Resistor value expression method

The resistance of a conductor to electric current is called resistance, represented by the symbol R, and its units are ohm, kiloohm, and megohm, represented by Ω, KΩ, and MΩ respectively.

1. The model naming method of resistors: The model of domestic resistors consists of four parts (not applicable to sensitive resistors)

The first part: Main name, expressed in letters, represents the name of the product, such as R for resistor and W for potentiometer.

The second part: materials, represented by letters, indicate what materials the resistor is made of, T-carbon film, H-synthetic carbon film, S-organic solid, N-inorganic solid, J-metal film, Y-nitride film, C-deposited film, I-glass glaze film, X-wire wound.

Part 3: Classification, generally represented by numbers, and some types are represented by letters, indicating what type the product belongs to. 1-General, 2-General, 3-UHF, 4-High Resistance, 5-High Temperature, 6-Precision, 7-Precision, 8-High Voltage, 9-Special, G-High Power, T-Adjustable.

Part 4: Serial number, expressed in numbers, indicating different varieties of the same product, to distinguish the product's appearance, dimensions and performance indicators, etc.

For example: RT 1 1 type ordinary carbon film resistor a1}

2. Classification of resistors

1. Wirewound resistors: general purpose wirewound resistors, precision wirewound resistors, high power wirewound resistors, high frequency wirewound resistors.

2. Thin film resistors: carbon film resistors, synthetic carbon film resistors, metal film resistors, metal oxide film resistors, chemical deposition film resistors, glass glaze film resistors, metal nitride film resistors.

3. Solid resistors: inorganic synthetic solid carbon resistors and organic synthetic solid carbon resistors.

4. Sensitive resistors: varistors, thermistors, photoresistors, force-sensitive resistors, gas-sensitive resistors, and humidity-sensitive resistors.

3. Main characteristic parameters

1. Nominal resistance: the resistance marked on the resistor.

2. Allowable error: The percentage of the difference between the nominal resistance and the actual resistance to the nominal resistance is called resistance deviation, which indicates the accuracy of the resistor.

The corresponding relationship between the allowable error and the accuracy level is as follows: ±0.5%-0.05, ±1%-0.1 (or 00), ±2%-0.2 (or 0), ±5%-level I, ±10%-level II, ±20%-level III

3. Rated power: The maximum power allowed to be dissipated by the resistor during long-term operation under normal atmospheric pressure of 90-106.6KPa and ambient temperature of -55℃～+70℃.

The rated power series of wirewound resistors are (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 4, 8, 10, 16, 25, 40, 50, 75, 100, 150, 250, 500

The rated power series of non-wirewound resistors are (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 5, 10, 25, 50, 100

4. Rated voltage: The voltage converted from resistance and rated power.

5. Maximum operating voltage: The maximum continuous operating voltage allowed. When working at low pressure, the maximum operating voltage is lower.

6. Temperature coefficient: The relative change in resistance caused by a 1°C change in temperature. The smaller the temperature coefficient, the better the stability of the resistor. The resistance increases with increasing temperature, which is a positive temperature coefficient, and vice versa, which is a negative temperature coefficient.

7. Aging coefficient: The percentage of relative change in resistance of a resistor under long-term load at rated power. It is a parameter that indicates the life of the resistor.

8. Voltage coefficient: The relative change in resistor for every 1 volt change in voltage within the specified voltage range.

9. Noise: An irregular voltage fluctuation generated in a resistor, including thermal noise and current noise. Thermal noise is caused by the irregular free movement of electrons inside the conductor, which causes the voltage at any two points of the conductor to change irregularly.

4. Resistor resistance marking method

1. Direct marking method: Use numbers and unit symbols to mark the resistance value on the surface of the resistor. The allowable error is directly expressed in percentage. If the deviation is not noted on the resistor, it is ±20%.

2. Text symbol method: Use a regular combination of Arabic numerals and text symbols to indicate the nominal resistance value, and the allowable deviation is also indicated by text symbols. The number before the symbol indicates the integer resistance value, and the number after it indicates the first decimal resistance value and the second decimal resistance value respectively.

Text symbols indicating allowable error

Text symbolDFGJKM

Allowable deviation ±0.5% ±1% ±2% ±5% ±10% ±20%

3. Digital method: A marking method that uses three digits to indicate the nominal value on a resistor. From left to right, the first and second digits are effective values, and the third digit is the exponent, i.e. the number of zeros, in ohms. Deviations are usually indicated by text symbols.

4. Color code method: Use different colored strips or dots to mark the nominal resistance and allowable deviation on the surface of the resistor. Most foreign resistors use the color code method.

Black-0, Brown-1, Red-2, Orange-3, Yellow-4, Green-5, Blue-6, Purple-7, Gray-8, White-9, Gold-±5%, Silver-±10%, Colorless-±20%

当电阻为四环时，最后一环必为金色或银色，前两位为有效数字， 第三位为乘方数，第四位为偏差。 当电阻为五环时，最後一环与前面四环距离较大。前三位为有效数字， 第四位为乘方数， 第五位为偏差。

5. Commonly used resistors

1. Potentiometer

A potentiometer is an electromechanical component that relies on the sliding of a brush on a resistor to obtain an output voltage that is related to the displacement of the brush.

1.1 Synthetic carbon film potentiometer

The resistor body is made of ground carbon black, graphite, quartz and other materials coated on the surface of the substrate. The process is simple and it is the most widely used potentiometer. The characteristics are high resolution, good wear resistance and long life. The disadvantages are current noise, large nonlinearity, poor moisture resistance and resistance stability.

1.2 Organic solid potentiometer

Organic solid potentiometer is a new type of potentiometer. It uses the method of heating and plastic pressing to press the organic resistance powder into the groove of the insulator. Compared with carbon film potentiometer, organic solid potentiometer has the advantages of good heat resistance, high power, high reliability and good wear resistance. However, it has large temperature coefficient, large dynamic noise, poor moisture resistance, complex manufacturing process and poor resistance accuracy. It is used to adjust voltage and current in miniaturized, highly reliable and highly wear-resistant electronic equipment and AC and DC circuits.

1.3 Metallic Glass Uranium Potentiometer

The metal glass uranium resistor paste is coated on the ceramic substrate according to a certain pattern by screen printing, and then sintered at high temperature. The characteristics are: wide resistance range, good heat resistance, strong overload capacity, moisture resistance, wear resistance, etc. It is a very promising potentiometer variety, but the disadvantages are large contact resistance and current noise.

1.4 Wirewound Potentiometer

The wire-wound potentiometer is made by winding constantan wire or nickel-chromium alloy wire as the resistor body on an insulating frame. The characteristics of the wire-wound potentiometer are small contact resistance, high precision, and small temperature coefficient. Its disadvantages are poor resolution, low resistance value, and poor high-frequency characteristics. It is mainly used as a voltage divider, variable resistor, zero adjustment and working point in instruments, etc.

1.5 Metal Film Potentiometer

The resistor body of the metal film potentiometer can be composed of alloy film, metal oxide film, metal foil, etc. It is characterized by high resolution, high temperature resistance, small temperature coefficient, small dynamic noise and good smoothness.

1.6 Conductive plastic potentiometer

The DAP (diallyl phthalate) resistor paste is coated on the insulating body by a special process, and then heated and polymerized into a resistor film, or the DAP resistor powder is thermoplastically pressed into the groove of the insulating substrate to form a solid body as the resistor body. The characteristics are: good smoothness, excellent resolution, good wear resistance, long life, low dynamic noise, extremely high reliability, and chemical corrosion resistance. It is used in servo systems of space devices, missiles, and aircraft radar antennas.

1.7 Potentiometer with switch

There are rotary switch potentiometers, push-pull switch potentiometers, and push-push switch potentiometers.

1.8 Pre-adjustable potentiometer

Once the pre-adjusted potentiometer is debugged in the circuit, the adjustment position is sealed with wax and it will not be adjusted under normal circumstances.

1.9 Slide Potentiometer

Use direct sliding method to change the resistance value.

1.10 Dual Potentiometer

There are heteroaxial double-connected potentiometers and coaxial double-connected potentiometers

1.11 Non-contact potentiometer

Contactless potentiometers eliminate mechanical contact, have long life and high reliability, and are divided into photoelectric potentiometers, magnetic sensitive potentiometers, etc.

2. Solid core carbon resistor

A solid resistor is made by mixing carbonaceous particles, strong conductive materials, fillers and binders.

Features: Low price, but large resistance error and noise voltage, poor stability, and currently less used.

3. Wirewound resistor

It is made of high-resistance alloy wire wound on an insulating frame and coated with a heat-resistant glaze insulation layer or insulating paint on the outside.

Wirewound resistors have a low temperature coefficient, high resistance accuracy, good stability, heat resistance and corrosion resistance. They are mainly used as precision high-power resistors. Their disadvantages are poor high-frequency performance and large time constant.

4. Thin film resistors

It is made by evaporating a certain resistivity material onto the surface of the insulating material. The main points are as follows:

4.1 Carbon Film Resistors

Made by depositing crystalline carbon on a ceramic rod skeleton. Carbon film resistors are the most widely used resistors at present due to their low cost, stable performance, wide resistance range, low temperature coefficient and voltage coefficient.

4.2 Metal film resistor.

The alloy material is deposited on the surface of the ceramic rod skeleton by vacuum evaporation.

Metal film resistors have higher precision, better stability, lower noise and lower temperature coefficient than carbon film resistors. They are widely used in instruments and communication equipment.

4.3 Metal Oxide Film Resistors

A layer of metal oxide is deposited on the insulating rod. Since it is an oxide itself, it is stable at high temperatures, resistant to thermal shock, and has a strong load capacity.

4.4 Synthetic membrane resistance

It is obtained by coating a conductive composite suspension on a substrate, so it is also called a paint film resistor.

Because its conductive layer has a granular structure, it has high noise and low precision. It is mainly used to make high-voltage, high-resistance, and small resistors.

5. Metal glass uranium resistor

The metal powder and glass uranium powder are mixed and printed on the substrate using screen printing.

Resistant to moisture, high temperature, small temperature coefficient, mainly used in thick film circuits.

6. Chip resistor SMT

Chip resistor is a form of metal glass uranium resistor. Its resistor body is made of high-reliability ruthenium series glass uranium material sintered at high temperature, and the electrode is made of silver-palladium alloy paste. It has small size, high precision, good stability, and good high-frequency performance because it is a sheet component.