The direct answer: the 120 ohm color code for a standard 4-band through-hole resistor is Brown, Red, Brown, Gold (or Silver/Brown for the tolerance band). For a 5-band precision resistor (1% tolerance), the code is Brown, Red, Black, Black, Brown. Unlike wire colors, these resistor band colors are universally standardized under IEC 60062 and do not change by region.
However, when building networks that rely on 120-ohm termination resistors—such as CAN bus or RS-485—the wire colors you use to connect them are heavily dependent on regional electrical codes. Below is the complete reference data for both the component and the surrounding wiring standards.
The 120 Ohm Color Code Reference Table
Use this table to verify the bands on your through-hole resistors. The multiplier band is where most calculation errors occur.
| Band Position | 4-Band (5% Tolerance) | 5-Band (1% Tolerance) | Value / Meaning |
|---|---|---|---|
| Band 1 (1st Digit) | Brown | Brown | 1 |
| Band 2 (2nd Digit) | Red | Red | 2 |
| Band 3 (3rd Digit) | N/A | Black | 0 (5-band only) |
| Multiplier Band | Brown (x10) | Black (x1) | 12 x 10 = 120 / 120 x 1 = 120 |
| Tolerance Band | Gold (±5%) | Brown (±1%) | Acceptable range: 114Ω - 126Ω |
- The 5-Band Multiplier Trap: Hobbyists often read a 5-band 120Ω resistor as a 4-band. They see Brown-Red-Black-Black and calculate 12 x 1 = 12 ohms. The third band (Black) is a significant digit (0), not the multiplier. The fourth band (Black) is the multiplier (x1).
- Faded Brown vs. Red: On cheap carbon film resistors exposed to high heat, the Brown (1) and Red (2) bands can oxidize and look nearly identical. Always verify the multiplier band position to orient the component correctly.
Component Standards vs. Regional Wire Colors (NEC, IEC, Old UK)
While the IEC 60062 standard dictates that a 120-ohm resistor is always Brown-Red-Brown globally, the cables used to wire these resistors into industrial networks (like RS-485 or CAN bus) must follow regional wiring standards. If you are pulling cable through conduit to connect remote termination nodes, you must adhere to local codes for power and data lines.
| Standard / Region | Mains Power (Line / Neutral / Ground) | Data / Control Pairs (Typical Practice) |
|---|---|---|
| IEC 60446 (EU / Global) | Brown / Blue / Green-Yellow | Grey & Brown (Pair 1), Black & Blue (Pair 2) |
| NEC / US Practice | Black (or Red/Blue) / White / Bare or Green | TIA-568: Orange & Orange-White (Pair 4 for RS-485) |
| Old UK (Pre-2004) | Red / Black / Green | Legacy telecom: White/Blue, White/Orange |
| Automotive CAN (ISO 11898) | Red (12V) / Black (GND) | CAN-H: White or Green / CAN-L: Blue or Yellow |
Safe Interpretation When Markings Are Faded or Missing
Resistors used in CAN bus termination or audio DAC filters often run warm. Over time, the epoxy coating yellows, and the color bands fade or burn off. Here is the exact decision path for identifying an unmarked or faded 120-ohm resistor.
- Visual Triage: If the resistor body is blistered, cracked, or the leads are discolored, it has failed open or drifted significantly. Do not attempt to measure it; desolder and replace it with a fresh 1/2W metal film resistor (e.g., Vishay MRS25 series).
- Out-of-Circuit Measurement: You cannot accurately measure a termination resistor while it is soldered into a CAN bus or RS-485 network. A standard CAN network has two 120-ohm resistors in parallel (one at each end of the bus), which will read 60 ohms on your multimeter. Desolder at least one leg of the suspect resistor to lift it from the parallel circuit before measuring.
- SMD Code Translation: If you are replacing a faded through-hole resistor with a surface-mount device (SMD), beware the SMD numbering trap.
121= 12 x 10^1 = 120 ohms (Correct)120= 12 x 10^0 = 12 ohms (Incorrect, common mistake)
For detailed testing procedures, refer to Fluke's guide on testing resistors, which emphasizes the dangers of in-circuit parallel path errors.
Practical Applications: Where 120 Ohm Resistors Matter
The 120-ohm value is not arbitrary; it is deeply tied to the physics of signal transmission and characteristic impedance.
- CAN Bus Termination: Standard twisted-pair cables used in automotive and industrial CAN networks have a characteristic impedance of approximately 120 ohms. Placing a 120-ohm resistor across the CAN-H and CAN-L lines at both physical ends of the bus absorbs the signal energy, preventing reflections that cause data corruption. For automotive environments, always use 1/2W or 1W resistors rated for high pulse loads, as 1/4W resistors can overheat and fail open during bus fault conditions.
- RS-485 Networks: Similar to CAN, RS-485 uses differential signaling over twisted pair. While CAT5e cable has an impedance closer to 100 ohms, legacy industrial installations often use 120-ohm termination to match older shielded twisted pair (STP) cables. According to All About Circuits, mismatching the termination resistor to the cable impedance by more than 10% can lead to severe signal ringing at baud rates above 500 kbps.
- Audio Line Drivers: In professional audio, 120-ohm series resistors are frequently placed at the output of operational amplifiers (like the NE5532) driving long cables. This isolates the op-amp from the capacitive load of the cable, preventing high-frequency oscillation and phase shift.
When sourcing replacements, prioritize metal film over carbon composition. Metal film resistors (like the Yageo MFR-25 series) offer tighter 1% tolerances and lower thermal noise, which is critical for maintaining the integrity of high-speed differential data buses.
