Selection of Resistors and Their Functions in Circuit Design
The resistors are one of the most commonly used devices in electronic products. Basically, as long as it is an electronic product, there will be resistance inside.
Resistors can be used as voltage dividers, shunts and load resistors in circuits, together with capacitors, they can form filters and delay circuits.
they can be used as sampling resistors in power supply circuits or control circuits; they can be used in semiconductor tube circuits.
The bias resistor determines the operating point; resistors with special properties such as varistor and thermistor are used to prevent surge voltage, suppress inrush current, realize over-temperature protection, etc.
The resistor is the most common device, and it is also an indispensable device in the circuit.
It is very important to choose and use the resistor well for the stable operation and reliability of the product.
Keywords: 0-ohm resistance, resistance parameters, the function of resistance
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 characteristics and parameters, and the points that need to be considered when using the circuit are also different.
Engineers who are new to circuit design, are likely to ignore some special parameters of resistance, resulting in product stability and reliability that cannot be guaranteed.
Only by correctly understanding the various parameters of the resistor and the precautions for selection, and fully understanding the real role played by the resistor in the circuit, can the quality of the product be guaranteed from the most basic circuit design at the bottom.
1 The basic parameters of the resistor:
Engineers who are new to hardware circuit design may have the first impression of resistance that the resistance of electrical conductors to current described in physical books is called resistance, which is represented by the symbol R, and the units are ohms, kiloohms, and megohms, respectively Ω. , KΩ, MΩ.
The main parameters of concern are:
1), nominal resistance: the resistance marked on the resistor;
2), allowable error: the ratio of the difference between the nominal resistance and the actual resistance to the nominal resistance is called resistance The value deviation, 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 also two important parameters that must be paid attention to in the design: rated power and withstand voltage.
These two parameters affect the reliability of the entire system. The impact is very large.
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 1W.
It is inappropriate to choose a commonly used chip resistor, such as 0805 or 1206 package because the rated power of the resistor is small.
And there is a problem, the rated power of the resistor should be more than 1W (the power margin of the resistor selected by 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 resistance.
For example, the AC-DC switching power supply module is at the input front end of the design, according to the requirements of the safety standard GB4943.1, after ensuring that the plug or connector is disconnected, the residual voltage on the input terminals L and N is within 1S Attenuate to 37% of the initial value.
Therefore, in the design, one or two MΩ-level impedance resistors are generally used for energy discharge, and the input terminal is a high voltage, that is, both ends of the resistor are subject to high voltage.
The withstand voltage value will fail when the low-voltage input terminal is high-voltage.
2 The role of resistors in the circuit:
2.1 Basic role:
Electronic engineers have all learned the basic role of resistors, that is, used as voltage dividers, shunts and load resistors in circuits; together with capacitors, they can form filters and delay circuits, and are used as sampling in power circuits or control circuits.
Resistors are used as bias resistors in semiconductor tube circuits to determine the operating point, etc.For these functions, there are many applications in the circuit, and they are also very important, so I won’t describe them too much.
The following mainly introduces the functions and precautions of 0Ω resistors and special resistors in electronic circuit design.
2.2 The role of the 0-ohm resistor on the circuit:
I believe that there are many new electricians. When looking at some electronic products designed by predecessors, they often see that there is a 0Ω resistor on the circuit.
Why should we design such a resistor? By searching and organizing the data, the main points are as follows:
1) Analog ground and digital ground are grounded at a single point
As long as it is the ground, it will eventually be received together, and then enter the earth.
If they are not connected together, they are “floating ground”, there is a voltage difference, and it is easy to accumulate charges and cause static electricity.
The ground is referenced to 0 potential, and all voltages are derived from the reference ground. The standard of the ground should be the same, so the various grounds should be shorted together.
The earth is believed to be the ultimate ground reference point, capable of absorbing all electrical charges, remaining stable at all times.
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 in the ground.
If the analogue ground and the digital ground are directly connected in a large area, it will cause mutual interference.
There are four ways to solve this problem:
- 1. Use magnetic beads to connect;
- 2. Use capacitors to connect;
- 3. Use inductors to connect;
- 4. Use 0-ohm resistors to connect.
The equivalent circuit of the magnetic bead is equivalent to a band-stop wave limiter, which can only significantly suppress the noise at a certain frequency.
The frequency of the noise needs to be estimated in advance in order to select an appropriate model.
In the case of uncertain or unpredictable frequency, the magnetic beads do not fit together; the capacitor blocks the direct traffic, causing floating; the inductance is large and has many stray parameters, which are unstable; 0-ohm resistance is equivalent to a very narrow current path, which can effectively limit loop current, so that noise is suppressed.
Resistors have attenuation in all frequency bands (0-ohm resistors also have impedance), which is stronger than magnetic beads.
2) For current loops when connected across
When the electrical ground plane is divided, the shortest return path of the signal is broken.
At this time, the signal loop has to be detoured, forming a large loop area, and the influence of the electric field and the magnetic field becomes stronger, and it is easy to be disturbed/disturbed.
Connecting a 0-ohm resistor across the partition can provide a short return path and reduce interference.
3) Configure the circuit
Generally, jumpers and DIP switches should not appear on the product. Sometimes users will tamper with the settings, which is easy to cause misunderstandings.
In order to reduce maintenance costs, 0-ohm resistors are used instead of jumpers to be welded on the board. The vacant jumper is equivalent to the antenna at high frequency, and the chip resistor is effective.
4) Other uses
For cross-line debugging/testing during wiring: At the beginning of the design, a resistor should be connected in series for debugging, but the specific value cannot be determined yet. After adding such a device, it is convenient to debug the circuit later.
If the result of debugging does not need to add a resistor, add a 0-ohm resistor. Temporary replacement of other SMD devices as temperature compensation devices is more often due to the need for EMC countermeasures.
Also, 0-ohm resistors have less parasitic inductance than vias, and vias also affect the ground plane (due to digging).
Summarized as follows:
1. There is no function in the circuit, but it is only on the PCB for debugging convenience or compatible design.
2. It can be used as a jumper. If a certain line is not used, just paste the resistor directly (does not affect the appearance)
3. When the parameters of the matching circuit are uncertain, use 0 ohms instead. During actual debugging, determine the parameters and replace them with components with specific values.
4. When you want to measure the current consumption of a certain part of the circuit, you can remove the resistance of 0 ohms and connect an ammeter, which is convenient for measuring the current consumption.
5. When wiring, if it can’t be routed, you can also add a 0 ohm resistor.
6. Under high frequency signals, it acts as an inductor or capacitor (related to the characteristics of external circuits), mainly to solve the EMC problem. Such as ground and ground, between power supply and IC Pin.
7. Single-point grounding (meaning that the protective grounding, working grounding, and DC grounding are separated from each other on the equipment, and each becomes an independent system).
2.3 The role of special resistors in the peripheral protection circuit of the power module
The most common special resistors are varistors and thermistors, which play a key role in the design and application of AC-DC switching power supplies.
Learn about the characteristics and specific functions of these two resistors:
The varistor MOV is one of the most commonly used devices in-circuit electromagnetic compatibility (EMC), and is widely used in electronic circuits to protect the circuit from possible damage to the circuit due to sudden voltage changes in the power supply system.
Its characteristics are generally understood that 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 disturbed by excessive instantaneous voltage. Thus protecting sensitive electronic components.
Circuit protection is to use the nonlinear characteristics of the varistor. When an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit.
The main parameters of varistor are varistor voltage, current capacity, junction capacitance, response time, etc. However, don’t think too much about the role of the varistor.
The varistor cannot provide complete voltage protection.
The energy or power that the varistor can withstand is limited and cannot provide continuous overvoltage protection. . A sustained overvoltage can destroy the protective device (varistor) and cause damage to the equipment.
The parts that the varistor cannot provide protection include inrush current during startup, overcurrent during a short circuit, voltage sag, etc.
These situations require other means of protection.
A thermistor is a temperature-related 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 lower the impedance.
The use of impedance’s sensitivity to temperature plays an important role in circuit design.
In the circuit, NTC mainly suppresses the startup current during the startup process of the circuit.
During the startup process of the system, due to the existence of power circuits, capacitive and inductive loads in the system, a very large inrush current will appear at the moment of a startup.
If the instantaneous anti-current capability of the device is not considered in the selection process of the circuit device, then the system will easily cause the device to be broken down during the operation of multiple startups, and adding NTC to the circuit is equivalent to the input loop startup.
Increasing the input impedance reduces the inrush current, and when the system is in a stable state, due to the heating of the NTC, according to its negative temperature characteristics, the impedance is reduced, and the loss on the NTC is also reduced, reducing the overall loss of the system.
PTC can act as a fuse in the circuit, so it has another name as a resettable fuse.
During the operation of the system, when the circuit is abnormal, resulting in a large current, if there is a PTC in series in this part of the circuit, it means that there is a large current flowing in the PTC, and the PTC generates heat.
According to its positive temperature characteristics, its impedance will become very large, making the impedance of the entire loop larger, so that the current in the loop will become smaller, which acts as a fuse.
According to its positive temperature characteristics, another role of PTC is to realize over-temperature protection in the circuit.
The knowledge of resistance covers a lot, not only can it be applied after knowing Ohm’s law, but also includes materials and their special properties,
Such as the resistance value of resistance elements is generally related to temperature, material, length, and cross-sectional area.
The physical quantity that measures the resistance affected by temperature is the temperature coefficient, which is defined as the percentage change of the resistance value when the temperature increases by 1 °C;
The main physical feature of the resistance is to change electrical energy into thermal energy, which can also be said to be an energy consumption Component, the current passes through it to produce loss, which is expressed in the form of heat energy;
Resistance usually plays the role of the voltage divider and shunt in the circuit;
For signals, both AC and DC signals can pass through the resistance.
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 particularities.