How to choose the resistance in the circuit design?
Resistor is one of the most commonly used devices in electronic products. Basically, as long as it is an electronic product, there is a resistor inside. The resistor can be used as a voltage divider, shunt and load resistor in the circuit; it can be used as a filter and delay circuit together with the capacitor; as a sampling resistor in the power supply circuit or control circuit; as a semiconductor tube circuit The bias resistor determines the operating point; the use of special-type resistors such as varistor, thermistor to achieve anti-surge voltage, suppression of inrush current, over-temperature protection and so on. Resistor is the most common device, and it is also an indispensable device in the circuit. Choosing a good resistor is crucial for the stable operation and reliability of the product.
Keywords: 0 ohm resistance, resistance parameter, resistance
There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors, and wirewound resistors. Special resistors include varistors, thermistors, and photoresistors. Different types of resistors have different characteristics and parameters, and the points to be considered when using the circuit are different. For engineers who are just touching the circuit design, it is very likely that some special parameters of the resistor will be ignored, resulting in the stability and reliability of the product. Correct understanding of the various parameters of the resistor and the considerations of the selection, and a comprehensive understanding of the real role of the resistor in the circuit, can ensure the quality of the product from the most basic circuit design.
1 basic parameters of the resistor:
Engineers who are new to hardware circuit design may have a first impression of resistance. The resistance of the conductor described in the physical book to the current is called resistance. It is represented by the symbol R, in ohms, kiloohms, and megaohms. , KΩ, MΩ said. The main parameters concerned are 1), nominal resistance: the resistance value indicated on the resistor; 2) allowable error: the percentage of the ratio of the nominal resistance to the actual resistance and the nominal resistance Value deviation, which represents 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 important parameters that must be paid attention to in the design: rated power and withstand voltage value. The reliability of these two parameters for the whole system. The impact is very large.
If the current flowing through the resistor in the circuit is 100mA and the resistance is 100Ω, then the power consumption on the resistor is 1W. Choosing a common chip resistor, such as 0805 or 1206, is not suitable, because the rated power of the resistor is small. And there was a problem. Therefore, the rated power of the selected resistor should be above 1W (the power margin of the circuit design selection resistor is generally more than 2 times), otherwise the power consumed on the resistor will cause the resistor to overheat and fail.
Similarly, if the withstand voltage value is not properly selected, the system design will fail due to the breakdown of the resistor. For example: the AC-DC switching power supply module is designed at the input front end. According to the requirements of the safety standard GB4943.1, after the plug or connector is disconnected, the residual voltage on the input terminals L and N is within 1S. Attenuation is 37% of the initial value. Therefore, in the design, one or two MΩ-level impedance resistors are used for energy bleed, and the input terminal is high voltage, that is, both ends of the resistor are subjected to high voltage. The withstand voltage value of the low voltage input terminal will cause failure. Table 1 below shows the parameters of common SMT thick film resistors. The final selection should be verified with the manufacturer of the optional device.
Note: For reference only, subject to the final manufacturer's description
2 the role of the resistor in the circuit:
2.1 Basic role:
Electronic engineers have learned the basic role of resistors, which are used as voltage dividers, shunts, and load resistors in circuits; they can be combined with capacitors to form filters and delay circuits, used as sampling in power or control circuits. Resistance; used as a bias resistor in a semiconductor tube circuit to determine the operating point, etc. For these functions, the application in the circuit is very much, and it is very important, and will not be described too much. The following mainly introduces the role of 0Ω resistors and special resistors in the design of electronic circuits and the precautions for use.
The role of 2.2 0 ohm resistor on the circuit:
I believe that there are many new electricians. When I look at the electronic products designed by some predecessors, I often see 0Ω resistors on the circuit. Why design such a resistor? It’s not a good idea to draw a board directly. Data search and organization, the main points are as follows:
1) Single grounding of analog ground and digital ground
As long as it is ground, it will eventually be received together and then into the earth. If they are not connected together, they are "floating", there is a pressure difference, and it is easy to accumulate electric charges and cause static electricity. The ground is referenced to 0 potential, all voltages are derived from reference ground, the ground standards are the same, so the various grounds should be shorted together. It is believed that the earth can absorb all the electric charges and always maintain stability, 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 supply on the board will eventually return to the power plant. If the analog ground and the digital ground are directly connected to each other, it will cause mutual interference. There is four ways to solve this problem: 1. Connect with magnetic beads; 2. Connect with capacitors; 3. Connect with inductors; 4. Connect with 0 ohm resistors.
The equivalent circuit of the magnetic bead is equivalent to the band-stopper, which only significantly suppresses the noise of a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance so that the appropriate model can be selected. For the case where the frequency is uncertain or unpredictable, the magnetic beads are not in contact; the capacitors are connected to each other to cause floating; the inductance is large, the stray parameters are large, and unstable; the 0 ohm resistance is equivalent to a very narrow current path, which can effectively limit The loop current is used to suppress noise. The resistor has an attenuation in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic beads.
2) For current loop when bridging
When the ground plane is divided, the shortest return path of the signal is broken. At this time, the signal loop has to be detoured to form a large loop area, and the influence of the electric field and the magnetic field becomes stronger, which is easy to interfere/interfere. By connecting a 0 ohm resistor across the partition, a shorter return path can be provided to reduce interference.
3) Configuration circuit
Generally, there should be no jumpers or dial switches on the product. Sometimes the user will tamper with the settings, which may cause misunderstanding. In order to reduce the maintenance cost, the 0 ohm resistor is used instead of the jumper to solder on the board. The vacant jumper is equivalent to the antenna at high frequencies, and the effect of the chip resistor is good.
4) Other uses
Wiring debugging/testing during wiring: At the beginning of the design, a resistor is required for debugging, but the specific value cannot be determined. After adding such a device, it is convenient to debug the circuit later. If the result of the debugging does not need to add a resistor. , add a 0 ohm resistor. Temporarily replacing other SMD devices as temperature compensation devices is more often required for EMC countermeasures. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the via, and the via also affects the ground plane (because the hole is to be dug).
Summarized as follows:
1. There is no function in the circuit, just for the convenience of debugging or compatible design on the PCB.
2, can be used for jumpers, if a certain line is not used, directly attach the resistor (does not affect the appearance)
3. When the matching circuit parameters are uncertain, replace it with 0 ohms. When the actual debugging is performed, 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 resistor and connect the ammeter to make it easier to measure the current.
5, in the wiring, if the actual cloth can not pass, you can also add a 0 ohm resistor.
6. Under high-frequency signals, it acts as an inductor or capacitor (related to external circuit characteristics), mainly to solve EMC problems. Such as ground and ground, power and IC Pin.
7. Single point grounding (refers to protective grounding, working grounding, and DC grounding are separated from each other on the equipment, and 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 effects of these two resistors:
The varistor MOV is one of the most commonly used devices in circuit electromagnetic compatibility EMC, and is widely used in electronic circuits to protect against circuit damage due to sudden voltage changes in the power supply system. The characteristic is generally understood that when the front end voltage is higher than the turn-on voltage of the varistor, the varistor is broken down, the resistance of the varistor is reduced, and the current is shunted to prevent the subsequent stage from being damaged or interfered by excessive transient voltage. This protects sensitive electronic components. Circuit protection is the use of the non-linear characteristics of varistor. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value to protect the latter circuit. The main parameters of the varistor are: varistor voltage, current capacity, junction capacitance, response time and so on.
However, do not think too much about the role of varistor, varistor can not provide complete voltage protection, varistor can withstand limited energy or power, can not provide continuous overvoltage protection . Sustained overvoltage can damage the protective device (varistor) and cause damage to the device. The parts that the varistor cannot provide protection include: inrush current at startup, overcurrent at short circuit, voltage drop, etc. 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 lower the impedance; PTC is the positive temperature coefficient thermistor, that is, the higher the temperature, the impedance The bigger. The use of impedance-temperature sensitive characteristics plays an important role in circuit design.
In the circuit, NTC mainly suppresses the starting current during the starting process of the circuit. During the system startup process, due to the existence of power circuit, capacitive and inductive load inside the system, a very large inrush current occurs at the moment of starting. If the instantaneous resistance of the device is not taken into account during the selection process of the circuit device, the system may easily cause breakdown of the device during multiple startup operations, and adding NTC to the circuit is equivalent to when the input circuit is started. Increasing the input impedance reduces the inrush current, and when the system is in a stable state, the NTC generates heat, and according to its negative temperature characteristic, the impedance is reduced, so that the loss on the NTC is also reduced, and the overall loss of the system is reduced.
The PTC can function as a fuse in the circuit, so there is another name for the self-recovering fuse. During the operation of the system, the circuit is abnormal, resulting in a large current. If there is a PTC in the string in the part of the circuit, then there is a large current flowing in the PTC, and the PTC generates heat. According to its positive temperature characteristics, The impedance will become very large, making the impedance of the entire circuit larger, thus making the current of the circuit smaller and acting as a fuse. According to its positive temperature characteristics, another function of the PTC is to achieve over-temperature protection in the circuit.
The knowledge of resistance covers a lot, not only knowing Ohm's law, but also including materials and their special properties. For example, the resistance of resistors is generally related to temperature, material, length, and cross-sectional area. The physical quantity that measures the magnitude of the resistance affected by temperature is the temperature coefficient, which is defined as the percentage change of the resistance value when the temperature rises by 1 ° C; the main physical characteristic of the resistance is that the electric energy is thermal energy, and it is also an energy consumption. The component, the current through which it produces loss, is expressed in the form of thermal energy; the resistor usually acts as a partial voltage and a shunt in the circuit; for the signal, both the AC and DC signals can pass through the resistor. As a hardware engineer, if you want to use components handy, you need to have an in-depth understanding of its material, electrical characteristics and its specificity.