Understanding of Carbon Film Resistors

Catalog

I What is a Carbon Film Resistor?

Carbon film resistors are prepared by applying a suspension liquid of carbon ink, graphite, and filler to an insulating substrate with an organic binder, and polymerizing it by heating. Gaseous hydrocarbons decompose in high temperature and vacuum, and carbon is deposited on porcelain rods or tubes to form a crystalline carbon film. Changing the thickness of the carbon film and changing the length of the carbon film by carving the groove can obtain different resistance values.

Carbon film resistors have low cost, poor electrical performance, and stability, and they are generally not suitable for general-purpose resistors. But because it is easy to make a high resistance film, it is mainly used as a resistor of high resistance and voltage, which is the same as high-voltage resistors.

Figure 1. A Carbon Film Capacitor

The common lead method of carbon film resistors are axial lead, tie lead, and no-lead ways. The resistance range from 1Ω to 10MΩ, and the rated power could be 0.125W, 0.25W, 0.5W, 1W, 2W, 5W, 10W, etc.

The main function of carbon film resistors is to hinder the flow of current. It is used in current limiting, shunting, step-down, voltage division, load and capacitor matching filters, and resistance matching.

II Structure and Features of Carbon Film Resistors

1. Basic Structure 

Carbon film resistors use special equipment to decompose gaseous hydrocarbons in high temperatures and vacuum. The decomposed carbon is evenly deposited on the circumferential surface of the ceramic cylinder or ceramic tube to form a crystalline carbon film. Then according to the required resistance value, change the thickness of the carbon film and select the pitch of the screw pitch of the spiral groove to determine the appropriate cross-sectional area and length of the carbon film. Next, attach a copper end cap, solder out the leads, and spray paint the surface. Its shape and structure are shown in the figure below.

Figure 2. Structure of a Carbon Film Resistor

In the figure, the width of the carbon film is inversely proportional to the resistance value, and its effective length is directly proportional to the resistance value. In addition, the thinner the carbon film is, the larger the resistance value will be. The precision of the carbon film resistor is relatively low, which is only up to ± 5%. A carbon film resistor is a kind of negative temperature coefficient resistor, which means its resistance value changes in a decreasing direction when the temperature increases.

2. Typical Characteristics 

(1) Accuracy: high accuracy, which is from 2% to 5%. The resistance value can be adjusted by cutting the thread of the film to make a precision resistor.

(2) Resistance Range: the resistance range is wide, generally from 2.1Ω to 10MΩ.

(3) Nominal resistance: E-48

(4) High limit voltage

(5) Excellent long-term stability. Changes in voltage have little effect on the resistance value. It also has a negative temperature coefficient.

Figure 3. Negative Temperature Coefficient

(6) Packing methods include bagging and packing in bulk

(7) Good high-frequency characteristics. It can be made into high-frequency resistors and ultra-high-frequency resistors. And the inherent noise electromotive force is small, which is below 10UV / V.

(8) The rated power includes 1 / 8W, 1 / 4W, 1 / 2W, 1W, 2W, 5W and 10W.

(9) The pulse load is stable and it has good adaptability to pulses. Its application range is very wide, which is suitable for AC, DC, and pulse circuits. 

III Carbon Film Resistors: Parameters&Error Rate

1. Parameters of Carbon Film Resistors

The resistance value on the resistor is called the nominal value. Its units include Ω (ohm), KΩ (kiloohm), and MΩ (megaohm). The conversion relationship is: 1MΩ = 1000KΩ, 1KΩ = 1000Ω. The nominal value is marked according to a series of national standards and is not arbitrarily calibrated by the producers. The resistance value of carbon film resistors ranges from 1Ω to 10MΩ.

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

(1) Allowable Error

The maximum allowable deviation range of the actual resistance of the resistor from the nominal value is called the allowable error. Error code: F, G, J, K.

(2) Rated Power

Assuming that the surrounding air is not circulating, in the case of continuous work for a long time without damaging or substantially changing the performance of the resistor, the allowable power consumption of the resistor under the specified ambient temperature is called rated power. The rated power of carbon film resistors is not marked on the shell of the resistor, but is distinguished by the length and diameter of the electron gun. Resistors with great lengths and large diameters have high power. Carbon film resistors generally have rated power of 0.125W, 0.25W, 0.5W, 1W, 2W, 5W, 10W, etc.

Figure 5. Construction of an Electron Gun

The size of ordinary carbon film resistors is large. In order to meet the needs of small-volume resistor devices, small carbon film resistors RTX has been produced. The power of RTX is only 0.125W, and most of them are made into color-coded resistors.

2. Error Rate of Carbon Film Resistors

The error rate of carbon film resistors is generally divided into three levels:

The first level is 5%,

The second level is 10%,

The third level is 20%.

Carbon film resistors are usually marked with the symbol RT.  R represents the resistor, and T represents the material is a carbon film. For example, if the housing of an electron gun is marked with the word RT47kI, which means that this is a carbon film resistor with a resistance of 47kΩ and a tolerance of ± 5%.

IV Marking Method of Carbon Film Resistors

1. Direct Marking Method

Use the numbers and unit symbols to mark the resistance value on the surface of the resistor. The allowable error is directly expressed as a percentage. If no deviation is noted on the resistance, they are all ± 20%.

Figure 6. Direct Marking

2. Text Symbol Method

Regular combinations of Arabic numerals and text symbols are used to indicate the nominal resistance value, and the allowable deviation is also expressed with text symbols. The number in front of the symbol indicates the integer resistance value, and the subsequent numbers, in turn, indicate the resistance value of the first decimal place and the second decimal place.

3. Digital Method

Mark three digital on the resistor to indicate the nominal value. The first and second digits are valid values, and the third digit is the exponent, which is the number of zeros, and its unit is Euro. Deviations are usually represented by text symbols.

4. Color Code Method

Use different color bands or dots to mark the nominal resistance value and allowable deviation on the surface of the resistor.

Black-0, brown-1, red-2, orange-3, yellow-4, green-5, blue-6, purple-7, gray-8, white-9, gold-0.1, and silver-0.01.

Figure 7. How to Read Resistor Color Codes

When the resistor has four rings, the last ring must be gold or silver. The first two digits are significant digits, the third digit is the exponentiation digit, and the fourth digit is the deviation.

When the resistor has five rings, the distance between the last ring and the previous four rings is large. The first three digits are significant figures, the fourth digit is the exponentiation digit, and the fifth digit is the deviation. This method is generally used in carbon film resistors.

V Carbon Film Resistor VS. Metal Film Resistors

Metal film resistors are made by nickel-chromium or similar alloy vacuum plating technology. Films are deposited on the surface of white porcelain rods, and the resistance values are adjusted after cutting to achieve the final requirements of precise resistance values. Metal film resistors provide a wide range of resistance values with high accuracy and a small tolerance range. They can also be applied to metal film fuse resistors.

What are Metal Film Resistors?

Carbon film resistors are widely used in electron, electric appliances, and information products. They have the cheapest price and high reliability in quality stability. They are made by separating carbon from the high-temperature vacuum, tightly attaching the carbon film to the surface of the porcelain rod, and coating the surface with epoxy resin on the surface for protection.

The difference between carbon film resistors and metal film resistors in appearance:

The metal film resistor has five color rings (1%), while the carbon film resistor has four color rings (5%). The metal film is blue, and the carbon film is khaki or other colors.

In the past, the national standard of micro-resistors was distinguished by color. Metal film resistors were red, and carbon film resistors were green. However, due to the improvement of technology and the appearance of fake gold films, these two methods are not very good, and can not differentiate these two types of resistors.

The better methods are the following two:

1. Scrape the protective paint with a blade. If it’s a carbon film resistor, the color of the exposed film is black; if it’s a metal film resistor, the color of the film is bright white.

2. Since the temperature coefficient of the metal film resistor is much smaller than the carbon film resistor, you can use a multimeter to measure the resistance value of the resistor. Then use a hot electric soldering iron to approach the resistor. If the resistance value changes greatly, it is a carbon film resistor, otherwise, it is a metal film resistor.

Figure 8. The Use of Electric Soldering Iron

VI Why Does the Resistance Increase and How Can We Increase it Manually?

1. Reasons for the Increase in Resistance

There are the following possible factors for the increase in the resistance value of carbon film resistors:

(1) Oxidation

Oxidation is a factor that has long-term effects. The oxidation process starts from the surface of the resistor and gradually deepens into the interior. The resistance value increases after the oxidation. And the thinner the resistance film is, the more significant the oxidation effect will be. If organic materials (plastics, resins, etc.) are used for coating or potting, the protective layer might be air permeable or getting wet. Though these materials can play a role in delaying oxidation or adsorbing gas, they will also bring some new aging factors related to the organic protective layer. Environmental conditions will also affect the intensity of oxidation, and high temperature and humidity will accelerate aging. If there is a lot of moisture in the environment, the weak part of the carbon film will also oxidize, damaging the thin film and increasing the resistance value, and even opening the circuit.

(2) Gas Adsorption and Desorption

When a thermally decomposed carbon film resistor made in a vacuum works directly under normal environmental conditions, it will adsorb part of the gas due to the increase in air pressure, which will increase the resistance value. If the semi-finished product that has no been engraved is preset at a proper time under normal pressure, the resistance stability of the finished resistor product will be improved.

Figure 9. Possible Adsorption Phases on Single-walled Carbon Nanotube Bundles

(3) Porous(Reflowed) Carbon Film

The film is defective or degraded, or there are mobile ions like Na, K, Cl, which makes the protective coating is defective.

(4) Loose Terminal Connection (reflow)

(5) Carbon Paste Quality

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.

VII Latest Application for Carbon Film Resistors

Carbon film resistors used to be the most widely used resistors in electron, electric appliances, and information products. They are the cheapest and have high reliability and stability in quality. Because they are lead-type resistors, they are convenient for manual installation and maintenance, and are also the cheapest among the lead-type resistors. Now they're mostly used in some low-value low-end products such as power supplies and adapters or products of early design.

Figure 11. Adapters

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