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Basic Parameters of Resistors
Release time:
Aug 02,2022
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 value: the resistance value marked on the resistor; 2), allowable error: the percentage of the ratio of the difference between the nominal resistance value and the actual resistance value to the nominal resistance value is called resistance The value deviation, which 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. Therefore, 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 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. The following table 1 shows the parameters of common SMT thick film resistors, and the final selection should be verified with the manufacturer of the purchased device.
Note: For reference only, the final selected manufacturer's instructions shall prevail
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