How to choose resistors in circuit design?

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How to choose resistors in circuit design? [Copy link]

There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors, wirewound resistors, etc.; special resistors include varistors, thermistors, photoresistors, etc. Different types of resistors have certain differences in their characteristic parameters, and the key points to be considered when using them in circuits are also different. If some special parameters of resistors are ignored in circuit design, the stability and reliability of the product may not be guaranteed. Correctly understand the various parameters of resistors and the selection precautions of different resistors, and fully understand the real role of resistors in circuits, so that the function and performance of products can be guaranteed from a basic level in circuit design.

1. Basic parameters of resistors

Speaking of resistors, our first impression should be what is described in physics books: the resistance of a conductor to current is called resistance. Resistors are represented by R in circuit schematics, and the unit is ohm (Ω). Commonly used units are ohms, kilo-ohms, mega-ohms, etc. (represented by Ω, KΩ, and MΩ, respectively).

The main parameters of resistors are:

1) Nominal resistance

The resistance value marked on the resistor.

2) Resistance deviation

The percentage obtained by dividing the difference between the nominal resistance value and the actual resistance value by the nominal resistance value is called resistance deviation, which indicates the accuracy of the resistor.

However, when carrying out actual circuit design, it is not enough to only focus on these two parameters. There are two other important parameters that must be given enough attention in the design: rated power and withstand voltage value. These two parameters have a great influence on the reliability of the entire circuit system. For example, if the current flowing through the resistor in the circuit is 100mA and the resistance value of the resistor is 100Ω, then according to the circuit power calculation formula P=I*I*R, it can be calculated that the power consumption of the resistor is 1W. At this time, if a commonly used chip resistor is selected, such as a package of 0805 or 1206, it is not suitable. The circuit will have problems due to the small rated power of the resistor. Therefore, the resistor should be selected with a rated power of more than 1W (in circuit design, the power margin when selecting the resistor should be more than twice the actual power consumption), otherwise the power consumed by the resistor will cause the resistor to overheat and fail. Similarly, when the withstand voltage value is not selected properly, the entire circuit system may 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 safety regulations (GB4943.1 standard), it is necessary to ensure that after the plug or connector is disconnected, the retained voltage on the input terminals L and N can decay to less than 37% of the initial value within 1S. Therefore, in the actual circuit design, when the withstand voltage value of the resistor is lower than the high voltage at the input terminal, it will fail.

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2. The role of resistance in the circuit 1. Basic role

Resistors are used as voltage dividers, shunts and load resistors in circuits; they can be used with capacitors to form filters and delay circuits, and as sampling resistors in power circuits or control circuits; they are used as bias resistors in semiconductor circuits to determine the operating point of the circuit, etc. For these functions, the applications in circuits are very numerous and very important.

The type of resistor to be used should be selected according to the role of the resistor in the circuit and the specific technical requirements. For example, for the voltage-reducing and current-limiting resistors, audio load resistors, etc. in the circuit, carbon film resistors can meet the requirements; if it is a sampling resistor in a voltage-stabilizing circuit, a timing resistor in a delay circuit, etc., it is best to use a metal film resistor; for the shunt and voltage-dividing resistors in measuring instruments, resistors with higher precision grades should be selected. We will not introduce these common functions in detail.

We will focus on the role of 0 ohm resistors and special resistors in electronic circuit design and precautions for use. 2. The role of 0 ohm resistor in the circuit I believe that when we look at the electronic products designed by our predecessors, we often see 0 ohm resistors in the circuit. Why do we need to design such a resistor? When drawing the circuit diagram, we can just connect it with a wire. Why do we need to add unnecessary details? There are many reasons for this, which we briefly introduce as follows: 1) Single-point grounding of analog ground and digital ground 34)]We know that in the circuit diagram, as long as it is ground, it will eventually be connected together and then connected to the ground. If it is not connected together, it is a "floating ground", there is a voltage difference, it is easy to accumulate charge, causing static electricity, the ground is the reference zero potential, all voltages are obtained by reference to the ground, the ground standard must be consistent, so various grounds should be short-circuited together. If the analog ground and the digital ground are connected over a large area, it will cause mutual interference. Not short-circuiting is not appropriate, there are four ways to solve this problem. Connect with a magnetic bead; connect with a capacitor; connect with an inductor; connect with a 0 ohm resistor.

Let's analyze these four connection methods one by one:

[color=rgb(34, 34, a) Connect with ferrite beads: The equivalent circuit of ferrite beads is equivalent to a band-stop filter, which can only significantly suppress noise at a certain frequency. When using it, it is necessary to estimate the noise frequency in advance in order to select the appropriate model; ferrite beads are not suitable for situations where the frequency is uncertain or unpredictable; b) Connect with capacitors: Capacitors block direct current and allow alternating current, which can easily cause floating ground; c) Connect with inductors: Inductors are large in size, have many stray parameters, and are unstable; d) Connect with 0 ohm resistors: 0 ohm resistors are equivalent to a very narrow current passing through, which can effectively limit the loop current and suppress noise. Resistors have an attenuation effect on all frequency bands (0 ohm resistors also have impedance), which is stronger than ferrite beads.

2) Used for current loops when jumpered

When the electric ground plane is split, the shortest 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. Jumping a 0 ohm resistor across the split area can provide a shorter return path and reduce interference. 3) Configuration circuit Generally, there should be no jumpers or DIP switches on the product, because once there are these manually operated switches, users will inevitably tamper with them, which will lead to setting errors, misunderstandings or failures. In order to reduce maintenance costs, 0 ohm resistors should be used instead of jumpers and soldered on the circuit board. The control jumper is equivalent to an antenna at high frequencies, so it is better to use chip resistors.

4) Other uses

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

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The summary is as follows:

It has no function in the circuit, but is only used on the PCB for debugging convenience or compatible design and other reasons.

It can be used as a jumper. If a certain section of the line is not used, just stick a 0 ohm resistor on it (it does not affect the appearance).

When the matching circuit parameters are uncertain, use a 0 ohm resistor instead. During actual debugging, after the parameters are determined, replace it with a component with a specific value. When you want to test the current of a certain part of the circuit, you can remove the 0 ohm resistor and connect an ammeter to facilitate current testing. When wiring, if you really can't go on, you can also add a 0 ohm resistor.

Under high-frequency signals, it acts as an inductor or capacitor (related to the characteristics of the external circuit), mainly to solve EMC problems, such as between ground and ground, power supply and IC Pin.

Single-point grounding (refers to protective grounding, working grounding, and DC grounding, which are separated from each other on the equipment and become independent systems). 3) The role of special resistors in the peripheral protection circuit of the power module The most common special resistors are thermistors, humidity resistors, varistors, etc. Varistors play a key role in the design and application of AC-DC switching power supplies.

The varistor MOV is one of the most commonly used devices in circuit electromagnetic compatibility (EMC). It is widely used in electronic circuits to protect the circuit from damage caused by transient voltage changes in the power supply system. Its characteristics are generally 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 is reduced, and the current is shunted to prevent the subsequent stage from being damaged or interfered by excessive transient voltage, thereby protecting sensitive electronic components. Circuit protection is to use the nonlinear characteristics of the varistor. When an overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the subsequent circuit. The main parameters of the varistor are varistor voltage, current capacity, junction capacitance, response time, etc.

However, don't overestimate the role of varistors. Varistors cannot provide complete voltage protection. The energy or power that a varistor can withstand is limited, and it cannot provide continuous overvoltage protection. Continuous overvoltage will destroy the protection device (varistor). The parts that varistors cannot provide protection for include inrush current during startup, overcurrent during short circuit, voltage drop, etc. These situations require other forms of protection.

34)]Thermistor is a temperature-dependent device, generally divided into two types, NTC is a negative temperature coefficient thermistor, 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 a very important role in circuit design.

NTC is mainly used in the circuit to suppress the starting current during the circuit startup process. During the system startup process, due to the existence of power circuits, capacitive and inductive loads inside the system, a very large impact current will appear at the moment of startup. If the instantaneous current resistance of the device is not considered during the circuit device selection process. Then, during the operation of multiple startups of the system, it is easy to cause the device to be broken down and damaged. Adding NTC to the circuit is equivalent to increasing the input impedance to reduce the impact current when the input circuit is started. When the system is in a stable state, due to the heating of NTC, according to its negative temperature characteristics, the impedance is reduced, so the loss on NTC is also reduced, reducing the overall loss of the system.

PTC can play the role of a fuse in the circuit, so it has another name: self-resetting fuse. During the operation of the system, if the circuit is abnormal and a large current appears, if there is a PTC in series in this part of the circuit, it is equivalent to a large current flowing through the PTC, and the PTC heats up. According to its positive temperature characteristics, its impedance will become very large, making the impedance of the entire circuit larger, thereby reducing the current in the circuit and playing the role of a fuse. According to its positive temperature characteristics, another function of PTC is to achieve over-temperature protection in the circuit.

3. Summary of the use of resistors in circuits

The knowledge of resistors covers a lot. It is not only about knowing Ohm's law, but also about the extremely special properties of materials. For example, the resistance value of a resistor element is not only related to temperature, material, and length, but also to the cross-sectional area. The physical quantity that measures the effect of temperature on resistance is the temperature coefficient, which is defined as the percentage change in resistance value when the temperature rises by 1°C. The main physical characteristic of a resistor is that it converts electrical energy into heat energy. It can also be said that it is an energy-consuming element. When current passes through it, it generates loss in the form of heat energy. Resistors usually play the role of voltage divider and current divider in circuits. For signals, both AC and DC signals can pass through resistors. As a hardware engineer, if you want to use components handy, you need to have a deep understanding of materials, electrical characteristics, and their particularities.

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懒猫爱飞

Very good, full of useful information, saved it, thanks for sharing!

langtuodianzi

Very detailed information, thanks for the analysis!

zileo0617

Thanks for the analysis!

shakencity

There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors, wirewound resistors, etc.; special resistors include varistors, thermistors, photoresistors, etc. Different types of resistors have different characteristic parameters, and the key points to consider when using them in circuits are also different. If certain special parameters of resistors are ignored in circuit design, the stability and reliability of the product may not be guaranteed. Correctly understand the various parameters of resistors and the selection precautions of different resistors, and fully understand the real role of resistors in circuits, so that the function and performance of products can be guaranteed from a basic level in circuit design.

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Lazy Cat Loves Flying posted on 2018-12-6 12:15 Very good, full of useful information, I have collected it, thank you for sharing!

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