Understanding of Carbon Film Resistors

I. What is a Carbon Film Resistor?

Carbon film resistors are manufactured by depositing a thin carbon film onto an insulating ceramic substrate through thermal decomposition of gaseous hydrocarbons under high temperature and vacuum conditions. The process involves decomposing hydrocarbon gases, which deposit carbon onto ceramic rods or tubes to form a crystalline carbon film. The resistance value can be adjusted by varying the thickness of the carbon film or by cutting helical grooves into the film to modify its effective length and cross-sectional area.

While carbon film resistors have relatively modest electrical performance and stability compared to more advanced resistor types, they offer significant cost advantages. They excel particularly in high-resistance and high-voltage applications, where the thin carbon film can be easily engineered to achieve resistance values in the megohm range.

Figure 1. Carbon Film Resistor

Common lead configurations for carbon film resistors include axial lead (most common), radial lead, and surface-mount varieties. The resistance range typically spans from 1Ω to 10MΩ, with rated power options of 0.125W, 0.25W, 0.5W, 1W, 2W, 5W, and 10W available depending on the physical size and construction.

The primary function of carbon film resistors is to impede current flow in electronic circuits. They are widely used for current limiting, voltage division, signal conditioning, load matching, filter networks with capacitors, and impedance matching applications across various electronic devices.

II. Structure and Features of Carbon Film Resistors

1. Basic Construction

Carbon film resistors utilize specialized equipment to thermally decompose gaseous hydrocarbons in a high-temperature, vacuum environment. The decomposed carbon is uniformly deposited onto the circumferential surface of a ceramic cylinder or tube, forming a crystalline carbon film. The resistance value is determined by adjusting the carbon film thickness and cutting a helical groove with specific pitch to achieve the desired cross-sectional area and effective length of the conductive path.

After the film deposition, metal end caps (typically nickel-plated copper) are attached, leads are soldered, and a protective epoxy coating is applied to the surface to protect against environmental factors and mechanical damage.

Figure 2. Internal Structure of a Carbon Film Resistor

As shown in the figure, the width of the carbon film is inversely proportional to the resistance value, while the effective length is directly proportional. Additionally, thinner carbon films produce higher resistance values. The tolerance of carbon film resistors typically ranges from ±5% to ±10%, with precision versions available at ±2% tolerance. Carbon film resistors exhibit a negative temperature coefficient (NTC), meaning their resistance decreases as temperature increases, typically around -200 to -1000 ppm/°C.

2. Typical Characteristics

(1) Tolerance: Available in tolerances from ±2% to ±10%, with ±5% being most common. The resistance value can be fine-tuned by cutting threads into the film to create precision resistors.

(2) Resistance Range: Wide resistance range, typically from 1Ω to 10MΩ, with some specialized versions reaching up to 100MΩ for high-voltage applications.

(3) Nominal Resistance: E-24, E-48, and E-96 series available depending on tolerance grade.

(4) High Voltage Rating: Can handle high voltages, with some types rated for several kilovolts depending on physical size and construction.

(5) Long-term Stability: Good long-term stability with voltage changes having minimal effect on resistance value. Features a negative temperature coefficient.

Figure 3. Negative Temperature Coefficient Behavior

(6) Packaging: Available in tape and reel, bulk, or individual bagging for through-hole mounting.

(7) High-Frequency Performance: Good high-frequency characteristics suitable for RF and ultra-high-frequency applications, with inherent noise voltage below 10μV/V.

(8) Rated Power: Available in 1/8W, 1/4W, 1/2W, 1W, 2W, 5W, and 10W ratings.

(9) Pulse Stability: Stable under pulse loading with good adaptability to pulse circuits. Wide application range suitable for AC, DC, and pulse circuits.

III. Carbon Film Resistors: Parameters and Error Rates

1. Key Parameters

The resistance value marked on the resistor is called the nominal value, expressed in Ω (ohms), kΩ (kilohms), or MΩ (megohms). The conversion relationship is: 1MΩ = 1,000kΩ and 1kΩ = 1,000Ω. Nominal values follow standardized series (E-series) established by international standards, not arbitrary values chosen by manufacturers. Carbon film resistors typically range from 1Ω to 10MΩ.

Figure 4. Mean Resistance, Design (Nominal) Resistance, and Factored Resistance

(1) Tolerance

The maximum allowable deviation of the actual resistance from the nominal value is called tolerance. Common tolerance codes: F (±1%), G (±2%), J (±5%), K (±10%), M (±20%).

(2) Rated Power

The rated power is the maximum power the resistor can continuously dissipate under specified ambient temperature (typically 70°C) without damage or significant performance degradation. For carbon film resistors, the power rating is not usually marked on the body but is indicated by the physical size—larger diameter and length correspond to higher power ratings. Common power ratings include 0.125W, 0.25W, 0.5W, 1W, 2W, 5W, and 10W.

Figure 5. Resistor Size Comparison by Power Rating

Miniature carbon film resistors with power ratings as low as 0.0625W (1/16W) are also available for space-constrained applications, often manufactured as color-coded resistors.

2. Error Rates and Tolerance Classifications

Carbon film resistors are available in several tolerance grades:

• ±2% (Precision grade)

• ±5% (Standard grade, most common)

• ±10% (General purpose)

• ±20% (Economy grade)

Carbon film resistors are typically marked with the designation "RT" where R represents resistor and T indicates carbon film construction. For example, "RT47kJ" denotes a carbon film resistor with 47kΩ resistance and ±5% tolerance.

IV. Marking Methods for Carbon Film Resistors

1. Direct Marking Method

Numbers and unit symbols are printed directly on the resistor surface to indicate the resistance value. Tolerance is expressed as a percentage. If no tolerance is marked, ±20% is assumed.

Figure 6. Direct Marking Method Example

2. Text Symbol Method

A combination of Arabic numerals and text symbols indicates the nominal resistance value. The number before the symbol represents the integer resistance value, while subsequent numbers represent decimal places. For example, "4R7" means 4.7Ω, and "4K7" means 4.7kΩ.

3. Digital Method

Three digits are marked on the resistor to indicate the nominal value. The first two digits are significant figures, and the third digit is the multiplier (number of zeros to add). The unit is ohms. For example, "473" represents 47,000Ω or 47kΩ.

4. Color Code Method

Different colored bands or dots indicate the nominal resistance value and tolerance. The color code is: Black-0, Brown-1, Red-2, Orange-3, Yellow-4, Green-5, Blue-6, Violet-7, Gray-8, White-9, Gold-0.1, Silver-0.01.

Figure 7. How to Read Resistor Color Codes

Four-Band Resistors: The last band is gold or silver (tolerance). First two digits are significant figures, third digit is the multiplier, and fourth digit is tolerance.

Five-Band Resistors: Used for higher precision. First three digits are significant figures, fourth digit is the multiplier, and fifth digit is tolerance. The spacing before the last band is typically larger.

V. Carbon Film Resistors vs. Metal Film Resistors

Metal film resistors are manufactured using vacuum deposition technology to apply a thin layer of nickel-chromium or similar alloy onto a ceramic substrate. The film is then cut with a helical groove to achieve the precise resistance value. Metal film resistors offer wider resistance ranges with superior accuracy, tighter tolerances (commonly ±1% or better), and better temperature stability than carbon film resistors.

Carbon film resistors are widely used in consumer electronics and general-purpose applications due to their low cost and reliable performance. They are manufactured by depositing carbon film onto ceramic substrates through thermal decomposition in high-temperature vacuum conditions. While less precise than metal film resistors, carbon film types offer excellent value for non-critical applications.

Visual Identification Methods

Color Bands: Metal film resistors typically use five color bands (±1% tolerance), while carbon film resistors commonly use four bands (±5% tolerance).

Body Color: Metal film resistors often have blue bodies, while carbon film resistors are typically beige, tan, or other colors. However, this is not always reliable as colors vary by manufacturer.

Reliable Differentiation Methods

Method 1 - Film Color Test: Carefully scrape away the protective coating with a blade. Carbon film resistors will reveal a black film layer, while metal film resistors will show a bright metallic (silvery) film.

Method 2 - Temperature Coefficient Test: Since metal film resistors have a much smaller temperature coefficient than carbon film resistors, measure the resistance value with a multimeter, then bring a hot soldering iron near (not touching) the resistor. A significant resistance change indicates a carbon film resistor, while minimal change suggests a metal film resistor.

Figure 8. Temperature Coefficient Testing with Soldering Iron

Key Comparison

VI. Why Resistance Increases and How to Adjust It

1. Reasons for Resistance Increase

Several factors can cause the resistance value of carbon film resistors to increase over time:

(1) Oxidation

Oxidation is a long-term aging mechanism that begins at the resistor surface and gradually penetrates deeper. The oxidation process increases resistance values, with thinner films being more susceptible. While organic protective coatings (plastics, resins, epoxy) can slow oxidation, they may also introduce moisture permeability issues. High temperature and humidity environments significantly accelerate oxidation and aging.

(2) Gas Adsorption and Desorption

Carbon film resistors manufactured in vacuum conditions may adsorb atmospheric gases when exposed to normal pressure, causing resistance increases. Proper aging of semi-finished products at atmospheric pressure before final assembly can improve long-term stability.

Figure 9. Gas Adsorption Effects on Carbon Film

(3) Film Defects and Contamination

Porosity or defects in the carbon film, presence of mobile ions (Na⁺, K⁺, Cl⁻), or inadequate protective coating can lead to degradation and resistance drift.

(4) Terminal Connection Issues

Poor contact between the carbon film and end caps, or loose terminal connections can cause intermittent or permanent resistance increases.

(5) Manufacturing Quality

The quality of raw materials, particularly the carbon deposition process and uniformity, significantly affects long-term stability.

Figure 10. Carbon Black Paste

Explanation: The longer the resistor is being used, the greater the temperature changes. Under normal temperature, the temperature of the resistor cannot be lower than the normal temperature, so the resistance value will only increase rather than decrease.

Reasons: Carbon film resistors have excellent long-term stability. The change in voltage has little effect on the resistance value. And it has a negative temperature coefficient, which means the higher the temperature is, the smaller the resistance will be.

2. Increase the Resistance Manually

Increase the resistance value by scraping the film. Take a resistor with a resistance value slightly smaller than the required value, scrape off the paint film on the surface until the carbon film is exposed. 

Measure the resistance while scraping, and stop when you reach the required resistance value.

The ideal range of "added value" should be controlled within 20% of the original resistance value, such as from 1kQ to 1.2kQ.

Note: Increasing the resistance value too much will affect the stability of the resistance.