The standard inductance color code uses a 4-band system modeled directly after the resistor color code, but the base unit of measurement is microhenries (µH) rather than ohms. To read an axial molded choke (like the classic Bourns 77F or Vishay IM series), start from the end with the widest or most widely spaced band and read toward the center: the first two bands are significant digits, the third is the multiplier, and the fourth is the tolerance.
The Complete Inductance Color Code Reference Table
The table below covers the standard EIA 4-band inductance color code. Keep this bookmarked for your bench; it applies to 95% of through-hole molded RF and power chokes you will encounter.
| Band Color | Significant Digit (Bands 1 & 2) | Multiplier (Band 3) in µH | Tolerance (Band 4) |
|---|---|---|---|
| Black | 0 | × 1 | — |
| Brown | 1 | × 10 | ± 1% |
| Red | 2 | × 100 | ± 2% |
| Orange | 3 | × 1,000 | ± 3% |
| Yellow | 4 | × 10,000 | ± 4% |
| Green | 5 | × 100,000 | — |
| Blue | 6 | — | — |
| Violet | 7 | — | — |
| Gray | 8 | — | — |
| White | 9 | — | — |
| Gold | — | × 0.1 | ± 5% |
| Silver | — | × 0.01 | ± 10% |
| None | — | — | ± 20% |
Decoding the Bands: What Each Row Means in Practice
Reading the inductor color code requires identifying the starting point. Manufacturers typically make the tolerance band (Band 4) slightly wider, or space it further away from the first three bands. If the spacing is perfectly symmetrical, look for a gold or silver band—these almost always denote tolerance and belong at the end.
Worked Example 1: Standard RF Choke
You have a small molded choke with the following bands: Orange, Orange, Brown, Gold.
- Band 1 (Orange): 3
- Band 2 (Orange): 3
- Band 3 (Brown): × 10
- Band 4 (Gold): ± 5% tolerance
Calculation: 33 × 10 = 330 µH (± 5%). This is a very common value for intermediate frequency (IF) filtering in older AM radio circuits.
Worked Example 2: Fractional Microhenry Values
You find a choke with bands: Green, Blue, Gold, Silver.
- Band 1 (Green): 5
- Band 2 (Blue): 6
- Band 3 (Gold): × 0.1
- Band 4 (Silver): ± 10% tolerance
Calculation: 56 × 0.1 = 5.6 µH (± 10%). This is where beginners stumble, expecting Gold to only mean tolerance. In the multiplier position, Gold and Silver create fractional µH values essential for VHF/UHF RF tank circuits.
Rows People Get Wrong (And How to Avoid Bench Mistakes)
Even experienced technicians misread inductors when rushing. Here are the most common pitfalls and how to sidestep them.
| Common Mistake | Why It Happens | The Correct Interpretation |
|---|---|---|
| Reading backwards | Bands are evenly spaced; tolerance band isn't obvious. | Look for Gold/Silver (always tolerance). If none, the first band is usually Black or Brown, as inductors rarely start with higher digits for standard E12 values. |
| Confusing nH and µH | SMD inductors often use nanohenries (nH) and printed text (e.g., 4N7). | Axial molded color bands almost universally use microhenries (µH). If you calculate 4700 µH, it's 4.7 mH, not 4.7 nH. |
| Misidentifying the core material | Assuming all tan/brown molded bodies are identical. | Body color sometimes hints at the core. A dark gray or black molded body often indicates an iron powder core (better for high current/power), while a light tan or green body usually indicates a ferrite core (better for high-frequency RF). Check the manufacturer datasheet for saturation current limits. |
| Ignoring the 'None' tolerance | Assuming a 3-band inductor is missing a band. | A 3-band inductor simply has no tolerance band, which defaults to ± 20%. This is common for cheap power line chokes where exact inductance isn't critical. |
Safe Interpretation When Markings Are Faded or Missing
Molded chokes in vintage gear, tube amplifiers, or industrial power supplies often suffer from heat discoloration, making the color bands impossible to read. When the visual inductance color code fails, you must rely on electrical measurement. However, you cannot simply slap a multimeter on it and trust the first number you see.
The LCR Meter Test Frequency Rule
Inductance is not a static value; it changes based on the test frequency due to core permeability shifts and parasitic winding capacitance. A 10 µH RF choke measured at 100 Hz on a cheap handheld meter might read 14 µH or fail to lock entirely. To get an accurate reading, set your LCR meter's test frequency to match the inductor's intended application:
- Power Line Chokes (>100 µH): Test at 100 Hz or 120 Hz. These use laminated iron or heavy iron-powder cores designed for 50/60 Hz mains filtering.
- Switching Supply Chokes (10 µH to 100 µH): Test at 1 kHz or 10 kHz. This matches the typical switching frequencies of older SMPS controllers.
- RF Chokes (<10 µH): Test at 100 kHz or 1 MHz. Ferrite cores used in RF applications exhibit wildly different permeability at low frequencies. Measuring a 2.2 µH RF choke at 100 Hz is useless data.
Checking for the 'Shorted Turn' Failure Mode
If you drop a molded choke on a concrete floor, the internal ferrite core can crack, or the copper winding can shift, causing two adjacent turns to touch. This creates a shorted turn.
Here is the trap: an LCR meter might still display an inductance value (often slightly lower than nominal), tricking you into thinking the part is fine. To catch this, you must measure the DC Resistance (DCR). A healthy 100 µH axial choke typically has a DCR between 2 Ω and 15 Ω, depending on the wire gauge. If your LCR meter shows the correct inductance but the DCR reads 0.2 Ω, the internal winding is shorted. The part will overheat and fail under load. Always verify both L and DCR when salvaging or troubleshooting unmarked inductors.
