In general, when choosing a resistor, we usually consider the resistance value. But should you choos

From a professional point of view, the selection of resistors is generally made as follows:

1. Resistance value;

2. Resistance type: wirewound resistance (generally used as a fuse), carbon film resistance (average accuracy, cheap, large size), metal film resistance (high precision, expensive, small size), cement resistance (worst accuracy, cheapest, larger size), SMD resistance (used in small power occasions, the accuracy can be very high, and different packaging power is different);

3. Power, 50% of the reserved amount can be retained during design;

4. Packaging, considering the aesthetics and production process, is a must;

5. Withstand pressure and current, if you want to use it safely, you must derate it;

6. **, it goes without saying;

If there are no very special circumstances, most circuits will not put forward requirements for the material of the resistor used, because this is tantamount to asking for trouble, the more versatile the device is selected, the more beneficial it is to reduce the cost of their products and enhance the convenience of future maintenance. Of course, when the resistance value and power of the resistor reach a certain range, the resistance material and process selected by the resistance manufacturer will indeed be different, and there is not much room for change at this time.

Generally, you can feel the material by looking at the power you need, and the power that can be achieved by the resistance of different materials is different.

Generally, when choosing a resistor, you generally don't define it, it's hard to say, but different materials will have different properties.

There are many kinds of resistor materials, and the common resistor materials are as follows:

1.Carbon materials: Carbon materials include graphite, activated carbon, bulk carbon, etc., which are often used to manufacture high-precision and low-noise resistors.

2.Metal materials: Metal materials such as iron, copper, chromium, nickel, tungsten, etc., are mainly used to manufacture high-power resistors, with good electrical conductivity and thermal conductivity.

3.Ceramic materials: Ceramic materials are resistant to high temperatures and corrosion, and are commonly used in the manufacture of high-precision fixed resistors and potentiometers.

4.Semiconductor materials: Semiconductor materials such as silicon, germanium, etc., are often used in the manufacture of voltage regulators and temperature sensors.

5.Clay materials: Clay materials include carbon clay, corona ceramics, porcelain bottle halo clay, etc., which are commonly used in the manufacture of high-voltage resistors.

The above are common resistive materials, each of which has its own characteristics and scope of application. Different circuit applications require different resistor materials.

When selecting resistors, only the nominal resistance, rated power, and resistance error are generally considered, and other parameters are only considered when there are special requirements.

1) The nominal resistance value of the resistor ref01ez indicates the resistance value on the resistor, and the nominal resistance value of the resistor is called the nominal resistance value of the resistor, and the state has unified regulations on the nominal resistance value of the resistor. The nominal resistance series of resistors is shown in Table 1-1.

2) The allowable power of the rated power, which is under long-term continuous load of the resistor without changing its performance under standard atmospheric pressure and a certain side-rise ambient temperature, is called the rated power. Table 1-2 shows the rated power series of resistors.

3) The maximum working voltage resistor is called the maximum working voltage under the condition of normal operation. When the working voltage of the resistor is too high, although the power loss does not exceed the specified value, arc spark discharge may occur inside the resistor, which will damage and deteriorate the resistor.

When selecting resistors, only the nominal resistance, rated power, and resistance error are generally considered, and other parameters are only considered when there are special requirements. (1) The nominal resistance of the resistor ref01ez

The resistance value marked on the resistor is called the nominal resistance value of the resistor, and the state has unified regulations on the nominal resistance value of the resistor. The nominal resistance series of resistors is shown in Table 1-1.

2) Rated power.

The allowable power of a resistor under standard atmospheric pressure and a certain ambient temperature without changing its performance for a long time is called the rated power. Table 1-2 shows the rated power series of resistors.

3) Maximum working voltage.

Under normal working conditions, the maximum voltage value that can be withstood by both ends of the resistor is called the maximum operating voltage. When the working voltage of the resistor is too high, although the power loss does not exceed the specified value, the arc spark discharge may occur inside the resistor, which will damage and deteriorate the resistor.

Sampling resistor, also known as current detection resistor, sampling resistor refers to current sampling and voltage sampling, and the current sampling series resistance value is small, its function is to accurately collect the current in the circuit, voltage sampling parallel resistance value of the resistor. This type of resistor is named according to the use and function of the product, so engineers call it a sampling resistor.

Sampling resistor effect.

The sampling resistance function is used as a reference, commonly used in the feedback circuit, taking the regulated power supply circuit as an example, in order to keep the output voltage constant, a part of the voltage should be taken from the output voltage as a reference (commonly used in the form of sampling resistance), if the output is high, the input terminal will automatically reduce the voltage to reduce the output; If the output is low, the input will automatically increase the voltage to increase the output. It is generally used in power supply products, or the power supply part of electronic, digital, and electromechanical products, and is powerful. Sampling resistors are commonly seen on many electronic products.

Characteristics of the sampling resistor.

Sampling resistance is generally divided into plug-in resistance, chip resistance, sampling resistance, low resistance value, high accuracy according to the requirements of the specific substrate, the general resistance value accuracy is within 1%, and the higher requirements of the use of precision resistor. Most of the plug-in resistors produced in domestic factories are made of constantan and manganese copper. However, most users need the high-precision resistors of the chip to achieve the sampling function to meet the automation requirements of small product production.

It is rare for a manufacturer to produce resistors with low temperature coefficients, high accuracy, and ultra-low resistance values to meet user requirements.

Generally, the sampling resistance value is less than 1 ohm, which belongs to the milliohm induction resistance, but some resistances, sampling voltages, etc., must choose a large resistance value resistor, but this kind of resistance base is large and the error is large. In this case, it is necessary to select a high-precision non-inductive resistor (which can achieve an accuracy of 1/10,000) to make the sampling data very reliable. Ultra-low resistance value resistors (ohms, 2 mOhms, 3 mOhms, 10 mOhms, etc.), SMD alloy resistors, high-power resistors (20W, 30W, 35W, 50W, 100W) and other products with a temperature coefficient of plus or minus 5ppm.

The <> alloy chip resistor adopts a special alloy that conforms to the high power electrical characteristics, high purity, high thermal conductivity, low temperature drift and high temperature resistance, and the integrated structure is formed without cutting, which greatly reduces the heat dissipation problem on the circuit board and can achieve almost no inductance value.

As a current carrier, the alloy resistor has accurate resistance, temperature stability, product safety, and stability is significantly higher than that of ordinary ceramic alloy resistors. At the same time, the thermal conductivity of metal is also its great advantage. Especially in the application of power supplies and other related products, when instantaneous impulse current, short-circuit current or pulse current is generated, the alloy is the first choice as a current medium to detect the current.

The selection of resistors should pay attention to the following aspects:

1. In high-gain amplification circuits, resistors with low noise electromotive force should be selected, such as metal film resistors, carbon film resistors and wirewound resistors.

2. Different types of resistors are selected according to the working frequency of the circuit. The distribution parameters of wirewound resistors are larger, and even if non-inductive wound resistors are used, their distribution parameters are much larger than those of non-wirewound resistors, so wirewound resistors are not suitable for operation in high-frequency circuits. In circuits below 5okhz, wirewound resistors can be selected because the distribution parameters of the resistors have little influence on the circuit operation.

Resistors in high-frequency circuits require as small distribution parameters as possible. Therefore, carbon film resistors, metal film resistors, and metal oxide film resistors should be used in high-frequency circuits up to hundreds of megahertz. In UHF circuits, UHF carbon film resistors should be selected.

3. The metal film resistor has good stability, high rated working temperature (+70, good high-frequency characteristics, low noise and electromotive force, and should be preferred in high-frequency circuits. For carbon film resistors with resistance values greater than 1m, metal film resistors should be used instead due to their poor stability.

4. Thin film resistors are not suitable for working in environments with high humidity (relative humidity greater than 80%) and low temperature (-40). For circuits operating under such environmental conditions, solid-core resistors or glass-glazed resistors should be used.

5. For low-resistance resistors that require good heat resistance and strong overload capacity, oxide film resistors should be selected.

6. For resistors that require high voltage resistance and high resistance value, synthetic film resistors or glass glaze resistors should be selected.

7. For resistors that require high power dissipation, low resistance value, low working frequency, and high precision requirements, wirewound resistors should be selected.

8. For the same type of resistor, when the resistance value is the same, the greater the power, the worse the high-frequency characteristics.

When selecting the resistor, it is not only required that its various parameters (rated power, resistance value, allowable deviation, withstand voltage, etc.) meet the use conditions of the circuit, but also consider the factors such as size and quality. The nominal resistance series should be selected. If the allowable deviation is more than 5%, the rated power of the selected resistor is 2 3 times the actual calculated value.

The type of resistor can also be selected according to the operating frequency of the circuit. The distributed inductance and distributed capacitance of the Rx type wirewound resistor are large, and they are only suitable for circuits with a frequency of 50kHz. RH type synthetic film resistors and RS type organic solid resistors can be used in circuits of tens of megahertz; RT type carbon film resistor can be used in circuits around 100MHz; On the other hand, RJ-type metal film resistors and R-type oxide film resistors can operate in high-frequency circuits up to hundreds of megahertz. In the selection process, the specific requirements of the circuit should be carefully analyzed.

In those circuits with high requirements for certainty, heat resistance and reliability, metal film or metal oxide film resistors should be selected; If high power, good heat resistance, and low co-group frequency are required, wirewound resistors can be selected; For general circuits that do not require special requirements, carbon film resistors can be used to reduce costs.

The resistance of the object r= l s

In the above equation, r is the resistance value – the commonly used unit

is resistivity – the common unit ·m

s is the cross-sectional area – a commonly used unit

l is the length of the wire - the common unit m

Resistivity is a physical quantity used to express the electrical properties of various substances. It depends on the properties of the material.

The resistivity of a wire made of a material with a length of 1 meter and a cross-sectional area of 1 square millimeter at room temperature (20 hours) is called the resistivity of this material. The unit of resistivity is the ohm-meter (·m or ohmm), and the common units are the ohm-millimeter and the ohm-meter.

The other 1 resistivity is not only related to the material of the conductor, but also to the temperature of the conductor. In the range where the temperature does not change much: the resistivity of almost all metals varies linearly with temperature, i.e. = o(1+at)

where t is the temperature in Celsius, o is the resistivity at o, and a is the temperature coefficient of resistivity.

2 Since resistivity changes with temperature, the physical state in which some electrical appliances are resisted must be stated. For example, the resistance of a 220 V -100 W electric light filament is 484 ohms when energized, and only about 40 ohms when unenergized.

3 Resistivity and resistance are two different concepts. Resistivity is a property that reflects the effect of a substance on the resistance of an electric current, and resistance is a property that reflects the effect of an object on the resistance of an electric current.

Resistance resistance is the ability in a substance to hinder the flow of electrons, i.e., the resistance value, in units. A resistor is a device that has a certain resistance to the flow of current. For the sake of convenience, resistors are often referred to simply as resistors.

The main parameters of the resistance 1The values marked on the nominal resistance and error resistors represent the nominal resistance value, and in order to facilitate the standardized production, not any kind of resistance can be produced. The resistance value of the resistor is mainly divided into four series according to its accuracy, which are E-6, E-12, E-24, and E-96.

The allowable error of the resistor refers to the error between the actual resistance value and the manufacturer's marked resistance, and the actual value within the allowable range of error is called a qualified resistor. 2.The rated power of the rated power resistor refers to the maximum power that the resistor can work continuously for a long time and meet the performance requirements when the resistor is working normally.

Above this value, the resistor will burn out due to excessive heat. 3.Maximum Operating VoltageThe maximum operating voltage is the operating voltage at which the resistor will not overheat or break down during prolonged operation.

If the voltage exceeds this value, sparks may occur inside the resistor, resulting in deviations in circuit performance. 4.High Frequency Characteristic ResistorIn high frequency operation, the resistor will become a DC resistor in series distributed inductor, parallel distributed capacitance element.

In this case, it is necessary to consider the influence of the intrinsic inductance and intrinsic capacitance of the resistor on the circuit.