will have more turns of thinner wire. This means its DCR will be higher (causing more heat) and its saturation current ($I_{sat}$) will be lower. If your load draws high peak currents, the 10 µH part might saturate where the 4.7 µH part would not. Always check the $I_{sat}$ of the higher-value substitute against your peak switch current.
How do I accurately measure common inductor values without a dedicated LCR meter?
You cannot accurately measure inductance with a standard digital multimeter; a DMM will only read the DC Resistance (DCR) of the copper wire. If you lack an LCR meter, you can use the resonant frequency method with an oscilloscope and a function generator. Wire the unknown inductor in parallel with a high-precision, known capacitor (e.g., a 100 nF C0G ceramic). Sweep the function generator's frequency across the circuit while monitoring the voltage on the scope. The frequency at which the voltage peaks is the resonant frequency ($f_r$). You can then calculate the inductance using the formula: $L = 1 / (4 \pi^2 f_r^2 C)$. For deeper component characterization, reviewing TDK's inductor technical notes provides excellent baseline data on parasitic behaviors that affect these measurements.
