The standard 4-band resistor 120 ohm color code is Brown, Red, Brown, Gold (for 5% tolerance) or Brown, Red, Brown, Silver (for 10%). For a precision 5-band 1% resistor, the code is Brown, Red, Black, Black, Brown. You calculate this by taking the first two significant digits (Brown = 1, Red = 2) and multiplying by the third band's multiplier (Brown = 10), yielding 12 × 10 = 120Ω. In a 5-band resistor, the third digit is Black (0), making the significant figures 120, multiplied by Black (1), which also equals 120Ω.
The Complete 120 Ohm Resistor Color Code Table
The table below maps the exact color bands and SMD markings for 120Ω resistors across all common commercial and precision formats. This assumes standard IEC 60062 color mapping for through-hole components.
| Format | Band 1 / Digit 1 | Band 2 / Digit 2 | Band 3 / Digit 3 | Multiplier | Tolerance | Marking Text |
|---|---|---|---|---|---|---|
| 4-Band (5%) | Brown (1) | Red (2) | N/A | Brown (×10) | Gold (±5%) | N/A |
| 4-Band (10%) | Brown (1) | Red (2) | N/A | Brown (×10) | Silver (±10%) | N/A |
| 5-Band (1%) | Brown (1) | Red (2) | Black (0) | Black (×1) | Brown (±1%) | N/A |
| 5-Band (2%) | Brown (1) | Red (2) | Black (0) | Black (×1) | Red (±2%) | N/A |
| SMD 3-Digit | 1 | 2 | N/A | 0 (×1) | ±5% | 121 |
| SMD 4-Digit | 1 | 2 | 0 | 0 (×1) | ±1% | 1200 |
Standard Variants: IEC, MIL-SPEC, and SMD Codes
While the colors themselves are universal, the application of those colors varies by standard and region. Assuming a single global standard exists is a common trap for hobbyists reading datasheets.
IEC 60062 (Global Commercial Standard)
The IEC 60062 standard governs almost all commercial through-hole resistors you buy from distributors like Digi-Key or Mouser. It defines the 4-band and 5-band systems detailed in the table above. Under this standard, the tolerance band is always spaced slightly further apart from the significant digit bands, giving you a visual cue for reading direction.
MIL-PRF-39008 (US Military / High-Reliability)
In US military and aerospace applications, you will frequently encounter 5-band resistors where the first four bands represent the value and the fifth represents tolerance, but with an added 6th band indicating reliability (failure rate). For a 120Ω MIL-SPEC resistor, the code is Brown-Red-Black-Black (120 × 1), followed by a tolerance band (e.g., Gold for 5%), and potentially a 6th band (e.g., Red for 0.1% failure rate per 1,000 hours). Always check for that 6th band before assuming a 5-band commercial part.
SMD EIA-96 and 3-Digit Codes
Surface mount devices (SMD) abandon paint for printed ink. A 120Ω 5% SMD resistor will simply say 121 (12 × 10^1). A 1% SMD will say 1200 (120 × 10^0). However, in the EIA-96 standard (used for 1% 0603 size resistors), 120Ω is represented by the code 12X or 12Y depending on the specific manufacturer's lookup table, where '12' is the significant figure code and the letter is the multiplier.
Rows and Bands People Get Wrong
Even experienced bench technicians misread resistor bands under poor lighting or when dealing with aged components. Here are the specific failure points for the 120Ω code:
- The Multiplier Trap (Brown vs. Black): In a 4-band 120Ω resistor, the third band is Brown (×10). Beginners often read Brown-Red-Brown as '1-2-1' (121Ω) instead of '12 × 10'. Remember: the third band in a 4-bander is always a multiplier, never a significant digit.
- Red vs. Orange Fading: Carbon film resistors (typically beige-bodied) are notorious for band oxidation. A Red band (2) exposed to high ambient heat over years can fade to look exactly like Orange (3). If you read Brown-Orange-Brown, you are looking at 130Ω. Always verify with a multimeter if the red band looks washed out.
- The 1% Tolerance Confusion: In a 5-band 1% resistor, the tolerance band itself is Brown. People see Brown-Red-Black-Black-Brown and get confused by the two Brown bands. The first Brown is '1', the last Brown is '±1%'. They serve entirely different functions.
- Gold and Silver Placement: Gold and Silver are never significant digits in standard IEC color codes. They are strictly multipliers (0.1, 0.01) or tolerance markers (5%, 10%). If you see a Gold band, it is almost always the final band, dictating your reading direction.
Frequently Asked Questions
What is the 120 ohm color code for a 1% tolerance resistor?
A 1% tolerance 120Ω resistor uses the 5-band system. The code is Brown, Red, Black, Black, Brown. The first three bands (Brown-Red-Black) give the significant digits 1-2-0. The fourth band (Black) is the multiplier (×1). The final band (Brown) indicates a 1% tolerance, meaning the actual resistance will measure strictly between 118.8Ω and 121.2Ω. You can find detailed tolerance mappings in standard reference guides like those provided by SparkFun.
How do I read a 120 ohm resistor if the color bands are burned or missing?
If visual identification fails, rely on your multimeter. Set your DMM to the lowest resistance range (usually 200Ω). For an accurate reading, you must remove the resistor from the circuit or lift at least one leg off the PCB pad. Measuring in-circuit will yield a falsely low reading because the meter will measure the 120Ω resistor in parallel with the rest of the circuit's impedance. If the out-of-circuit reading shows 'OL' (open loop), the internal film has fractured and the resistor is dead.
Can I use a standard 120 ohm 1/4W resistor across 120V AC mains as a bleeder?
No. This is a severe fire hazard. A standard 1/4W (0.25W) through-hole resistor is typically rated for a maximum working voltage of 250V, but its power dissipation is the limiting factor here. Using Ohm's Law (P = V² / R), applying 120V AC across a 120Ω resistor results in 120 Watts of heat dissipation. A 1/4W resistor will instantly overheat, smoke, and potentially catch fire. Furthermore, 120V AC has a peak voltage of ~170V. If you need a 120Ω bleeder or snubber resistor for mains voltage, you must use a high-wattage, flameproof wirewound resistor (e.g., 5W or 10W) specifically rated for mains isolation, and ensure local electrical codes permit the installation.
