How to choose a resistor in circuit design

There are many types of resistors. Commonly used resistors include carbon film resistors, cement resistors, metal film resistors, wirewound resistors, etc. Special resistors include varistors, thermistors, and photoresistors. Different types of resistors have different characteristics and different characteristics. The key points to be considered when using the circuit are different. If some special parameters of the resistor are omitted in the circuit design, the stability and reliability of the product may not be guaranteed. Correct understanding of the various parameters of the resistor and the selection of different resistors, a comprehensive understanding of the real role of the resistor in the circuit, in order to ensure the function and performance of the product from the basic level in the circuit design.

Basic parameters of the resistor:

Speaking of resistance, our first impression should be described in the physics book: the resistance of the conductor to the current is called resistance. The resistance is represented by R in the circuit schematic diagram, and the unit is ohm (Ω). Commonly used are ohms, kiloohms, megohms, etc. (indicated by Ω, KΩ, MΩ, respectively).

The main parameters of the resistor are:

1) Nominal resistance

The resistance value indicated on the resistor.

2) Resistance deviation

The difference between the nominal resistance and the actual resistance divided by the nominal resistance is called the resistance deviation, which represents the accuracy of the resistor.

In the actual circuit design, it is not enough to focus on these two parameters. There are two important parameters that must be paid enough attention in the design: rated power and withstand voltage values. These two parameters are for the whole The reliability of the circuit system has a great impact.

For example, if the current flowing through the resistor in the circuit is 100 mA and the resistance of the resistor is 100 Ω, then according to the circuit power calculation formula P=I*I*R, the power consumption on the resistor can be calculated as 1 W. A resistor, such as a package of 0805 or 1206, is not suitable, and the circuit may have problems due to the low rated power of the resistor. Therefore, the resistor should be selected to have a rated power of more than 1W (in the circuit design, the power margin when the resistor is selected should be more than twice the actual power consumption), otherwise the power consumed by the resistor will cause the resistor to overheat and fail.

Similarly, when the withstand voltage value is not properly selected, the entire circuit system may be malfunctioned due to the breakdown of the resistor. For example, the AC-DC switching power supply module is designed at the input front end. According to the safety requirements (GB4943.1 standard), the retention voltage at the input terminals L and N can be guaranteed at 1S after the plug or connector is disconnected. The internal attenuation is less than 37% of the initial value. Therefore, in the actual circuit design, when the withstand voltage of the resistor is lower than the high voltage of the input terminal, it will fail.