What is a Resistor?
Resistors are commonly referred to as resistors in daily life. It is a current-limiting component. After the resistor is connected to the circuit, the resistance of the resistor is usually two pins, which can limit the current flowing through the branch connected to it. Those whose resistance cannot be changed are called fixed resistors. Variable resistance is called a potentiometer or variable resistor. The ideal resistor is linear, that is, the instantaneous current through the resistor is proportional to the applied instantaneous voltage. Variable resistor for voltage division. On the exposed resistor body, one or two movable metal contacts are pressed tightly. The position of the contact determines the resistance between either end of the resistor and the contact.
- Chinese name
- Resistor
- Foreign name
- Resistor
- nickname
- resistance
- Features
- Limit current
- Resistors are commonly referred to as resistors in daily life. It is a current-limiting component. After the resistor is connected to the circuit, the resistance of the resistor is usually two pins, which can limit the current flowing through the branch connected to it. Those whose resistance cannot be changed are called fixed resistors. Variable resistance is called a potentiometer or variable resistor. The ideal resistor is linear, that is, the instantaneous current through the resistor is proportional to the applied instantaneous voltage. Variable resistor for voltage division. On the exposed resistor body, one or two movable metal contacts are pressed tightly. The position of the contact determines the resistance between either end of the resistor and the contact.
- The terminal voltage and current have a definite function relationship, and the two-terminal device that reflects the ability of electric energy to be converted into other forms is represented by the letter R in ohms. Actual devices such as light bulbs, heating wires, resistors, etc. can be represented as resistor elements.
- The resistance value of a resistance element is generally related to temperature, material, length, and cross-sectional area. The physical quantity that measures the resistance affected by temperature is the temperature coefficient, which is defined as the percentage change in resistance value when the temperature increases by 1 ° C. The main physical characteristic of a resistor is to change electrical energy into thermal energy. It can also be said that it is an energy consuming element, and the internal energy is generated when current passes through it. The resistor usually plays the role of voltage division and current distribution in the circuit. For signals, both AC and DC signals can pass through resistors.
Resistor composition
- Made of resistive material, has a certain structural form,
- Resistor
Resistor basics
- The resistor is composed of a resistor body, a skeleton, and a lead-out terminal (the resistor body and the skeleton of a solid-core resistor are combined into one), and it is only the resistor that determines the resistance value. For resistors with uniform cross section, the resistance value is
- resistance
- Where is the resistivity of the resistive material (ohm · cm); L is the length of the resistor (cm); A is the cross-sectional area of the resistor (cm2).
- The thickness d of the thin film resistor is very small, which is difficult to measure, and varies with the thickness. Therefore, it is regarded as a constant related to the thin film material, and is called a film resistance. In fact, it is the resistance of the square film, so it is also called square resistance (Euro / square). For uniform film
- Film resistance
- Where W is the width (cm) of the film. Usually R s should be within a limited range. Too large R s will affect the stability of the resistor performance. Therefore, the cylindrical resistor is etched, and the planar resistor is etched with a circuitous pattern to expand its resistance range and fine-tune the resistance.
- The volt-ampere characteristic uses a graphical curve to indicate the relationship between the voltage and current at the end of the resistor. When the voltage and current are proportional (the characteristic is a straight line), it is called linear resistance, otherwise it is called nonlinear resistance.
- Parameters and characteristics The main parameters of resistance characteristics are the nominal resistance value and its allowable deviation, rated power, load characteristics, temperature coefficient of resistance, etc.
- Nominal resistance : Design resistance value marked on the resistor with a number or color code.
- Bell Labs (2 photos)
- The method of labeling the resistance value and allowable deviation of a resistor includes the direct labeling method, the color labeling method, and the text symbol method.
- Direct standard method
- The resistance value and error of the resistor are directly printed on the resistance with numbers and letters (no error is marked as allowable error ± 20%). There are also manufacturers using custom marking methods, such as:
- 3 3 means the resistance value is 3.3 and the allowable error is ± 5%
- 1 K 8 means a resistance value of 1.8 K and an allowable error of ± 20%
- 5 M 1 means 5.1 M resistance, tolerance is ± 10%
- Color scale method
- The color rings of different colors are coated on the resistor (or capacitor) to indicate the nominal value and allowable error of the resistor (capacitor). The values corresponding to each color are shown in Table B303. Figure T301 shows the rules for identifying the color marks of fixed resistors.
- Identification method of four-ring resistance
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- Identification of five-ring resistance
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- How to use the above table: four-ring resistance: one ring of digits (ten digits) "red" two ring of digits (one digit) "orange" * multiplier "black" error "gold"
- Example; red orange black gold = 23 * 10 ^ 0 = 23 (± 5%)
- Five-ring resistance : One ring number (hundred digits) "Red" Second ring number (ten digits) "Blue" Three ring number (single digit) "Green" * Multiplier "Black" Error
- Example: Red Blue Green Black Brown = 265 * 10 ^ 0 = 265 (± 1%)
- Allowable deviation: The maximum allowable deviation between the actual resistance value and the nominal resistance value, expressed as a percentage. Commonly used are ± 5%, ± 10%, ± 20%, precision is less than ± 1%, and high precision can reach 0.001%. Accuracy is determined by both allowable deviations and irreversible resistance changes.
- Rated power: The maximum power that a resistor can dissipate continuously at rated temperature (maximum ambient temperature) t R. The maximum operating voltage is also specified for each type of resistor, that is, when the resistance is high, the maximum operating voltage cannot be used even if it does not reach the rated power.
- Identification of the rated power of the resistor The rated power of the resistor refers to the maximum power allowed by the resistor in a DC or AC circuit for long-term continuous operation. There are two ways of marking: resistors above 2W are directly printed on the resistor body; resistors below 2W represent power in terms of their volume. When the resistor power is represented on the circuit diagram, the symbol shown in Figure T302 is used:
- Load characteristics When the working ambient temperature is lower than t R, the resistor cannot be used beyond its rated power. When t R is exceeded, the load power must be reduced. Load characteristics are specified for each resistor. In addition, the load is allowed to be reduced correspondingly at low pressures. Under pulse load, the average pulse power is much lower than the rated power, and generally there are other regulations.
- The temperature coefficient of resistance is within the specified ambient temperature range. The average relative change in resistance value for each 1 ° C change in temperature is expressed in ppm / ° C. In addition to the above parameters, there are non-linearity (the extent to which the current and the applied voltage characteristics deviate from the linear relationship), voltage coefficient (each change in the applied voltage, the relative change rate of the volt resistance), and current noise (resistance in the resistor due to The ratio of the effective value of the noise potential generated by the flow to the test voltage is represented by the current noise index), and the high-frequency characteristics (due to the influence of the distributed capacitance and the distributed inductance in the resistor body, the resistance value decreases as the operating frequency increases Curves, long-term stability (the process of resistors undergoing irreversible changes in resistance values under the influence of environmental conditions during long-term use or storage) and other technical indicators.
Resistor classification
Resistors are classified by volt-ampere characteristics
- For most conductors, at a certain temperature, its resistance remains almost constant and becomes a certain value. This type of resistance is called linear resistance. The resistance of some materials changes significantly with current (or voltage), and its volt-ampere characteristic is a curve. This type of resistance is called a non-linear resistance. Non-linear resistance under a given voltage (or current), the ratio of voltage to current is the static resistance at this operating point, and the slope on the volt-ampere characteristic curve is the dynamic resistance. The way to express nonlinear resistance characteristics is more complicated, but these nonlinear relationships have been widely used in electronic circuits. [1]
Resistors by material
- a. Wire-wound resistors are made of resistance wires wound into resistors. High-resistance alloy wires are wound on the insulating skeleton, and the outside is coated with a heat-resistant glaze insulating layer or insulating paint. 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. The disadvantages are poor high-frequency performance and large time constants.
- b. Carbon composite resistors are made of carbon and synthetic plastics.
- c. The carbon film resistor is formed by plating a layer of carbon on a porcelain tube, and the crystalline carbon is deposited on the ceramic rod skeleton. Carbon film resistors are the most widely used resistors at low cost, stable performance, wide resistance range, low temperature coefficient and voltage coefficient.
- d. The metal film resistor is formed by plating a layer of metal on a porcelain tube, and the alloy material is evaporated on the surface of the ceramic rod skeleton by a vacuum evaporation method.
- Metal film resistors have higher accuracy than carbon film resistors, good stability, low noise, and low temperature coefficient. It is widely used in instrumentation and communication equipment.
- e. The metal oxide film resistor is plated with a layer of tin oxide on the porcelain tube, and a layer of metal oxide is deposited on the insulating rod. Because it is an oxide itself, it is stable at high temperatures, resistant to heat and shock, and has a strong load capacity according to purpose. There are general, precision, high frequency, high voltage, high resistance, high power and resistance networks.
Resistors Special Resistors
- 1. Fuse resistor: It is also called a fuse resistor. Under normal circumstances, it plays the dual role of resistance and fuse. When the circuit fails and its power exceeds the rated power, it will blow like a fuse to disconnect the connection circuit. Fuse resistors generally have small resistance values (0.33 to 10K) and low power. Common types of fuse resistors are: RF10 type, RF111-5 symbol type of fuse resistor, RRD0910 type, RRD0911 type and so on.
- 2. Sensitive resistor: It means that its resistance value is sensitive to a certain physical quantity (such as temperature, humidity, light, voltage, mechanical force, and gas concentration, etc.). When these physical quantities change, the resistance of the sensitive resistor is Will change with the change of physical quantity, showing different resistance values. According to the sensitivity to different physical quantities, sensitive resistors can be divided into thermal, humidity, light, pressure, pressure, magnetic and gas sensitive and other types of sensitive resistors. The materials used for sensitive resistors are almost all semiconductor materials. This type of resistor is also called a semiconductor resistor.
- The resistance of the thermistor changes with temperature. The resistance to temperature rise is a negative temperature coefficient (NTC) thermistor. The most commonly used are negative temperature coefficient thermistors, which can be divided into ordinary negative temperature coefficient thermistors; voltage regulator negative temperature coefficient thermistors; temperature measuring negative temperature coefficient thermistors and so on. Photoresistor is the resistance of the resistor changes with the intensity of the incident light.
- When the incident light increases, the photoresistor decreases, and when the incident light decreases, the resistance value increases.
Resistor related introduction
Resistor selection
- 1. There are many types of fixed resistors to choose. Which kind of material and structure resistor to choose should be determined according to the specific requirements of the application circuit. High-frequency circuits should use non-wound resistors with small distributed inductance and distributed capacitance, such as carbon film resistors, metal resistors and metal oxide film resistors, thin film resistors, thick film resistors, alloy resistors, and corrosion-resistant coatings Resistors, etc. Low-noise resistors, such as metal film resistors, carbon film resistors, and wire-wound resistors, should be used for high-gain small-signal amplifier circuits. Synthetic carbon film resistors and organic solid resistors with high noise should not be used.
- The resistance value of the selected resistor should be close to a nominal value calculated in the application circuit, and the standard series resistor should be preferred. The tolerance of resistors used in general circuits is ± 5% ~ ± 10%. For resistors used in precision instruments and special circuits, precision resistors should be used. For resistors with an accuracy of less than 1%, such as 0.01%, 0.1%, and 0.5%, resistors of the order of magnitude should use Jebson resistors. The rated power of the selected resistor should meet the requirements of the resistor's power capacity in the application circuit. Generally, the power of the resistor should not be increased or decreased at will.
- If the circuit requires a power resistor, its rated power can be 1 to 2 times higher than the power required by the actual application circuit. 2.Fuse resistor selection
- Fuse resistor A resistor with a protective function. Should choose its dual performance when selecting, according to the specific requirements of the circuit to choose its resistance and power parameters. It is necessary to ensure that it can blow quickly under overload, and that it can work stably for a long time under normal conditions. If the resistance value is too large or the power is too large, neither can protect it.
- Three basic principles for resistor selection:
- Choose resistors manufactured to high-level standards from production lines certified by a certification body.
- Choose resistors produced by manufacturers with functional advantages, quality advantages, efficiency advantages, functional price ratio advantages, and service advantages.
- Select a manufacturer that can meet the above requirements and order resistors directly from them.
Resistor considerations
- The resistance should be checked before use. Checking its performance is to measure whether the actual resistance value is consistent with the nominal value and whether the error is within the allowable range. The method is to use the resistance of the multimeter to measure.
- Resistor
- 1. The range should be determined according to the measured resistance value, so that the pointer indicates the middle section of the scale line, which is convenient for observation.
- 2. After determining the resistance range, zero adjustment is required. The method is to short the two test leads (direct contact), adjust the "zero adjustment" appliance so that the pointer is accurately pointed at "0" on the scale, and then measure the resistance. Resistance value. In addition, be careful not to touch the ends of the resistor or the metal part of the test lead with your hands. Otherwise it will cause test error.
- The resistance value measured with a multimeter is close to the nominal value. It can be considered that the quality is basically good, and if the difference is too large or not at all, it is bad.
Resistor detection
- 1. Appearance inspection
- For fixed resistors, first check the signs clearly, the protection paint is intact, no scorching, no scars, no cracks, no corrosion, and the resistor is in close contact with the pins. For the potentiometer, you should also check that the rotating shaft is flexible, properly tightened, and feels comfortable. If there is a switch, check whether the switch operates normally.
- 2.Multimeter detection
- Detection of fixed resistance
- The resistance of the multimeter is used to measure the resistance. For measuring the resistance of different resistance values, different multipliers of the multimeter are selected. For pointer multimeters, because the indication of electrical blocking is non-linear, the larger the resistance value, the denser the indication. Therefore, when selecting a suitable range, the deflection angle of the needle should be larger, indicating at 1/3 to 2/3 full scale. , The reading is more accurate. If the measured resistance value exceeds the tolerance range of the resistance, the resistance value is infinite, the resistance value is 0, or the resistance value is unstable, it means that the resistor is broken.
- During the measurement, pay attention that the hand holding the resistor should not be in contact with the two pins of the resistor. This will make the resistance presented by the hand in parallel with the measured resistance and affect the accuracy of the measurement. In addition, you cannot use a multimeter to electrically block the detection circuit in the case of power failure.
- Resistors (2 photos)
- Detection of fuse resistance and sensitive resistance
- The fuse resistance generally has a resistance of only a few to several tens of ohms. If the measured resistance is infinite, it has blown. You can also check the quality of the fuse resistance online and measure the voltage to ground at both ends. If one end is the power supply voltage and the other end is 0 volts, the fuse resistance has blown.
- There are many types of sensitive resistors. Taking thermistors as an example, they are divided into positive temperature coefficient and negative temperature coefficient thermistors. For positive temperature series (PTC) thermistors, the resistance value is generally not large at room temperature. Use a heated electric soldering iron to approach the resistance during the measurement. At this time, the resistance value should increase significantly, indicating that the resistance is normal. The component is damaged, the negative temperature thermistor is the opposite.
- The photoresistor has a large resistance value measured in the absence of light (covering the light with your hand or object), and a lighted needle indicates a significant decrease in resistance value. If there is no change, the component is damaged.
- Detection of variable resistance and potentiometer
- First measure whether the resistance between the two fixed ends is normal. If it is infinite or zero ohms, or it is significantly different from the nominal value, which exceeds the tolerance range, it means that it is damaged; the resistance of the resistor is normal, and then a multimeter The test lead is connected to the sliding end of the potentiometer, and the other test lead is connected to any fixed end of the potentiometer (adjustable resistor). Slowly rotate the shaft to observe whether the meter needle changes smoothly. When rotating from one end to the other end, the resistance value is zero. If the value of Europe changes to the nominal value (or vice versa) and there is no jump or jitter, it means that the potentiometer is normal. If there is a jump or jitter during the rotation, it means that the sliding point is not in good contact with the resistor.
- 3.Measure resistance with electric bridge
- If the precise measurement of the resistance is required, it can be tested by a bridge (digital). Insert the resistance into the measuring end of the bridge element, select the appropriate range, and then read the resistance of the resistor from the display. For example, when using a resistance wire to make a self-made resistor or processing a fixed resistor to obtain a more accurate resistance value, it is necessary to use a bridge to measure the resistance of the self-made resistor.
Resistor application
- For different applications, the purpose of applying a varistor is not the same as the voltage / current stress on the varistor, so the requirements for the varistor are also different. It is important to distinguish this difference for correct use. of. According to the purpose of use, varistors can be divided into two categories: varistors for protection and varistors for circuit functions.
- Protection varistor
- 1. Differentiate between power supply protection, signal line and data line protection varistor, they must meet the requirements of different technical standards.
- Potentiometer
- 3. According to the different overvoltage characteristics of the varistor, the varistor can be divided into three types: surge suppression type, high power type and high energy type.
- Surge suppression type : refers to a varistor used to suppress transient overvoltages such as lightning overvoltages and operating overvoltages. The appearance of such transient overvoltages is random, non-periodic, and the peak value of current and voltage may be very high. Big. Most varistors fall into this category.
- High power type : refers to a varistor used to absorb the continuous pulse group that appears periodically, such as a varistor connected in parallel to a switching power converter. Here, the impulse voltage appears periodically, and the period can be known. The energy value can generally be calculated It appears that the peak value of the voltage is not large, but because of the high frequency of occurrence, its average power is quite large.
- High-energy type : It refers to a varistor used to absorb the magnetic energy in large induction coils such as generator excitation coils, lifting electromagnet coils, etc. For this type of application, the main technical indicator is energy absorption capacity.
- The protection function of the varistor can be used repeatedly in most applications, but it is sometimes made into a one-time protection device such as a current fuse. For example, a varistor with a short-circuit contact connected in parallel to the load of some current transformers.
Resistor outlook
- The development direction of the resistor is:
- 1. Miniaturization and high reliability;
- 2. Discrete small resistors are still widely used, but will further reduce the size, improve performance, and reduce prices; 3. In consumer electronics, carbon film resistors still dominate, while precision resistors will Metal film resistors are the mainstays, most of the small power wirewound resistors will be replaced;
- 4. In order to adapt to the development of circuit integration and planarization, the demand for chip resistors will increase significantly; general-purpose types will tend to develop thick-film resistors, while precision types will still lean towards thin-film metal films and Metal foil resistors; 5. Develop a combined resistance network;
Resistor Ideal Resistor
- In an ideal resistor, the resistance value does not change with voltage or current, nor does it change due to sudden changes in current. Real resistors cannot achieve this. Today's internal design enables resistors to exhibit relatively small changes in resistance under extreme voltages or currents (and other environmental factors such as temperature).
- Limitations of realistic resistors
- Each resistor has an upper limit on the voltage or current that it can withstand (mainly depends on the size of the resistor). If the voltage or current exceeds this range, the resistance of the resistor will change first (there can be drastic changes in some resistors), and then the resistor will be damaged due to overheating and other conditions. Most resistors will indicate the rated electrical power, while others will provide the rated current or voltage.
- In addition, the actual resistor itself has a small amount of inductance or capacitance in addition to the resistance, which makes its performance different from the ideal resistor.
Resistor identification method
- Identification method of color ring resistance
- Metal foil resistors (3 photos)
- The first and second rings with four color circles respectively represent the first two digits of the resistance value; the third ring represents the magnification; and the fourth ring represents the error. The key to fast identification is to determine the resistance value within a certain order of magnitude according to the color of the third ring, such as a few K or a few dozen K, and then "generation" the numbers read out in the first two rings, This way you can read the numbers quickly.
- (1) Remember the numbers represented by each color of the first and second rings. It can be remembered like this: brown 1, red 2, orange 3, yellow 4, green 5, blue 6, purple 7, gray 8, white 9, black 0. Read them in this way, and repeat them a few times to remember.
Remembering the range of resistance represented by the color of the third ring is the key to quick understanding. specifically is:
Gold: what time is
Black: several dozen
Brown: hundreds of tens of
Red: what time is k
Orange: several dozen k
Yellow: hundreds of tens of k
Green: What time is M
Blue: several dozen M
From the order of magnitude, they can be divided into three major levels in the body, namely: gold, black, and brown are ohmic; red, orange, and yellow are in the thousands of euros; green and blue are in the megaohms Class. This division is for easy memory.
(3) When the second ring is black, the color represented by the third ring is an integer, that is, several, dozens, hundreds of k, etc. This is a special case when reading. Please pay attention. For example, the third ring is red, so its resistance is a whole few k.
(4) Remember the error represented by the color of the fourth ring, that is: 5% for gold; 10% for silver; 20% for colorless.
Here are some examples:
Example 1: When the four color rings are yellow, orange, red, and gold in sequence, because the third ring is red and the resistance range is a few k, the numbers "4" and "2" are represented by the two colors of yellow and orange 3 , and its reading is 4.3 k. The first ring is gold to indicate an error of 5%.
Example 2: When the four color rings are brown, black, orange, and gold in sequence, because the third ring is orange and the second ring is black again, the resistance value should be a few tens of k. The number represented by brown is "1" Plug in and read 10 k. The fourth ring is gold with an error of 5%.
- Identification method of chip resistor
- SMD components have small size, light weight, high installation density and strong shock resistance. Anti-dry
- The resistance of chip resistors is the same as that of general resistors, which are marked on the resistor body. There are three nominal methods of resistance values, but the nominal methods are not exactly the same as general resistors.
1. Digital cable nominal method (generally rectangular chip resistors use this nominal method)
The digital cable nominal method is to use three digits on the resistor body to indicate its resistance value.
For example: "472 '" means "4700"; "151" means "150".
If it is a decimal. "R" means "decimal point". And occupy one significant digit, the remaining two are significant digits.
For example: "2R4" means "2.4"; "R15" means "0. 15".
- Four-digit notation:
- The first three digits represent significant digits, and the fourth digit represents the magnification.
- 2702 = 27000 = 27k
- 2. Color ring nominal method (generally cylindrical fixed resistors use this nominal method)
- The chip resistance is the same as the general resistance, and most of them use four rings (sometimes three rings) to indicate the resistance value. The first and second rings are significant digits, and the third ring is the magnification (color circle codes are shown in Table 1). For example: "Brown Green Black" means "15"; "Blue Gray Orange Silver" means "68k" error ± 10%.
3 E96 digital code and letter mixed nominal method Digital code and letter mixed nominal method also uses three digits to indicate the resistance value, that is, "two digits plus one letter", where two digits represent the E96 series resistance code. For details, see Schedule 2. The third digit is the magnification expressed by the letter code (see Table 3). For example: 51D means 332 × 103; 332k; 249Y means 249 × 10-2; 2.49.
Resistor effect
- Low-power resistors are usually made of carbon film encapsulated in a plastic case, while high-power resistors are usually wire-wound resistors made by winding a high-resistance metal wire around a ceramic core.
- If the resistance of a resistor is close to zero ohms (for example, a large cross-section wire between two points), the resistor has no hindrance to the current. The circuit in parallel with such a resistor is short-circuited and the current is infinite. If a resistor has infinite or large resistance, the loop connected in series with the resistor can be regarded as an open circuit and the current is zero. The resistor commonly used in the industry lies between two extreme cases. It has a certain resistance and can pass a certain current, but the current is not as large as in a short circuit. The current limiting effect of the resistor is similar to the role of the small diameter pipe connected between two large diameter pipes to limit the water flow. Resistance, the English name resistance, is usually abbreviated as R, it is a basic property of a conductor, and it is related to the size, material and temperature of the conductor. Ohm's law states that I = U / R, then R = U / I, the basic unit of resistance is ohms, which is represented by the Greek letter "", and has this definition: when a volt is added to a conductor, an ampere current The corresponding resistance value. The main function of a resistor is to prevent current from flowing. In fact, "resistance" refers to a property, and the resistance usually referred to in electronic products refers to a component such as a resistor. Ohm is often referred to as Europe. Common units used to express resistance values are kiloohms (k), megohms (M), and milliohms (m ).
Resistor development
- 1885 British C. Bradley invented a molded carbon solid-core resistor. In 1897, British T. Gabriel and A. Harris made carbon film resistors from carbon ink. From 1913 to 1919, the British W. Swann and the German F. Kruger invented metal film resistors. In 1925, German Siemens-Harsk company invented a thermal decomposition carbon film resistor, which broke the monopoly market of carbon-based solid-core resistors. After the advent of the transistor, the requirements for miniaturization and resistance stability of resistors were more stringent, which promoted the development of various new types of resistors. American Bell Labs developed TaN resistor in 1959. Since the 1960s, new technologies such as drum magnetron sputtering and laser resistance fine-tuning have been adopted, and some products have developed in the areas of planarization, integration, miniaturization, and flakes.