Common electronic components-resistance (simple detection)

I really need to move anywhere. I recently repaired the board. Just reviewing the knowledge of electronics, strong electricity has been dry for a long time, and many things are blurred.

The appearance of the resistor is correct, the color is even and shiny, and there is no crack, scorch and corrosion, so it can be preliminarily judged that the resistor is normal. Then measure the resistance value of the resistor and compare it with the nominal resistance. If it is within the error range, the resistor is in good condition. Let's look at common resistances.

1 metal film resistance

Metal film resistance mainly uses vacuum deposition technology to form a nickel-chromium alloy coating on ceramic rods, and then processes spiral grooves on the coating to accurately control the resistance. Metal film resistance can be said to be a good resistance with high precision, good temperature characteristics, low noise and strong stability. Its physical diagram is as follows:

? The nominal resistance of the metal film is as follows:

First, find the wrong color ring, so as to arrange the color ring order.

The colors most commonly used to represent the resistance error are: gold, silver and brown, especially the gold ring and silver ring, which are rarely used as the first ring of the resistance color ring, so as long as there are gold rings and silver rings in the resistance, it can basically be regarded as the last ring of the color ring resistance.

Brown rings are often used as error rings and significant digital rings, and often appear in the first ring and the last ring at the same time, making it difficult for people to identify who is the first ring. In practice, it can be judged according to the interval between color rings: for example, for the resistance of five-channel color rings, the interval between the fifth ring and the fourth ring is wider than that between the first ring and the second ring, so the arrangement order of color rings can be judged accordingly.

The first color circle is one hundred digits, the second color circle is ten digits, the third color circle is one digit, the fourth color circle is the power of color, and the fifth color circle is the error rate.

As shown in the above figure, for example, brown, black, red and brown, the resistance is100×10 2 =10k, and the error is 1%. The fluctuation of the standard value (1%× 100K) shows that the resistance is acceptable, that is, it is a good resistance between 99k-10/0k. (See the comparison table below for numerical values:)

? The method of detecting the resistance quality of metal film is as follows: first, read the corresponding nominal resistance, select the ohm file corresponding to the universal board according to the nominal resistance, and compare it with the nominal resistance. Within the error range, the resistance is normal. Out of the error range, the resistance is damaged.

Open circuit measurement, only need to measure once; In road survey, exchange pens and measure twice to get the larger value.

2, metal oxide film resistance

Its mechanism is to form tin oxide film on ceramic rod. In order to increase the resistance, an antimony oxide film can be added to the tin oxide film, and then a spiral groove can be machined on the oxide film to accurately control the resistance. The biggest advantage of metal oxide film resistance is high temperature resistance. Its physical diagram is as follows:

3. Carbon film resistance

Before the carbon film resistor, there was a carbon composite resistor, which had almost no advantages (low accuracy, poor temperature characteristics and high noise) except good voltage resistance, so it was eliminated. As an improvement, the carbon film resistor mainly forms a carbon mixture film on ceramic rods, for example, the thickness of the carbon film and the carbon concentration in it can control the resistance; In order to control the resistance more accurately, spiral grooves can be machined on the carbon film, and the more spirals, the greater the resistance; Finally, metal leads are added and a resin package is formed. However, due to the material, its temperature characteristics are still not very good, and its temperature characteristics are negative. The higher the temperature, the lower the resistance. Its physical diagram is as follows:

First, let's understand the following three questions:

? Q 1: What is the difference between carbon film resistance and metal film resistance?

Answer 1: ① metal film has five color rings (1%), while carbon film has four color rings (5%); ② Under the same power condition, the volume of metal film resistance is much smaller than that of carbon film resistance, which is about half of that of carbon film resistance.

? Q 2: How to distinguish between carbon film resistance and metal film resistance?

Answer 2: There are usually two methods: ① Scrape off the protective paint with a blade, and the exposed paint film is black, which is carbon film resistance; If the color of the film is bright white, it is a metal film resistor; (2) Because the temperature coefficient of metal film resistance is much smaller than that of carbon film resistance, the resistance can be measured by multimeter, and then the resistance can be approached by hot ferrochrome. If the resistance changes greatly, it is carbon film resistance, and vice versa.

? Q 3: What are the applicability of carbon film resistance and metal film resistance?

Answer 3: ① If it is used within the rated parameters, there is no difference in service life and reliability between them; ② The working conditions are complicated. Sometimes, when there is short-term overcurrent or the temperature changes in a large range, the resistance of metal film is better, and the resistance of metal film is stronger than that of carbon film mainly in terms of accuracy and temperature drift. ③ The pulse load stability of carbon film resistance is good (the change process of physical quantity that suddenly changes in a short time and then quickly returns to the initial value is called pulse).

? The nominal resistance of the carbon film is as follows:

The first colored circle is ten digits, the second colored circle is single digits, the third colored circle is the power of color, and the fourth colored circle is the error rate.

Taking the above figure as an example, the resistances of brown, black, yellow and gold are10×104 =100k. An error of 5% indicates the resistance value, and a fluctuation from the standard value (5%× 100K) indicates that the resistance value is acceptable. Just look at the numerical comparison table in the text.

? The method of detecting carbon film resistance is the same as that of detecting metal film resistance.

4. Cement resistance

Cement resistor is a resistor filled with refractory mud. It is mainly composed of core, packaging filler, solder lead and ceramic shell. Cement resistor has the characteristics of large volume, impact resistance, moisture resistance, heat resistance, good heat dissipation and low price. But the precision and stability of its resistance are poor.

The core structure belongs to winding type, and the winding resistance is that Ni-Cr alloy wires are wound on the alumina ceramic substrate, and the resistance is controlled one by one. Winding resistance can be made into precision resistance with tolerance of 0.005% (cement resistance generally does not belong to this category), and the temperature coefficient is very low. The disadvantage is that the parasitic inductance of winding resistance is relatively large and cannot be used at high frequency. Its physical diagram is as follows:

Cement resistor is a kind of high-power resistor, so it is widely used in workplaces that allow high power and high current, such as resistors used to limit the starting resistance of motors instead of the load output, and also in various power products and various instruments and equipment.

? The reading method of the nominal resistance of cement resistance can be clearly seen on the component shell. Here we mainly talk about the meaning of Chinese and English code names of resistors.

Numerically: r stands for decimal point, k is 100, and m is100000;

Error values: e is 0.5%, f is 1%, g is 2%, j is 5%, k is 10%, and m is 20%;

In the above example, the resistance is: the power is 5W, the resistance value is 12K, and the error value is 5%.

? The test method of cement resistance is the same as that of metal film resistance.

5, metal foil resistance

The resistance of metal foil is formed by melting Ni-Cr alloy in vacuum, then it is made into metal foil by rolling, and then it is bonded to alumina ceramic substrate. Then the shape of metal foil is controlled by photolithography process, so as to control the resistance. Metal foil resistance is the best resistance with controllable performance at present. Its physical diagram is as follows:

6, patch resistance

① Thin film resistance

Thin film resistor is a chromium-nickel thin film formed on alumina ceramic substrate by vacuum deposition, and its thickness is usually only 0. 1um, which is only one thousandth of that of thick film resistor. Then, the thin film is etched into a certain shape by photolithography, which is very accurate and can control the performance of thin film capacitor well.

② Thick film resistance

The screen printing method used in thick film resistor is to paste a layer of silver palladium electrode on ceramic substrate, and then print a layer of ruthenium dioxide between the electrodes as resistor. The resistive film of thick film resistor is usually thick, about 100 micron. Thick film resistor is the most widely used resistor at present, and its price is cheap, and its tolerance is 5% and 1%. Most products use 5% and 1% patch thick film resistors. Its physical diagram is as follows:

? How to read the nominal resistance value of the SMD resistor: On the surface of the SMD resistor, three (four) digits are used to represent the resistance value, where the first and second digits (the third digit) are significant digits, and the third digit represents the number of zeros behind, and the unit is Ω. When there is a decimal point, it is represented by "r" and occupies a significant number. Nominal resistance code representation.

Take the three numbers printed on the resistor for example: "472" means "4700Ω"151"means"150Ω ". If they contain English (r, k, m) with numerical meaning, they occupy a valid number. For example, "2R4" means "2.4ω" and "r15" means "0. 15ω".

The mark on the patch resistor in the above figure is 4 digits (indicating that the error is1%): "24r0" means 24.0ω.

? The method of detecting patch resistance is the same as that of detecting metal film resistance.

7. Exclusion

Row resistance (row resistance for short) is a kind of combined resistance which integrates discrete resistors arranged according to certain rules. The row resistor is small in size and convenient to install, which is suitable for situations where multiple resistors have the same resistance value and one pin is connected to the same position of the circuit. Its physical diagram is as follows:

? How to read the nominal resistance of a row resistor: white dots represent male * * * pins, usually on both sides; The first letter represents the circuit type. On the surface, the row resistor is represented by three numbers, where the first and second numbers are significant numbers, the third number indicates that the number is followed by zero, and the last letter indicates the error value.

The circuit types are as follows: a. A plurality of resistors share one end, and the left end of the shared end leads out; B. The resistors are led out separately and are not connected with each other; C, each resistor is connected end to end, and each end is led out; D, the common terminals of all resistors are led out from the middle of the common terminals; E. All resistors have a common terminal, and both ends of the common terminal are led out; F and g are more complicated.

Example in the above figure: A 103J, where a represents the common terminal of multiple resistors, and the left end of the common terminal is led out; The resistance value is10k; ; The error value is 5%.

? Check the row resistance: For the row resistance with known pin arrangement sequence, connect one contact pin to the male pin, and measure each resistance with another contact pin in turn, and its resistance value should meet the nominal value.

For row resistors with unknown pin arrangement order, first connect the red probe to any pin of the resistor to be tested, and then use the black probe to test the other pins. If the values obtained are the same, it means that the red probe is connected to the male pin of the measured row resistance.

8. Insurance resistance

Safety resistor has dual functions of resistor and fuse. When the surface temperature reaches 500 ~ 600℃ due to overcurrent, the resistance layer will peel off and fuse. Therefore, the safety resistor can be used to protect other components in the circuit from damage, so as to improve the safety and economy of the circuit. Safety resistors can be divided into two types: unrepairable and repairable.

The unrecoverable physical diagram is as follows:

The physical diagram of PTC is as follows:

? As can be seen from the above figure, they have many forms, but at present, the symbols are not uniform, but they have the following characteristics:

(1) They are obviously different from the labels of general resistors, which is easy to judge in the circuit diagram.

(2) It is generally used in low-voltage or high-voltage circuits with large current capacity of power supply circuits or secondary power supplies.

(3) Generally, it has low resistance (several ohms to tens of ohms) and low power (1/8~ 1W).

(4) In the circuit, the safety resistor is welded on the circuit board with long legs (generally, the resistor is welded close to the circuit board) and far away from the circuit board, which is convenient for heat dissipation and differentiation.

? Let's mainly look at the method of detecting the quality of fuse resistance: use multimeter R× 1 or R× 100 when measuring. If the resistance is greatly out of range or fails, the safety resistor is damaged.

? 9. Electrothermal Regulator

Thermistor is a kind of sensitive element, which can be divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient (NTC) according to different temperature coefficients. Positive temperature coefficient thermistor (PTC) has higher resistance at higher temperature. Negative temperature coefficient devices (NTC) have lower resistance at higher temperatures, and they all belong to semiconductor devices. Its physical diagram is as follows:

PTC is also mentioned above, which can be used as a fuse in the circuit. There are many forms of thermistors, so I won't elaborate on the meaning of specific models here. Let's take a look at the method of detecting the quality of thermistor: measure its normal temperature resistance with a multimeter, compare it with the nominal resistance, and then put the heated electric soldering iron near the thermistor to see if its change is normal.

10 photoresistor

Photosensitive resistor is a kind of resistor made by photoelectric effect of semiconductor, and its resistance value varies with the intensity of incident light. The intensity and resistance of incident light decrease, the incident light is weak, and the resistance increases. Its physical diagram is as follows:

Its dark resistance can reach several to several tens of megohms, while its bright resistance is only several hundred to several tens of kiloohms. According to this characteristic, we can easily get a method to detect the quality of photoresistors:

The light receiving surface of the photoresistor is perpendicular to the incident light, and its bright resistance is measured. If the value is large, the internal circuit is open. Cover the light receiving surface completely with black paper and measure its dark resistance. If this value is small, it will be short-circuited. Shake the black paper on the light receiving surface to intermittently receive the light from the west, and observe the change of its resistance. If there is no change, the photosensitive material will be damaged.

1 1 rheostat

Rheostat is a kind of voltage limiting protection device. Based on the nonlinear characteristics of varistor, overvoltage appears between the two electrodes of varistor, which is equivalent to a resistor with infinite resistance.

The voltage protection function of rheostat in the circuit can generally be used with fuse for lightning strike or other overvoltage protection. Usually used for lightning protection; When overvoltage occurs, the varistor will be broken down, showing a short circuit state, thus clamping the voltage at both ends at a low level. At the same time, the overcurrent caused by short circuit will burn the fuse in front or force the air switch to trip, thus forcibly cutting off the power supply. Its physical diagram is as follows:

In addition, varistor can not provide complete voltage protection, and the energy or power that varistor can bear is limited, so it can not provide continuous overvoltage protection. Continuous overvoltage will damage the varistor and damage the equipment, and there may be a fire hazard. The parts that varistor can't provide protection are: surge current at startup, overcurrent at short circuit, voltage drop, etc. These situations require other protection methods. Moreover, due to its inherent parasitic capacitance, it is not suitable for high frequency occasions.

? Method for testing the quality of varistor: Measure the positive and negative insulation resistance between the two pins of varistor with multimeter R* 1k, which is infinite and normal, otherwise it means that the leakage current is large. When the measured resistance is small, the varistor is broken down.

12 potentiometer

The potentiometer is mainly composed of elements with three terminals, in which there is a constant resistance between two fixed terminals, and the other end slides on the resistor body through the brush, so the resistance values from the sliding end and the fixed end of the potentiometer will change with the position of the sliding end. Its physical diagram is as follows:

? Method for detecting the quality of potentiometer: When measuring, contact the red and black probes of multimeter with the stator pins (that is, the pins on both sides) respectively, and the reading of multimeter should be the nominal resistance value of potentiometer (as shown in the above picture, 100ω). If the multimeter reading differs greatly from the nominal resistance, the potentiometer is damaged.

When the nominal resistance of the potentiometer is normal, measure its variable resistance and whether the contact between the movable contact and the resistor (fixed contact) is good. At this time, one probe of the multimeter is connected to the moving contact pin (usually the middle pin) and the other probe is connected to a contact pin (two pins).

After connecting the probe, the multimeter should display zero or nominal resistance, and then rotate the rotating shaft of the multimeter from one extreme position to another extreme position, and the resistance should continuously change from zero (or nominal resistance) to nominal resistance (or zero). When the shaft handle of the potentiometer rotates or slides, if the pointer of the multimeter moves smoothly or the displayed number changes evenly, it means that the potentiometer under test is good; If the resistance reading of the multimeter jumps when rotating the shaft handle, it means that the moving contact of the potentiometer under test has a fault of poor contact.

To sum up:

According to the experience of repairing PCB, resistor is the most numerous component on PCB, but it is not the component with the highest damage rate. Resistance damage is the most common in open circuit, and it is rare that the resistance becomes larger and the resistance becomes smaller. General damage is common in low resistance (below 100ω) and high resistance (above 100ω).

When the low resistance resistance is damaged, it will often burn and turn black, while when the high resistance resistance is damaged, there is almost no trace. To sum up, in many cases, we only need to pay attention to the resistance components whose measured value is greater than the nominal resistance, which can greatly improve the maintenance efficiency.

In addition, the packaging color of the resistor does not correspond to the type of the resistor. It used to be related, but now manufacturers do it casually, pay attention to identification.

At the end of the article, the power supply problem is mentioned. When the resistor works in the circuit, the power it bears shall not exceed the rated power of the resistor. In order to ensure the normal operation of the circuit without being damaged, there must be a choice. Usually, the rated power of the resistor used should be more than twice the actual power, so as to ensure the reliability of the resistor working in the circuit for a long time.

Resistors with different power have different package sizes, so we can know the power by measuring the package size of resistors and comparing it with the technical manual.