Like current and voltage, resistance is the basic parameter of a circuit. Resistors are the most used components in circuits.
As the name implies, a resistor is a component that hinders the flow of electric current and is the "dead rival" to electric current. When we apply a voltage to a conductor, the charge will flow through the conductor in a specific direction, and charge collision will occur during the flow of charge, which will cause energy loss and limit the amount of current passing through. Like mechanical friction that opposes movement, resistors oppose the movement of electric charge (current).
The current in the circuit is similar to the water flow in the pipe. If the pipe is long enough, the water flow will become slow enough because of resistance. The same is true for resistors. In a conductor, if we increase the length of the conductor, the number of charge collisions will increase and the movement of the charge will be further reduced.
In physics, resistance is used to express the size of the conductor's obstructive effect on current. The greater the resistance of the conductor, the greater the resistance of the conductor to the current. Different conductors usually have different resistances, and resistance is a characteristic of the conductor itself.
Resistive components are energy-consuming components that hinder current. The main physical characteristic of a resistor is to transform electrical energy into thermal energy. It can also be said that it is an energy-consuming element, and heat energy is generated when the current passes through it.
The resistance of a conductor is generally related to temperature, material, length, and cross-sectional area. For example, under the same conditions, the longer the conductor, the greater the resistance; the larger the cross-sectional area, the smaller the resistance.
Resistance is not necessarily a bad thing.
The main function of resistors is to limit current and reduce the voltage (serial voltage division, parallel shunting). In addition, resistors, like the friction that helps us walk and drive, can convert electrical energy into heat and provide us with various conveniences.
From the structure, there are fixed resistors and variable resistors.
The basic parameters of fixed resistance are resistance and tolerance. Tolerance refers to changes caused by changes in temperature and light.
Variable resistors include variable resistors and physical quantity resistance sensors. Variable resistors include adjustable potentiometers and sliding rheostats. Physical quantity resistance sensors include thermal sensors, photosensitive sensors, pressure-sensitive sensors, and magnetic sensors.
From the manufacturing materials, including carbon resistors, carbon film resistors, metal film resistors, thick film resistors, foil resistors, wire-wound resistors.
Carbon resistors are relatively old structures. The accuracy is low, and it is usually used where high-energy pulses are generated.
Wire wound resistors are the oldest structure. The resistance is accurate and is usually used in high-power applications. The small resistance is still very reliable.
Nowadays, metal and metal oxide resistors are used more frequently. The resistance value and tolerance are relatively stable, and the temperature coefficient is relatively good.
What are the parameters of resistance?
The main parameters of the resistor are resistance value and rated power.
The resistance value is abbreviated as resistance value, and its basic unit is ohm, abbreviated as ohm (Ω). Commonly used units are kiloohms (KΩ) and megaohms (MΩ). The conversion relationship between them is 1MΩ=1000kΩ, 1kΩ=1000Ω. There are three ways to indicate the resistance value on the resistor, namely, direct labeling method, color labeling method and digital labeling method.
In electronic manufacturing, either 4-ring or 5-ring resistors can be used. In circuits such as frequency selection and bias circuit, resistors with small errors should be selected as much as possible. If necessary, an ohmmeter can be used to check the selection.
Rated power is another main parameter of resistors. Commonly used resistors have 1/8W, 1/4W, )/2W, 1W, 2W, 5W, etc.
In operation, a resistor with a rated power equal to or greater than the circuit request should be selected. What is not marked in the circuit diagram means that the power consumption of the resistor is very small, so there is no need to think about it.
The precision of the resistance is generally 1% and 5%, and the precision is 0.1%. The price of 0.1% is about ten times that of 1%, and the price of 1% is about 1.3 times that of 5%.
The voltage that can be applied at both ends of the resistor is determined by the rated power. It is necessary to ensure that the power does not exceed the rated power, and the other is the withstand voltage of the resistor. Although the power of the resistor body does not exceed the rated power, the too high voltage will cause instability of the resistor, creepage between the resistor pins, and other faults. When using it, a reasonable resistor should be selected according to the voltage used. The withstand voltage of some packages includes 0603=50V, 0805=100V, 1206 to 2512=200V, 1/4W plug-in=250V. Moreover, in a long-term application, the voltage on the resistor should be more than 20% less than the rated withstand voltage value, otherwise, problems will easily occur over a long period of time.
The temperature coefficient of resistance is a parameter that describes the change of resistance with temperature. This is mainly determined by the material of the resistor. Generally, the package of thick film chip resistors above 0603 can achieve 100ppm/℃, which means that when the ambient temperature of the resistor changes by 25 degrees Celsius, the resistance value may change by 0.25%.
In more demanding precision instruments, metal film resistors are used. It is easy for them to achieve a temperature drift of 10 to 20 ppm, and of course, it is more expensive. In short, the temperature coefficient is definitely a very important parameter in the precision applications of instruments. Inaccurate resistance can be adjusted during calibration, and the change of resistance with external temperature cannot be controlled.
Failure mode: various failure phenomena and their manifestations.
Failure mechanism: It is the physical, chemical, thermodynamic or another process that leads to failure.
The main failure modes and failure mechanisms of resistors are:
A multimeter can be used to measure the resistance value and determine whether the resistance is good or bad.
The actual resistance value can be measured by connecting the two test leads (regardless of positive and negative) to the pins at both ends of the resistor. In order to improve the measurement accuracy, the range should be selected according to the nominal value of the resistance being measured. If the measurement result is very different from the market value on the resistor, it means that the resistor has been damaged. (Analog multimeter needs to be calibrated to 0 before measurement, a digital multimeter is not used)
In addition, resistance is affected by many factors. Therefore, the following factors must be considered when measuring resistance:
A resistance of 0 ohms is common in circuit design, and everyone is often confused: Since it is a resistance of 0 ohms, it is a wire, why should it be installed? (In fact, the resistance of a zero-ohm resistor is not "0Ω", but very small, almost 0.)
There are many reasons.
First, zero-ohm resistance can be used for single-point grounding of analog ground and digital ground. All the grounds on the circuit board need to be connected together, but if the analog ground and digital ground are directly connected in a large area, it will cause mutual interference. However, using a zero-ohm resistor to short-circuit all grounds can solve this problem.
Second, zero-ohm resistance can be used in PCB design instead of wires and jumpers.
For example, the use of zero-ohm resistors in PCB mass production can reduce costs. In the PCB production process, we will use an automatic insertion machine to pick and place components such as diodes, capacitors, inductors, and resistors to reduce production costs. Here, zero-ohm resistors can be used to replace wires and jumpers. But if there is no zero-ohm resistance, we will have to turn on another automatic wire placement machine or have to manually place the wires, resulting in high production costs and long production time.
For another example, zero-ohm resistance can effectively reduce the risk of PCB design being copied. Some people use reverse engineering strategies to copy other people's PCB designs. In this case, zero-ohm resistance is the best alternative to wires, which can confuse and prevent PCBs from being copied. Designers and manufacturers will place 0-Ω resistors where there is no resistance value or use different color codes for the resistors.
Finally, the zero-ohm resistance also has the functions of fuse, wire crossing during wiring, debugging/testing, and temperature compensation device.