How to choose resistors in circuit design?

Resistors are one of the commonly used devices in electronic products. Basically, as long as it is an electronic product, there will be resistors inside. Resistors can be used as voltage dividers, shunts and load resistors in circuits; they can form filters and delay circuits together with capacitors; they are used as sampling resistors in power circuits or control circuits; they are used as bias resistors in semiconductor tube circuits to determine the working point; resistors with special properties such as varistors and thermistors are used to prevent surge voltages, suppress impact currents, and achieve over-temperature protection, etc. Resistors are common devices, and they are also indispensable devices in circuits. Choosing and using resistors well is crucial to the stable operation and reliability of products.

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 certain differences in their characteristic parameters, and the points that need to be considered when using them in circuits are also different. For engineers who are new to circuit design, it is very likely that some special parameters of resistors will be ignored, resulting in the stability and reliability of the product not being guaranteed. Only by correctly understanding the various parameters of resistors and the precautions for selection, and fully understanding the real role of resistors in circuits, can the quality of products be guaranteed from the bottom-level basic circuit design.

1 Basic parameters of resistors:

Engineers who are new to hardware circuit design may have the impression that resistors are described in physics books as the resistance of conductors to current, which is called resistance, represented by the symbol R, and the units are ohms, kilo-ohms, and megohms, represented by Ω, KΩ, and MΩ respectively. The main parameters of concern are 1) nominal resistance: the resistance indicated 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 circuit design, it is not enough to only pay attention to these two parameters. There are two other important parameters that must be taken seriously in the design: rated power and withstand voltage value. These two parameters have a great impact on the reliability of the entire system.

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. It is inappropriate to choose commonly used chip resistors, such as 0805 or 1206 packages, which will cause problems due to the small rated power of the resistor. Therefore, the rated power of the selected resistor must be above 1W (the power margin of the resistor selected in the 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 due to the breakdown of the resistor. For example: In the design of the input front end of the AC-DC switching power supply module, according to the requirements of the safety regulations GB4943.1 standard, after the plug or connector is disconnected, the retained voltage on the input terminals L and N decays to 37% of the initial value within 1S. Therefore, during the design, one or two MΩ-level impedance resistors are generally used in parallel for energy discharge, and the input terminal is high voltage, that is, the two ends of the resistor must withstand high voltage. When the resistance value of the resistor is low and the input terminal is high voltage, failure will occur. The following table 1 is the parameters of common SMT thick film resistors. When the final selection is made, it is necessary to verify with the manufacturer of the selected device.

2 The role of resistors in circuits:

2.1 Basic role:

Electronic engineers have learned the basic role of resistors, that is, they are used as voltage dividers, shunts and load resistors in circuits; they can form filters and delay circuits with capacitors, and are used as sampling resistors in power circuits or control circuits; they are used as bias resistors in semiconductor tube circuits to determine the working point, etc. For these roles, there are many applications in circuits and they are also very important, so I will not describe them too much. The following mainly introduces the role of 0Ω resistors and special resistors in electronic circuit design and precautions for use.

2.2 The role of 0 ohm resistors in circuits:

I believe that many new electricians will often see 0Ω resistors in circuits when looking at some electronic products designed by their predecessors. Why design such a resistor? Isn't it better to just connect a piece on the drawing board? Why add unnecessary details? Through searching and sorting out the information, the key points are as follows:

1) Single-point grounding of analog ground and digital ground

As long as it is ground, it must be connected together and then enter the earth. If they are not connected together, it is a "floating ground", there is a voltage difference, it is easy to accumulate charge and cause static electricity. The ground is the reference 0 potential, all voltages are obtained by reference to the ground, the ground standard must be consistent, so various grounds should be short-circuited together. People believe that the earth can absorb all charges and always maintain stability, and it is the ultimate ground reference point. Although some boards are not connected to the ground, the power plant is connected to the ground, and the power on the board will eventually return to the power plant and enter the ground. If the analog ground and the digital ground are directly connected over a large area, it will cause mutual interference. It is not appropriate not to short-circuit, there are 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 a band-stop limiter, which only has a significant suppression effect on the noise of 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 are not suitable; capacitors block direct current and pass alternating current, resulting in floating ground; inductors are large in size, have many stray parameters, and are unstable; 0 ohm resistors are equivalent to very narrow current paths, which can effectively limit loop currents and suppress noise. Resistors have an attenuation effect in all frequency bands (0 ohm resistors also have impedance), which is stronger than magnetic beads.

2) Used for current loops when jumper

When the ground plane is split, the short return path of the signal is broken. At this time, the signal loop has to take a detour, forming a large loop area, and the influence of the electric and magnetic fields becomes stronger, which is easy to interfere/be interfered. Jumpering a 0 ohm resistor on the split area can provide a shorter return path and reduce interference.

3) Configure the circuit

Generally, jumpers and dip switches should not appear on the product. Sometimes users will mess with the settings, which can easily cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should be used instead of jumpers and soldered on the board. Empty jumpers are equivalent to antennas at high frequencies, and chip resistors are effective.

4) Other uses

For cross-line debugging/testing during wiring: When starting to design, a resistor is needed for debugging, but the specific value cannot be determined yet. After adding such a device, it is convenient for debugging the circuit in the future. If the debugging result does not require adding a resistor, add a 0 ohm resistor. Temporarily replace other SMD devices as temperature compensation devices, more often for the needs of EMC countermeasures. In addition, the parasitic inductance of a 0 ohm resistor is smaller than that of a via, and the via will also affect the ground plane (because the via needs to be dug).