There are many parameters of the resistor, usually we are generally concerned about the value, accuracy, amount of power, these three indicators are appropriate. It is true that in digital circuits, we do not need to pay attention to too many details, after all, there are only 1 and 0 inside the digital, not much counting the minuscule impact. But in analog circuits, when we use a precise voltage source, or analog-to-digital conversion of signals, or amplify a weak signal, a small change in the resistance value will have a great impact. In the time of pounding with the resistor, of course, is in the occasion of processing analog signals, and later on, according to the analog circuit applications to analyze the impact of each parameter of the resistor.
The amount of resistance value of the resistor – the amount of resistance value of the resistor selection is often fixed by the application, such as a LED lamp current limit, or a current signal sampling, the resistance value of the resistor basically no other options. But some occasions, there are a variety of choices for the resistor, such as the amplification of a voltage signal, as shown in the figure, the amplification is related to the ratio of R2 to R3, and has nothing to do with the value of R2 and R3. At this time, the choice of resistance of the resistor is still based on: the greater the resistance of the resistor, the greater the thermal noise, the worse the performance of the amplifier; the smaller the resistance of the resistor, the greater the work is the current, the greater the current noise, the worse the performance of the amplifier; this is the reason why many amplification circuits are tens of K resistance, there is a need to use a large resistance value, or the use of voltage followers, or the use of T-networks to avoid.
Non-Inverting Amp
The precision of the resistor – the precision of the resistor is well understood, here do not verbose. Resistor accuracy is generally 1% and 5%, precision to 0.1%, etc. The price of 0.1% is about ten times more than 1%, and 1% is about 1.3 times more than 5%. Generally, the accuracy code A=0.05%, B=0.1%, C=0.25%, D=0.5%, F=1%, G=2%, J=5%, K=10%, M=20%.
The resistor’s frontal power – the power of the resistor would have been very simple, but often easy to use improperly. For example, the 2512 chip resistor, the quota power is 1W, according to the specifications of the resistor, the temperature exceeds 70 degrees Celsius, the resistor should be reduced to use. 2512 chip resistor in the end how much power can be used, at room temperature, if the PCB pads without special heat dissipation treatment, 2512 chip resistor power to 0.3W, the temperature may be more than 100 or even 120 degrees Celsius. . In 125 degrees Celsius temperature, according to the temperature derating curve, the amount of 2512 power needs to be derated to 30%. This situation in any package resistors need to pay attention to, do not believe in the nominal power, the key position is best to double check to avoid leaving hidden problems.
Resistor withstand voltage value – resistor withstand voltage value is generally less mentioned, especially for newcomers, often have little concept, thinking that capacitors only have withstand voltage value. The voltage that can be applied to both ends of the resistor, one is determined by the amount of power, to ensure that the power does not exceed the amount of power, the other is the resistance of the resistor voltage value. Although the power of the resistor body does not exceed the rated power, too high a voltage can lead to resistor instability, creepage between resistor pins, and other failures, so it is necessary to choose a reasonable resistor according to the voltage used. Some of the package withstand voltage values include: 0603 = 50V, 0805 = 100V, 1206 to 2512 = 200V, 1/4W plug-in = 250V. And, time applications, the voltage on the resistor should be smaller than the quota withstand voltage value of more than 20%, otherwise it is easy to have problems after a long time.
Temperature coefficient of resistance – 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 thick film chip resistor 0603 package above can do 100ppm / ℃, meaning that the resistor ambient temperature change of 25 degrees Celsius, the resistance value may change by 0.25%. If it is a 12bit ADC, a 0.25% change is 10 LSB. Therefore, for an op-amp like the AD620, which relies on only one resistor to adjust the amplification, many old engineers will not use it for convenience, they will use a conventional circuit to adjust the amplification by the ratio of two resistors. When the resistors are the same type of resistors, the change in resistance value caused by temperature will not bring about a change in the ratio, and the circuit will be more stable. In more demanding precision instrumentation, metal film resistors will be used, their temperature drift to 10 to 20ppm is easy, but of course, it is also more expensive. In short, in the precision applications of the instrument class, the temperature coefficient is definitely a very important parameter, the resistance is not accurate can adjust the parameters at school, the change in resistance with the external temperature is not controlled.
The structure of the resistor – the structure of the resistor is more, here to mention the application that can be thought of. The starting resistor of the machine is generally used to pre-charge the large capacity aluminum electrolytic, and then close the relay to turn on the power after filling the aluminum electrolytic. This resistor needs to be shock resistant, and it is best to use a large wirewound resistor. The amount of power of the resistor is not very important, but the instantaneous power is high, and ordinary resistors are difficult to meet the requirements. High voltage applications, such as resistors for capacitor discharge, where the actual operating voltage exceeds 500V, it is best to use high voltage vitreous enamel resistors rather than ordinary cement resistors. Spike absorption applications, such as silicon controlled modules at both ends need to parallel RC to do absorption, to do dv/dt protection, it is best to achieve non-inductive wirewound resistors, so as to have good absorption performance of spikes and not easily damaged by shocks.
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Post time: May-19-2022