Abstract:
Resistors are among the most commonly used components in electronic products; virtually every electronic product contains resistors. Resistors function as voltage dividers, shunts, and load resistors in circuits; they can be used with capacitors to form filters and delay circuits; they serve as sampling resistors in power supply or control circuits; they are used as bias resistors in semiconductor transistor circuits to determine the operating point; and special resistors such as varistors and thermistors are used to prevent surge voltage, suppress inrush current, and provide over-temperature protection, among other things. Resistors are the most common and indispensable components in circuits; selecting and using resistors correctly is crucial for the stable operation and reliability of products.
0 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. Different types of resistors have different characteristic parameters, and the points to consider when using them in circuits also differ. Engineers new to circuit design may overlook certain special parameters of resistors, leading to compromised product stability and reliability. A correct understanding of the various parameters of resistors and the considerations for selection, along with a comprehensive understanding of the true role resistors play in circuits, is essential to ensuring product quality from the most basic circuit design level.
1. Basic parameters of resistors:
For engineers new to hardware circuit design, the first impression of resistors might be the resistance described in physics textbooks: the opposition of a conductor to the flow of current, represented by the symbol R, and measured in ohms, kiloohms, and megaohms, respectively Ω, KΩ, and MΩ. The main parameters of interest are: 1) Nominal resistance: the resistance value marked on the resistor; 2) Tolerance: the percentage difference between the nominal and actual resistance values, expressed as the nominal resistance, indicating the resistor's accuracy. However, in circuit design, focusing solely on these two parameters is insufficient. Two other crucial parameters must be given due attention: rated power and withstand voltage. These two parameters significantly impact the reliability of the entire system.
If the current flowing through a resistor in a circuit is 100mA and the resistance is 100Ω, then the power consumption across the resistor is 1W. Choosing a common surface-mount resistor, such as one with a 0805 or 1206 package, is unsuitable because the rated power is too low. Therefore, the rated power of the resistor should be at least 1W (the power margin for resistor selection in circuit design is generally more than twice the rated power); otherwise, the power consumed by the resistor will cause it to overheat and fail.
Similarly, an inappropriate voltage rating can also lead to system design failure due to resistor breakdown. For example, in the input front-end design of an AC-DC switching power supply module, according to the safety standard GB4943.1, the residual voltage on the input terminals L and N should decay to 37% of its initial value within 1 second after the plug or connector is disconnected. Therefore, in the design, one or two MΩ-level resistors are typically connected in parallel for energy dissipation. However, the input terminal is high voltage, meaning the resistor must withstand high voltage. If the resistor's voltage rating is lower than the high voltage at the input terminal, it will fail. Table 1 below shows the parameters of common SMT thick-film resistors; the final selection should be verified with the component manufacturer.
2. The role of resistors in circuits:
2.1 Basic Functions:
Electronic engineers have all learned about the basic functions of resistors: in circuits, they function as voltage dividers, current dividers, and load resistors; together with capacitors, they can form filters and delay circuits; in power supply or control circuits, they serve as sampling resistors; and in semiconductor transistor circuits, they are used as bias resistors to determine the operating point. These applications in circuits are numerous and very important, so we won't go into too much detail here. Below, we will mainly introduce the roles of 0Ω resistors and special resistors in electronic circuit design and the precautions for their use.
2.2 The role of a 0-ohm resistor in a circuit:
Many new electricians, when looking at electronic products designed by experienced engineers, often see 0Ω resistors on the circuit boards. Why design such a resistor? Why not just connect it directly to the board? After researching and organizing information, the key points are as follows:
1) Single-point grounding for analog and digital grounds
All grounding devices must eventually be connected together and then grounded. If they are not connected, they are "floating grounds," which create voltage differences, easily accumulate charge, and cause static electricity. Ground is the reference zero potential; all voltages are derived from ground. Ground standards must be consistent, therefore all grounding devices should be shorted together. It is believed that the earth can absorb all charge and remain stable, making it the ultimate ground reference point. Although some circuit boards are not grounded, power plants are, and the power supplied to the boards will eventually return to the power plant and be grounded. Directly connecting analog and digital grounds over a large area can cause mutual interference. Not shorting them is also problematic. There are four ways to solve this problem: 1. Use a ferrite bead; 2. Use a capacitor; 3. Use an inductor; 4. Use a 0-ohm resistor.
The equivalent circuit of a ferrite bead is similar to a band-stop filter, which significantly suppresses noise only at a specific frequency. Therefore, the noise frequency needs to be estimated beforehand to select the appropriate model. Ferrite beads are unsuitable for situations where the frequency is uncertain or unpredictable; capacitors block DC and pass AC, causing floating ground; inductors are large, have many stray parameters, and are unstable; a 0-ohm resistor acts as a very narrow current path, effectively limiting loop current and suppressing noise. Resistors have attenuation effects across all frequency bands (even a 0-ohm resistor has impedance), which is superior to ferrite beads.
2) Used in current loops when bridging.
When the ground plane is segmented, the shortest return path for signals is broken. The signal loop must then detour, creating a large loop area. This strengthens the influence of electric and magnetic fields, making it easier to interfere with or be interfered with. Connecting a 0-ohm resistor across the segmented area can provide a shorter return path and reduce interference.
3) Circuit configuration
Generally, jumpers and DIP switches should not be present on the product. Users sometimes tamper with settings, which can easily lead to misunderstandings. To reduce maintenance costs, 0-ohm resistors should be soldered onto the board instead of jumpers. Unused jumpers act like antennas at high frequencies; surface-mount resistors are more effective in these situations.
4) Other uses
For debugging/testing during routing: At the beginning of the design, a resistor is often added in series for debugging, but the exact value is not yet determined. Adding this component facilitates later circuit debugging. If the debugging results show that the resistor is not needed, a 0-ohm resistor can be added. It can also temporarily replace other surface-mount components as a temperature compensation device, often for EMC mitigation purposes. Additionally, a 0-ohm resistor has lower parasitic inductance than a via, and vias also affect the ground plane (because they require drilling).
In summary:
1. It has no function in the circuit; it is only on the PCB for reasons such as ease of debugging or compatibility design.
2. It can be used as a jumper. If a section of the circuit is not used, you can simply attach this resistor (it will not affect the appearance).
3. When the matching circuit parameters are uncertain, use 0 ohms as a substitute. During actual debugging, determine the parameters and then substitute with components of 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 an ammeter, which makes it easier to measure the current consumption.
5. When wiring, if it is really impossible to run the wires, you can add a 0-ohm resistor.
6. Under high-frequency signals, it acts as an inductor or capacitor (depending on the characteristics of the external circuit), mainly to solve EMC problems. For example, between ground and ground, between power supply and IC pins.
7. Single-point grounding (refers to protective grounding, working grounding, and DC grounding being separated from each other on the equipment, each becoming an independent system).
2.3 The role of special resistors in the peripheral protection circuit of the power supply module
The most common special resistors are varistors and thermistors, which play a crucial role in the design and application of AC-DC switching power supplies. Let's understand the characteristics and specific functions of these two types of resistors:
Varistors (MOVs) are among the most commonly used components in electromagnetic compatibility (EMC) circuits. They are widely used in electronic circuits to protect circuits from potential damage caused by transient voltage surges in the power supply system. Simply put, when the upstream voltage exceeds the varistor's threshold voltage, the varistor breaks down, its resistance decreases, and it diverts current, preventing subsequent stages from being damaged or interfered with by excessive transient voltage, thus protecting sensitive electronic components. Circuit protection utilizes the non-linear characteristics of varistors; when an overvoltage occurs between the varistor's terminals, it clamps the voltage to a relatively fixed value, thereby protecting downstream circuits. The main parameters of a varistor include: varistor voltage, current carrying capacity, junction capacitance, and response time.
However, don't overestimate the importance of varistors. Varistors cannot provide complete voltage protection; their energy or power tolerance is limited, and they cannot provide continuous overvoltage protection. Sustained overvoltage will damage the protection device (varistor) and harm the equipment. Varistors also cannot protect against: inrush current during startup, overcurrent during short circuits, and voltage dips; these situations require other forms of protection.
A thermistor is a temperature-dependent device, generally divided into two types: NTC (negative temperature coefficient) thermistors, meaning their impedance decreases as temperature increases; and PTC (positive temperature coefficient) thermistors, meaning their impedance increases as temperature increases. Utilizing the temperature-sensitive characteristic of impedance plays an important role in circuit design.
In circuits, the primary function of an NTC (Network Current Tractor) is to suppress the inrush current during circuit startup. During system startup, due to the presence of power circuits, capacitive and inductive loads, a very large inrush current occurs at the moment of startup. If the instantaneous current withstand capability of the components is not considered during component selection, the components can easily be damaged during repeated startup operations. Adding an NTC to the circuit increases the input impedance during startup, reducing the inrush current. When the system is in a stable state, the NTC generates heat, and due to its negative temperature characteristic, its impedance decreases, thus reducing losses on the NTC and overall system losses.
In circuits, a PTC (Positive Temperature Coefficient) acts as a fuse, hence its other name, a resettable fuse. During system operation, if a circuit malfunctions and experiences a large current, a PTC connected in series in that part of the circuit effectively allows a large current to flow through it. The PTC heats up, and due to its positive temperature coefficient (PTC), its impedance increases significantly, increasing the overall circuit impedance and thus reducing the current in the circuit, effectively acting as a fuse. Based on its PTC characteristic, another function of a PTC is to provide over-temperature protection in circuits.
3. Conclusion:
The knowledge of resistors is vast, extending far beyond simply knowing Ohm's Law. It includes understanding the materials and special properties of resistors. For example, the resistance value of a resistor is generally related to temperature, material, length, and cross-sectional area. The physical quantity that measures the effect of temperature on resistance is the temperature coefficient, defined as the percentage change in resistance value for every 1°C increase in temperature. The main physical characteristic of a resistor is its conversion of electrical energy into heat energy; it can also be considered an energy-consuming component, as current passing through it incurs losses, manifested as heat. Resistors typically function as voltage dividers and current dividers in circuits. For signals, both AC and DC signals can pass through resistors. As a hardware engineer, to use components effectively, a deep understanding of their materials, electrical characteristics, and special properties is essential.
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