There are many kinds of resistors, such as carbon film resistor, cement resistor, metal film resistor and wire wound resistor. Special resistors include piezoresistors, thermistors and photoresistors. Different types of resistors have certain differences in their characteristic parameters, and the points to be considered in circuit use are also different. For engineers who are new to circuit design, some special parameters of resistance are likely to be ignored, which leads to the failure to guarantee the stability and reliability of products. Only by correctly understanding the parameters of resistors and the points for attention in selection, and fully understanding the real role of resistors in the circuit, can the quality of products be guaranteed from the basic circuit design at the bottom.
Basic parameters of resistance:
Engineers who are new to hardware circuit design may have the first impression of resistance, that is, the blocking effect of electric conductors on current described in physics books is called resistance, and it is represented by the symbol R. The units are ohms, kiloohms and megaohms, which are represented by ω, kω and mω respectively. The main parameters of concern are 1) Nominal resistance: the resistance value marked on the resistor; 2) Allowable error: The percentage of the difference between the nominal resistance and the actual resistance and the ratio of the nominal resistance is called resistance deviation, which indicates the accuracy of the resistor. In the design of the circuit, it is not enough to only 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, which have great influence on the reliability of the whole system.
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 1 W. It is inappropriate to choose the commonly used chip resistor, such as 0805 or 1206 package, which will cause problems due to the low rated power of the resistor. Therefore, the rated power of the selected resistor should be more than 1W (the power margin of the selected resistor in circuit design is generally more than 2 times), otherwise the power consumed by the resistor will cause the resistor to overheat and fail. Similarly, if the withstand voltage value is not selected properly, the system design will fail because the resistance is broken down. For example, at the input front end of the AC-DC switching power supply module, according to the requirements of safety standard GB4943.1, after the plug or connector is disconnected, the voltage on the input terminals L and N will be attenuated to 37% of the initial value within 1S. Therefore, one or two resistors with Mω impedance will be connected in parallel to discharge the energy, and the input terminal is high voltage, that is, both ends of the resistor will bear high voltage. When the resistor The following table 1 is the parameters of common SMT thick film resistors, and the final selection should be checked with the manufacturers who choose the devices.
Table 1 Common SMT Thick Film Resistors:
Note: For reference only, the final manufacturer’s instructions shall prevail.
The function of resistance in the circuit:
1 Basic function: Electronic engineers have learned the basic function of resistance, that is, it is used as a voltage divider, shunt and load resistor in the circuit; Together with the capacitor, it can form a filter and a delay circuit, and be used as a sampling resistor in a power supply circuit or a control circuit. It is used as a bias resistor in semiconductor transistor circuits to determine the operating point, etc. For these functions, the applications in circuits are very numerous and important, so I will not describe them too much. Here, we mainly introduce the functions of 0Ω resistors and special resistors in the design of electronic circuits and the points for attention.
The function of 0Ω resistor on the circuit: I believe there are many new electricians who often see 0 Ω resistor on the circuit when looking at some electronic products designed by their predecessors. Why do you want to design such a resistor? It’s better to draw a board directly and connect it with one piece, so why gild the lily? By searching and sorting out the data, the main points are as follows:
Single point grounding of analog ground and digital ground:
As long as it is the ground, it will eventually be connected together, and then it will enter the ground. If they are not connected together, they are “floating to the ground”, and there is a pressure difference, which is easy to accumulate charges and cause static electricity. The ground is the reference 0 potential, all voltages are obtained by reference to the ground, and the ground standards should be consistent, so all kinds of grounds should be shorted together. People think that the earth can absorb all the electric charges and remain stable all the time, which is the ultimate 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 and be grounded. If the analog ground and digital ground are directly connected in a large area, it will lead to mutual interference. There are four ways to solve this problem: 1. Connect with magnetic beads; 2. Connect with capacitor; 3. Connect with inductance; 4. Connect with 0 ohm resistor.
The equivalent circuit of magnetic beads is equivalent to band-stop wave limiter, which can only significantly suppress the noise at a certain frequency point. When using, it is necessary to estimate the noise frequency in advance so as to select the appropriate model. When the frequency is uncertain or unpredictable, the magnetic beads do not match; Capacitors are isolated from each other, resulting in floating ground; Large inductance, many stray parameters and instability; Ohm resistor is equivalent to a narrow current path, which can effectively limit the loop current and suppress the noise. Resistors have attenuation in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.
Used for current loop when crossing:
When the electric ground plane is divided, the shortest return path of the signal is broken. At this time, the signal circuit has to detour, forming a large loop area, and the influence of electric and magnetic fields becomes stronger, which is easy to be disturbed. Connecting 0 ohm resistor across the dividing area can provide a shorter return path and reduce interference.
Configuration circuit:
Generally, jumper and dip switch should not appear on the product. Sometimes users will tamper with the settings, which is easy to cause misunderstanding. In order to reduce the maintenance cost, 0 ohm resistor should be used instead of jumper wire to be welded on the board. The vacant jumper is equivalent to an antenna at high frequency, and the patch resistor has a good effect.
Other uses:
For wire crossing debugging/testing: at the beginning of design, a resistor should be connected in series for debugging, but the specific value can’t be determined yet. After adding such a device, it is convenient for future circuit debugging. If the result of debugging doesn’t need to add a resistor, add a 0 ohm resistor. Temporarily replacing other SMD devices as temperature compensation devices is more often due to the need of EMC countermeasures. In addition, the 0 ohm resistance is smaller than the parasitic inductance of the via hole, and the via hole will also affect the ground plane (because the hole needs to be dug). The summary is as follows:
1. It doesn’t have any function in the circuit, but it’s on the PCB for debugging convenience or compatible design.
2. It can be used as jumper. If a certain line is not used, just stick the resistor directly (without affecting the appearance).
3. When the parameters of the matching circuit are uncertain, it should be replaced by 0 ohm. In actual debugging, the parameters should be determined and then replaced by components with specific values.
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, which is convenient for measuring the current consumption.
5. When wiring, you can also add a resistance of 0 ohms if the cloth can’t pass.
6. Under high-frequency signals, it can be used as an inductor or capacitor (related to external circuit characteristics), mainly to solve EMC problems. Such as ground to ground, power supply and IC Pin.
7. Single-point grounding (protective grounding, working grounding and DC grounding are separated from each other on the equipment and become independent systems).
Function of special resistance in peripheral protection circuit of power supply module;
The most common special resistors are varistor and thermistor, which play a key role in the design and application of AC-DC switching power supply. Learn about the characteristics and specific functions of these two resistors:
Varistor MOV is one of the most commonly used devices in EMC of circuit, which is widely used in electronic circuits to protect the circuit from the possible damage caused by the sudden voltage change of power supply system. Its characteristic is generally understood that when the front-end voltage is higher than the starting voltage of the varistor, the varistor is broken down, and the resistance of the varistor decreases, thus shunting the current, preventing the later stage from being damaged or interfered by excessive instantaneous voltage, thus protecting sensitive electronic components. Circuit protection is to make use of the nonlinear characteristics of varistor. Overvoltage appears between the two poles of varistor, and varistor can clamp the voltage to a relatively fixed voltage value, so as to protect the subsequent circuit. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.
However, don’t think too much of the function of the varistor. The varistor can’t provide complete voltage protection, and its energy or power is limited, so it can’t provide continuous overvoltage protection. Continuous overvoltage will damage the protection device (varistor) and damage the equipment. Other parts that the varistor can’t provide protection are: surge current during startup, overcurrent during short circuit, sudden voltage drop, etc., which need other ways of protection.
Thermistor is a kind of temperature-related device, which is generally divided into two types. NTC is negative temperature coefficient, that is, the higher the temperature, the smaller the impedance. PTC is a positive temperature coefficient thermistor, that is, the higher the temperature, the greater the impedance. The sensitivity of impedance to temperature plays an important role in circuit design.
NTC is mainly used in the circuit to restrain the starting current during the starting process of the circuit. When the system is started, there are power circuits, capacitive and inductive loads in the system, so there will be a very large impact current at the starting moment. If the instantaneous current resistance of the device is not considered in the process of circuit device selection, the device will be easily damaged during the operation of the system starting many times. Adding NTC to the circuit is equivalent to increasing the input impedance and reducing the impact current when the input circuit is started. When the system is in a stable state, the impedance decreases due to the negative temperature characteristics of NTC, thus reducing the loss on NTC and the overall loss of the system.
PTC can act as a fuse in the circuit, so it has another name, self-recovery fuse. During the operation of the system, when the circuit is abnormal, resulting in a large current, if there is a PTC in this part of the circuit in series, it means that there is a large current flowing through the PTC, and the PTC heats up. According to its positive temperature characteristics, its impedance will become very large, which will make the impedance of the whole circuit become larger, thus making the current of the circuit smaller, thus acting as a fuse. According to its characteristic of positive temperature, another function of PTC is to realize over-temperature protection in the circuit.
Summary:
Knowledge of resistance covers a lot, which can be applied well not only after knowing Ohm’s law, but also materials and their special properties. For example, the resistance value of resistance elements is generally related to temperature, material, length and cross-sectional area. The physical quantity to measure the resistance affected by temperature is temperature coefficient, which is defined as the percentage of resistance value change when the temperature rises by 1℃. The main physical characteristic of resistance is that the transformation energy is heat energy, or it can be said that it is an energy-consuming element, through which the current will produce loss, which is expressed in the form of heat energy; Resistance usually plays the role of voltage division and shunt in the circuit; For signals, both AC and DC signals can pass through resistors, etc. As a hardware engineer, if you want to use components with ease, you need to have a deep understanding of their materials, electrical characteristics and particularity.
