Engineers who are new to hardware circuit design may have the first impression of resistance that the resistance described in the physics book that the conductors hinder current is called resistance. It is represented by the symbol R, and the unit is ohm, kiloohm, and megaohm, respectively. , KΩ, MΩ means. The main parameters of concern are 1). Nominal resistance: the resistance value indicated on the resistor; 2). Allowable error: the percentage of the difference between the nominal resistance and the actual resistance to the ratio of the nominal resistance. Value deviation, which indicates the accuracy of the resistor. In the design of the circuit, it is not enough to pay attention to these two parameters. There are two other important parameters that must be paid attention to in the design: rated power and withstand voltage. These two parameters have an effect on the reliability of the entire system. The impact is very big.

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 commonly used chip resistors, 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 selected to be above 1W (the power margin of the resistor selected in the circuit design is generally more than 2 times), otherwise the power consumed on the resistor will overheat the resistor and cause it to fail.

Similarly, if the withstand voltage is not properly selected, the system design will fail due to the breakdown of the resistance. For example: AC-DC switching power supply module is designed at the input front end, according to the requirements of the safety standard GB4943.1, after ensuring that the plug or connector is disconnected, the stagnant voltage on the input terminals L and N is within 1S Attenuation to 37% of the initial value. Therefore, in the design, one or two MΩ impedance resistors are generally used to discharge energy, and the input end is high voltage, that is, both ends of the resistor must withstand high voltage. In the case of high voltage at the low-voltage input end, failure will occur. The following table 1 is the parameters of common SMT thick film resistors. Please check with the manufacturer of the purchased device when selecting the model.