Resistors are one of the commonly used components in electronic products, and basically any electronic product will have resistors inside. Resistors can be used as voltage dividers, splitters, and load resistors in circuits; It can form filters and delay circuits together with capacitors; Used as a sampling resistor in power or control circuits; Used as a bias resistor in semiconductor tube circuits to determine the operating point; Using resistors with special properties such as varistors and thermistors to prevent surge voltage, suppress surge current, and achieve over temperature protection, etc. Resistors are ordinary devices and indispensable components in circuits. Choosing and using resistors well is crucial for the stable operation and reliability of products.

There are many types of resistors, and commonly used resistors include carbon film resistors, cement resistors, metal film resistors, and wire wound resistors; Special resistors include varistors, thermistors, photoresistors, etc. Different types of resistors have certain differences in their characteristic parameters, and the points that need to be considered when using circuits are also different. For engineers who are new to circuit design, they may overlook certain special parameters of resistance, resulting in the instability and reliability of the product not being guaranteed. A correct understanding of the various parameters and selection considerations of resistors, as well as a comprehensive understanding of the true role that resistors play in circuits, is necessary to ensure the quality of products from the basic circuit design at the bottom level.

Basic parameters of resistance:

Engineers who are new to hardware circuit design may have the impression of resistance as described in physics books, where the hindering effect of conductors on current is called resistance, represented by the symbol R in ohms, kiloohms, and megaohms, respectively represented by Ω, K Ω, and M Ω. The main parameters of concern are 1) nominal resistance value: 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 nominal resistance is called the resistance deviation, which represents the accuracy of the resistor. In circuit design, it is not enough to only focus on these two parameters. There are two important parameters that must be taken seriously in the design: rated power and withstand voltage value, which have a significant impact on the reliability of the entire system.

If the current flowing through the resistor in the circuit is 100mA and the resistance value is 100 Ω, then the power consumption on the resistor is 1W. Choosing commonly used surface mount resistors, such as those packaged as 0805 or 1206, is not appropriate and may cause problems due to the low rated power of the resistor. Therefore, the rated power of the selected resistor should be above 1W (the power margin of the selected resistor in circuit design is generally more than twice), otherwise the power consumed by the resistor will cause it to overheat and fail.

Similarly, if the withstand voltage value is not selected appropriately, it can also lead to system design failure due to resistance breakdown. For example, in the design of AC-DC switching power supply modules, according to the requirements of the GB4943.1 standard, the residual voltage on the input terminals L and N decays to 37% of the initial value within 1 second after the plug or connector is disconnected. Therefore, in the design, one or two M Ω impedance resistors are generally connected in parallel for energy discharge, and the input terminal is high voltage, that is, both ends of the resistor need to withstand high voltage. When the withstand voltage of the resistor is low and the input terminal is high voltage, failure will occur. The following Table 1 shows the parameters of common SMT thick film resistors, which need to be verified with the manufacturer of the selected device during final selection.

The role of resistors in circuits:

2.1 Basic function:

Electronic engineers have learned the basic function of resistors, which are used as voltage dividers, splitters, and load resistors in circuits; It, together with capacitors, can form filters and delay circuits, and can be used as sampling resistors in power or control circuits; In semiconductor tube circuits, it is used as a bias resistor to determine the operating point. For these functions, there are many and important applications in circuits, so we will not describe them in detail. The following mainly introduces the role and usage precautions of 0 Ω resistors and special resistors in electronic circuit design.

2.2 The role of a 0 ohm resistor in a circuit:

I believe there are many new electricians who often see 0 Ω resistors on the circuit when looking at electronic products designed by predecessors. Why design such a resistor? Just connect it directly to the drawing board, why add unnecessary details? Through searching and organizing information, the key points are as follows:

1) Analog ground and digital ground single point grounding

As long as it is the ground, it must eventually be connected together and enter the earth. If not connected together, it is a "floating ground" with a pressure difference, which can easily accumulate charges and cause static electricity. Ground is the reference zero potential, and all voltages are derived from the reference ground. The standard for ground should be consistent, so all types of ground should be short circuited together. People believe that the earth can absorb all charges and maintain stability, making it the ultimate reference point for the earth. Although some boards are not grounded, the power plant is grounded, and the power supply on the board will eventually return to the ground of the power plant. If analog ground and digital ground are directly connected over a large area, it will cause mutual interference. There are four ways to solve this problem: 1. Connect with magnetic beads; 2. Connect with capacitors; 3. Connect with inductance; 4. Connect with a 0 ohm resistor.

The equivalent circuit of magnetic beads is equivalent to a band stop limiter, which only significantly suppresses noise at a certain frequency point. When using it, it is necessary to estimate the noise frequency in advance in order to select the appropriate model. For situations where the frequency is uncertain or unpredictable, magnetic beads do not match; Capacitor isolation leads to direct crossing, causing floating ground; Inductance has a large volume, multiple stray parameters, and is unstable; A 0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. Resistors have attenuation effects in all frequency bands (even 0-ohm resistors have impedance), which is stronger than magnetic beads.

2) Used for current circuits during cross connection

When the electric ground plane is divided, it causes a short signal return path to break. At this time, the signal circuit has to detour, forming a large loop area, and the influence of electric and magnetic fields becomes stronger, making it easy to interfere/be interfered with. Connecting a 0-ohm resistor across the partition can provide a shorter return path and reduce interference.

3) Configure circuit

Generally, jumper wires and dip switches should not appear on the product. Sometimes users may tamper with the settings, which can cause misunderstandings. In order to reduce maintenance costs, 0-ohm resistors should be used instead of jumper wires and soldered onto the board. Vacant jumper wires are equivalent to antennas at high frequencies, and using patch resistors has a good effect.

4) Other uses

For cross line debugging/testing during wiring: At the beginning of the design, a resistor needs to be connected in series for debugging, but the specific value cannot be determined yet. Adding such a device will facilitate future circuit debugging. If the debugging result does not require adding a resistor, a 0 ohm resistor can be added. Temporarily replacing other surface mount devices as temperature compensation devices is often necessary for EMC countermeasures. In addition, a 0 ohm resistance is smaller than the parasitic inductance of a via, and the via can also affect the ground plane (because it needs to be dug).

The summary is as follows:

1. There is no function in the circuit, only on the PCB for debugging convenience or compatibility design reasons.

2. It can be used as a jumper. If a certain section of the line is not needed, simply attach the resistor (without affecting the appearance)

3. When the matching circuit parameters are uncertain, use 0 ohms instead. During actual debugging, determine the parameters and then 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 it to an ammeter, which is convenient for measuring the current consumption.

5. When wiring, if it is really impossible to connect, you can also add a 0 ohm resistor.

6. Under high-frequency signals, it serves as an inductor or capacitor (related to external circuit characteristics) mainly to solve EMC problems. Between ground, power supply, and IC pins.

7. Single point grounding (referring to the separation of protective grounding, working grounding, and DC grounding on equipment, each becoming an independent system).

2.3 The role of special resistors in the peripheral protection circuit of power modules

Common special resistors include varistors and thermistors, which play a key role in the design and application of AC-DC switching power supplies. Let's understand the characteristics and specific functions of these two resistors:

Varistor MOV is one of the commonly used devices in circuit electromagnetic compatibility (EMC), widely used in electronic circuits to protect circuits from potential damage caused by transient voltage changes in power supply systems. Its characteristic is commonly understood as 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 decreases, and the current is diverted to prevent the subsequent stage from being damaged or interfered by excessive instantaneous voltage, thus protecting sensitive electronic components. Circuit protection is the use of the nonlinear characteristics of varistors. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed value, thereby protecting the subsequent circuit. The main parameters of varistors include: varistor voltage, current carrying capacity, junction capacitance, response time, etc.

However, don't overthink the role of varistors. Varistors cannot provide complete voltage protection, and the energy or power they can withstand is limited, so they cannot provide sustained overvoltage protection. Continuous overvoltage can damage the protective device (varistor) and cause damage to the equipment. The parts that varistors cannot provide protection for include: surge current during startup, overcurrent during short circuit, voltage drop, etc. These situations require other forms of protection.

Thermistor is a temperature dependent device, generally divided into two types. NTC is a negative temperature coefficient thermistor, which means that the higher the temperature, the smaller the impedance; PTC is a positive temperature coefficient thermistor, meaning that the higher the temperature, the greater the impedance. The sensitivity of impedance to temperature has played an important role in circuit design.

NTC is mainly used in circuits to suppress the starting current during the startup process. During the system startup process, due to the presence of power circuits, capacitive and inductive loads inside the system, a very large surge current will occur at the moment of startup. If the instantaneous current resistance of the circuit components is not considered during the selection process, the system is prone to breakdown and damage during multiple start-up operations. Adding NTC to the circuit is equivalent to increasing the input impedance and reducing the surge current when the input circuit is started. When the system is in a stable state, due to the heating of NTC, the impedance decreases based on its negative temperature characteristics, resulting in reduced losses on NTC and overall system losses.

PTC can act as a fuse in circuits, so it is also known as a self-healing fuse. During the operation of the system, if there is an abnormality in the circuit that leads to a large current, if there is a PTC in series in that part of the circuit, it is equivalent to a large current flowing through the PTC. The PTC heats up, and according to its positive temperature characteristics, its impedance will become very large, causing the impedance of the entire circuit to increase, thereby reducing the current of the circuit and acting as a fuse. According to its positive temperature characteristics, another function of PTC is to achieve over temperature protection in circuits.