introduction
There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors and wire wound resistors; special resistors include varistors, thermistors, and photoresistors. The characteristic parameters of different types of resistors have certain differences, and the points that need to be considered when the circuit is used are also different. For engineers who are new to circuit design, they are likely to ignore some special parameters of resistance, resulting in unguaranteed product stability and reliability. A correct understanding of the various parameters of the resistor and the precautions for selection, and a comprehensive understanding of the true role of the resistor in the circuit, can ensure the quality of the product from the most basic circuit design at the bottom.
1 Basic parameters of resistance:
Engineers who are new to hardware circuit design may have the first impression of resistance that the resistance described in the physics book that the conductors hinder current is called resistance. It is represented by the symbol R, and the unit is ohm, kiloohm, and megaohm, respectively. , KΩ, MΩ means. The main parameters of concern are 1). Nominal resistance: the resistance value indicated on the resistor; 2). Allowable error: the percentage of the difference between the nominal resistance and the actual resistance to the ratio of the nominal resistance. Value deviation, which indicates the accuracy of the resistor. In the design of the circuit, it is not enough to pay attention to these two parameters. There are two other important parameters that must be paid attention to in the design: rated power and withstand voltage. These two parameters have an effect on the reliability of the entire system The impact is very big.
If the current flowing through the resistor in the circuit is 100mA and the resistance value is 100Ω, then the power consumption on the resistor is 1W. It is inappropriate to choose a commonly used chip resistor, such as 0805 or 1206 package, because the rated power of the resistor is small. And there is a problem. Therefore, the rated power of the resistor should be selected to be above 1W (the power margin of the resistor selected in the circuit design is generally more than 2 times), otherwise the power consumed on the resistor will overheat the resistor and cause it to fail.
Similarly, if the withstand voltage is not properly selected, the system design will fail due to the breakdown of the resistance. For example: AC-DC switching power supply module is designed at the input front end, according to the requirements of the safety standard GB4943.1, after ensuring that the plug or connector is disconnected, the stagnant voltage on the input terminals L and N is within 1S Attenuation to 37% of the initial value. Therefore, in the design, one or two MΩ impedance resistors are generally used to discharge energy, and the input end is high voltage, that is, both ends of the resistor must withstand high voltage. In the case of high pressure at the low-voltage input end, failure will occur. The following table 1 is the parameters of common SMT thick film resistors, and the final selection should be verified with the manufacturer of the component.
Table 1 Commonly used SMT thick film resistors
Note: For reference only, the manufacturer’s instructions for the final selection shall prevail
2 The role of resistance in the circuit:
2.1 Basic functions:
Electronic engineers have learned the basic role of resistance, which is used as a voltage divider, shunt and load resistance in a circuit; it can be used as a filter and delay circuit with a capacitor, and used as a sampling in a power circuit or a control circuit. Resistance; Used as a bias resistor in a semiconductor tube circuit to determine the operating point, etc. For these functions, the circuit is used in many applications, and it is also very important, so I won't do too much description. The following mainly introduces the function of 0Ω resistance and special resistance in electronic circuit design and the precautions for use.
2.2 The role of 0 ohm resistor in the circuit:
I believe that there are many new electricians who often see 0Ω resistors in the circuit when they look at some electronic products designed by their predecessors. Why do you want to design such a resistor? It's not enough to connect the drawing board directly. Why do you add more to it? The main points of data search and sorting are as follows:
1) Single-point grounding of analog ground and digital ground
As long as it is the ground, it will eventually be connected together, and then into the earth. If they are not connected together, it is "floating", there is a pressure difference, and it is easy to accumulate charges and cause static electricity. The ground is referenced to the 0 potential, and all voltages are derived from the reference ground. The ground standards must be consistent, so various grounds should be short-circuited together. It is believed that the earth can absorb all electric charges and maintain stability at all times, which is the ultimate ground reference point. Although some boards are not connected to the earth, the power plant is connected to the earth, and the power on the board will eventually return to the power plant to the ground. If the analog ground and the digital ground are directly connected in a large area, it will cause mutual interference. There are four ways to solve this problem if it is not short-circuited or appropriate. There are four ways to solve this problem: 1. Connect with a magnetic bead; 2. Connect with a capacitor; 3. Connect with an inductance; 4. Connect with a 0 ohm resistor.
The equivalent circuit of the magnetic bead is equivalent to a band-stop wave limiter, which only has a significant suppression effect on the noise of a certain frequency point. The noise frequency must be estimated in advance in order to select the appropriate model. For situations where the frequency is uncertain or unpredictable, the magnetic beads do not match; the capacitors are separated from each other and cause floating; the inductance is large, and there are many stray parameters, and it is unstable; 0 ohm resistance is equivalent to a very narrow current path, which can effectively limit The loop current makes the noise suppressed. Resistance has an attenuation effect in all frequency bands (0 ohm resistance also has impedance), which is stronger than magnetic beads.
2) Used for current loop when jumper
When the electrical ground plane is divided, the shortest return path of the signal is broken. At this time, the signal loop has to be detoured, forming a large loop area, and the influence of electric and magnetic fields becomes stronger, and it is easy to interfere/be interfered. Connecting a 0 ohm resistor across the partition can provide a shorter return path and reduce interference.
3) Configuration circuit
Generally, jumpers and DIP switches should not appear on the product. Sometimes users will change the settings randomly, which is easy to cause misunderstanding. In order to reduce maintenance costs, 0 ohm resistors are used instead of jumpers to be soldered on the board. Vacant jumpers are equivalent to antennas at high frequencies, and the use of patch resistors is effective.
4) Other uses
For cross-wire debugging/testing during wiring: At the beginning of the design, a resistor should be connected in series for debugging, but the specific value cannot be determined yet. After such a device is added, it is convenient for future circuit debugging. If the result of debugging, no resistor is needed. , Add a 0 ohm resistor. Temporary replacement of other SMD devices as temperature compensation devices is more often due to the need for EMC countermeasures. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the via, and the via will also affect the ground plane (because of the need to dig a hole). Summarized as follows:
1. There is no function in the circuit, just for debugging convenience or compatible design on the PCB.
2. It can be used as a jumper, if a certain section of the line is not used, just stick the resistor directly (not affecting the appearance)
3. When the matching circuit parameters are uncertain, replace it with 0 ohms. In actual debugging, determine the parameters and replace them with specific numerical components.
4. When you want to measure the current consumption of a certain part of the circuit, you can remove the 0 ohm resistance and connect the ammeter to facilitate the measurement of the current consumption.
5. When wiring, if it is really impossible to deploy, you can also add a 0 ohm resistor.
6. Under high-frequency signals, it can be used as an inductor or capacitor (related to the characteristics of the external circuit), mainly to solve the EMC problem. Such as between ground and ground, power supply and IC Pin.
7. Single-point grounding (referring to the separation of protective grounding, working grounding, and DC grounding on the equipment, each becomes an independent system).
2.3 The role of special resistors in the peripheral protection circuit of the power module
The most common special resistors are varistors and thermistors, which play a key role in the design and application of AC-DC switching power supplies. Understand the characteristics and specific functions of these two resistors:
Varistor MOV is one of the most commonly used devices in circuit electromagnetic compatibility EMC. It is widely used in electronic circuits to prevent possible damage to circuits due to transient voltage changes in the power supply system. Its characteristics are generally understood as when the front-end voltage is higher than the turn-on voltage of the varistor, the varistor is broken down, and the resistance of the varistor decreases and the current is shunted to prevent the subsequent stage from being damaged or interfered by excessive instantaneous voltage. This protects sensitive electronic components. Circuit protection is to use the non-linear characteristics of the varistor. When an overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
However, do not think too much about the role of varistors. Varistors cannot provide complete voltage protection. The energy or power that varistors can withstand is limited and cannot provide continuous overvoltage protection. . Continuous overvoltage will destroy the protective device (varistor) and cause damage to the equipment. The parts that the varistor cannot provide protection include: inrush current when starting up, overcurrent when short-circuited, and voltage dips. These situations require other means of protection.
Thermistor is a temperature-related device, generally divided into two types, NTC is a negative temperature coefficient thermistor, that is, the higher the temperature, the smaller the impedance; PTC is a positive temperature coefficient thermistor, that is, the higher the temperature, the impedance Bigger. The use of impedance's sensitivity to temperature plays an important role in circuit design.
NTC in the circuit mainly suppresses the starting current during the starting of the circuit. When the system is started, due to the power circuit, capacitive and inductive loads inside the system, a very large inrush current will appear at the moment of starting. If the instantaneous anti-current capability of the device is not considered in the selection process of the circuit device, then the system will easily cause the device to be broken down during the operation process of multiple startups, and adding NTC to the circuit is equivalent to when the input loop is started. Increase the input impedance to reduce the impact current, and when the system is in a stable state, due to the NTC heating, according to its negative temperature characteristics, the impedance is reduced, so the loss on the NTC is also reduced, reducing the overall loss of the system.
PTC can function as a fuse in the circuit, so it has another name for self-recovery fuse. During the operation of the system, when the circuit is abnormal and causes a large current, if there is a PTC in this part of the circuit, it means that there is a large current flowing in the PTC, and the PTC generates heat. According to its positive temperature characteristics, its The impedance will become very large, so that the impedance of the entire loop becomes larger, so that the current of the loop becomes smaller, which acts as a fuse. According to its positive temperature characteristics, another function of PTC is to realize over-temperature protection in the circuit.
3 Conclusion:
The knowledge of resistance covers a lot, not only can you apply Ohm’s law well after knowing Ohm’s law, it also includes the material and its special properties. For example, the resistance value of a resistance element is generally related to temperature, material, length, and cross-sectional area. , The physical quantity that measures the magnitude of resistance affected by temperature is the temperature coefficient, which is defined as the percentage change in the resistance value when the temperature rises by 1°C; the main physical feature of resistance is that electrical energy is transformed into thermal energy, which can also be said to be an energy consumption Component, the current passing through it produces loss, which is expressed in the form of heat; the resistance usually plays the role of voltage division and shunting in the circuit; for the signal, both AC and DC signals can pass through the resistance. As a hardware engineer, if you want to use the components handily, you need to have an in-depth understanding of their materials, electrical characteristics and their particularities.
