Reading an inductor color code is nearly identical to reading a resistor, but the base unit is microhenries (µH) rather than ohms, and the tolerance bands follow slightly different rules. Below is the definitive reference for identifying axial and molded RF chokes on your workbench.
The Complete Inductor Color Code Reference Table
The standard 4-band system (governed by the EIA) uses the first two bands for significant digits, the third band as a multiplier in microhenries (µH), and the fourth band for tolerance. Read the bands from left to right, usually starting from the band closest to the lead wire.
| Color | Band 1 & 2 (Digit) | Band 3 (Multiplier in µH) | Band 4 (Tolerance) |
|---|---|---|---|
| Black | 0 | 1 | 20% |
| Brown | 1 | 10 | — |
| Red | 2 | 100 | — |
| Orange | 3 | 1,000 | — |
| Yellow | 4 | 10,000 | — |
| Green | 5 | — | — |
| Blue | 6 | — | — |
| Violet | 7 | — | — |
| Gray | 8 | — | — |
| White | 9 | — | — |
| Gold | — | 0.1 | 5% |
| Silver | — | 0.01 | 10% |
Worked Example: An inductor with bands Brown - Black - Red - Silver.
Brown = 1, Black = 0 (Digits: 10). Red = 100 multiplier. Silver = 10% tolerance.
Calculation: 10 × 100 = 1,000 µH (or 1 mH) ± 10%. This is a standard 1mH power choke, commonly seen in older switching power supplies or audio crossover networks.
Standard Variants: EIA Commercial vs. MIL-PRF Military
While the EIA commercial standard dominates modern hobbyist and consumer electronics, you will encounter military and regional variants when repairing legacy telecom, aviation, or vintage Asian audio equipment. Recognizing which standard applies to your region or equipment prevents catastrophic miscalculations in tuned circuits.
Under the MIL-PRF-15305 standard, military-grade RF inductors often feature a double-width first band. This wide band does not represent a digit; it simply indicates MIL-spec qualification. If you mistake a wide silver band for two separate silver bands, you will misread a 5-band sequence and calculate the wrong inductance by an order of magnitude.
MIL-PRF-15305 (US Military)
Military chokes (often molded in distinct olive-drab or tan epoxy) use the same digit/multiplier logic as EIA but add a 5th band in some sub-specifications to indicate failure rate levels or temperature coefficients. Furthermore, MIL-spec parts are rigorously tested for Q-factor stability at high frequencies. If you are replacing a MIL-spec choke in an RF transmitter, substituting a commercial EIA part with the same nominal µH value may fail due to core losses at VHF/UHF frequencies.
JIS and Vintage Asian Dot Codes
Older Japanese Industrial Standard (JIS) components, particularly from 1970s and 1980s Sony or Panasonic radios, sometimes abandon axial bands entirely in favor of a 3-dot system painted on a cylindrical ferrite core. In this system, the dots are read like a resistor, but the base unit is often nanohenries (nH) or microhenries (µH) depending on the physical size of the coil. Always cross-reference the physical footprint; a 2mm long coil with a '101' dot code is 100nH, while a 10mm long coil with the same code is 100µH.
The Rows People Get Wrong and Faded Band Recovery
Even experienced bench technicians make errors when translating inductor bands, primarily because they carry over assumptions from resistor color codes. Here are the specific failure modes and how to resolve them.
The Multiplier Decimal Trap (Gold & Silver)
On a resistor, a gold multiplier means 0.1 ohms. On an inductor, a gold multiplier means 0.1 µH. If you see bands Red-Violet-Gold-Gold, the value is 27 × 0.1 = 2.7 µH ± 5%. Technicians rushing often read the gold multiplier as a tolerance band and assume the previous band was the multiplier, resulting in a calculation of 270 µH. Always count exactly four bands; if the third band is gold or silver, it is a fractional multiplier, not a tolerance marker.
The 'Missing' Tolerance Band
Many cheap, mass-produced axial inductors (like the Bourns 78F series or generic Fastron clones) only have three color bands. According to EIA conventions, if the fourth tolerance band is omitted, the default tolerance is ±20%. Do not assume it is 5% or 10%. In a buck converter circuit, a 20% swing on a 47µH inductor means the actual value could be anywhere from 37.6µH to 56.4µH, which can push your ripple current beyond the IC's safe operating area.
Safe Interpretation of Faded or Burnt Bands
Inductors in power supplies run hot. Over time, the epoxy coating yellows, and brown bands turn red, or black bands fade to a muddy gray. Never guess a faded inductor value in an RF tank circuit or a switching regulator. Guessing wrong in an RF circuit destroys the Q-factor and detunes the oscillator; guessing wrong in a power supply can cause inductor saturation and destroy the switching MOSFET.
The Recovery Protocol:
- Desolder one leg: Lift one lead of the inductor from the PCB to eliminate parallel circuit interference.
- Select the right LCR meter frequency: Do not just clip on a multimeter. Use a dedicated LCR meter (like the DER EE DE-5000 or Keysight U1733C). Set the test frequency to match the component's application.
- Power Chokes (Buck/Boost): Test at 1 kHz or 100 Hz.
- Audio Crossovers: Test at 1 kHz.
- RF Chokes / IF Transformers: Test at 1 MHz or higher.
- Check the DC Resistance (DCR): While you have the meter out, measure the series resistance. If the DCR is significantly higher than the datasheet specifies for that physical size, the internal windings have partially shorted or degraded, and the part must be replaced regardless of its inductance reading.
For exact specifications on commercial molded chokes, always refer to the manufacturer's datasheet, such as the Bourns 78F series documentation, which outlines the precise physical dimensions and DCR limits that color codes cannot convey. For a deeper understanding of how these passive components behave under AC load, review the foundational theory at Electronics Tutorials.
