Resistors Secret - Resistor Parameters You Should Know
A resistor is a common component, but there are unusual ways. Today, after almost ten years of designing electronic devices, I, a little electrician, have realized some truths, and I will share them with you here.
There are many parameters of resistors. Usually we pay attention to rating, accuracy and rated power. These three indicators are suitable. It's true that in digital circuits we don't have to pay too much attention to detail, after all, there are only numbers 1 and 0, so we don't care about minor influences. But in an analog circuit, when we use a precise voltage source, or do A/D conversion of a signal, or amplify a small signal, a small change in resistance will have a big impact. When it comes to bargaining with resistors, this is of course case for dealing with analog signals. Later, we will analyze effect of resistor settings according to application of analog circuits.
Rated resistance of resistor. The choice of resistor value is often determined by application, such as limiting current of an LED lamp or sampling a specific current. signal, resistor value There are no other options in principle. But in some cases, there are many resistor options, for example, to amplify voltage signal, as shown in figure, gain is related to ratio of R2 and R3, and has nothing to do with values of R2 and R3. P3. At present, there is still reason to choose a resistor value: larger resistance value, greater thermal noise and worse performance of amplifier, smaller resistance value, greater operating current. , and larger current noise. The worse performance of amplifier, so resistance of many amplifying circuits is tens of kΩ, where a large resistance value is required, either use a voltage follower or use a T-type network to avoid this.
Resistor Accuracy - Resistor accuracy is easy to understand, so I won't repeat myself here. The resistance accuracy is usually 1% and 5%, and accuracy is 0.1%. The price of 0.1% is about ten times price of 1%, and price of 1% is about 1.3 times price of 5%. Typically, accuracy codes: A=0.05%, B=0.1%, C=0.25%, D=0.5%, F=1%, G=2%, J=5%, K =10%, M=20%. . Resistor Wattage Rating—The power rating of a resistor is very simple, but it's often easy to misuse it. For example, 2512 resistor has a power rating of 1W. According to resistor specification, at temperatures above 70 degrees Celsius, resistor must be denominated. How much power can a 2512 resistor use? At room temperature, if PCB pad does not have special heat dissipation treatment, when power of 2512 resistor reaches 0.3W, temperature may exceed 100 or even 120 degrees Celsius. At a temperature of 125 degrees Celsius, according to temperature derating curve, nominal power of 2512 must be reduced to 30%. In this case, you need to pay attention to any packed resistors. Do not be superstitious about power rating. It is best to repeatedly confirm positions of keys in order to avoid hidden dangers. Resistors Withstand Voltage Value—— The voltage withstand value of resistors is rarely mentioned at all, especially for beginners, they often have no idea, thinking that capacitors have voltage withstand values. The voltage that can be applied to both ends of resistor is determined by power rating. To ensure that power does not exceed power rating, other is voltage withstand value of resistor. Although power of resistor body does not exceed rated power, too high voltage will lead to resistance instability, current leakage between resistor terminals and other malfunctions. When using it, you must select a reasonable resistor according to voltage used. The withstand voltage of some packages includes: 0603=50V, 0805=100V, 1206-2512=200V, 1/4W plug-in=250V. In addition, in application of time, voltage across resistor must be more than 20% less than rated withstand voltage, otherwise problems will easily occur after a long time. Temperature Coefficient of Resistance. The temperature coefficient of resistance is a parameter that describes how resistance changes with temperature. This is mainly determined by material of resistor. Generally, thick-film chip resistor packages above 0603 can reach 100 ppm/℃, which means that when ambient temperature of resistor changes by 25 degrees Celsius, resistance value can change by 0.25. %. If it's a 12-bit ADC, change by 0.25%is 10 least significant digits. So op amps like AD620 rely on just one resistor to adjust gain. Many older engineers will not use it for convenience. They will use conventional circuitry to adjust gain through ratio of two resistors. When resistors are same type resistor When change in resistance value caused by temperature does not cause a proportional change, circuit is more stable. More demanding instruments will use metal film resistors. It is easy to achieve temperature drift of 10 to 20 ppm, but of course it is more expensive. In short, in precision application of instruments, temperature coefficient is certainly a very important parameter. If resistance is not accurate, parameter can be adjusted during calibration, and change in resistance with external temperature cannot be controlled. Resistor Structure — There are many resistor designs, here are some applications to remember. The starting resistor of machine is generally used to pre-charge large capacity aluminum electrolysis with a resistor. After aluminum electrolysis is full, close relay and turn on power to work. This type of resistor must be shock resistant. It is best to use a large wirewound resistor. The power rating of resistor is not very important, but instantaneous power is very high. Ordinary resistors are difficult to meet requirement. For high voltage applications such as capacitor discharge resistors, actual operating voltage is greater than 500V, and high voltage glaze resistors are better instead of conventional cement resistors. For example, to apply peak absorption, both ends of thyristor module must be connected in parallel with RC for absorption and protection against dv/dt. It is best to use non-inductive wirewound resistors so that they can have good absorption. performance for peaks and not easily damaged by impact.
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