How to choose a resistor
Nov 02, 2019| Stay Charging Safely with SChitec
How to choose a resistor
The application of surface mount technology (SMT) is very common, and the proportion of electronic products assembled by SMT has exceeded 90%. China has been involved in SMT technology since the 1980s. With the development of small SMT production equipment, the application scope of SMT is further expanded. In the fields of aviation, aerospace, instrumentation, machine tools, etc., SMT is also used to produce various electronic products or components with small quantities.
In recent years, in addition to the development of new products by electronic product developers using chip devices, maintenance personnel have begun to repair a large number of electronic products assembled by SMT technology.
At present, the models of the chip resistors are not uniform, and are set by the manufacturers themselves, and the models are particularly long (consisting of more than a dozen English letters and numbers). If you can correctly propose various parameters and specifications of the chip resistor during the purchase, it is convenient to purchase (or order) the required resistance.
There are five parameters for the chip resistor, namely size, resistance, tolerance, temperature coefficient and packaging.
1. The size series of chip resistor series is generally available in seven sizes and is represented by two size codes. One size code is an EIA (American Electronics Industry Association) code represented by a 4-digit number. The first two digits and the last two digits indicate the length and width of the resistor, in inches. The other is a metric code, also represented by a 4-digit number, in millimeters. Different sizes of resistors have different power ratings. Table 1 lists the code and power ratings for these seven resistor sizes.
2. Resistance Series The nominal resistance is determined by series. Each series is divided by the tolerance of the resistor (the smaller the tolerance is, the more the resistance is divided), the most commonly used is E-24 (the tolerance of the resistance is ±5%), as shown in Table 2. .
The resistance value is represented by three digits on the surface of the chip resistor, wherein the first digit and the second digit are valid digits, and the third digit represents the number of subsequent zeros. When there is a decimal point, it is represented by "R" and takes up one effective digit. The nominal resistance code representation method is shown in Table 3.
3. The tolerance of the chip resistance (carbon film resistance) has 4 levels, namely F class, ±1%; G class, ±2%; J grade, ±5%; K grade, ±10%.
4. Temperature coefficient The temperature coefficient of the chip resistor has 2 levels, ie w level, ±200ppm/°C; X level, ±100ppm/°C. Only resistors with tolerances of class F are used for class x, while resistors for other classes are generally of class w.
5. The packaging is mainly available in bulk and in tape.
The operating temperature range of the chip resistor is -55--+125°C. The maximum operating voltage is related to the size: 0201 is the lowest, 0402 is 0602 for 0402 and 150V for 0805, and other dimensions are 200V.
6. The number on the surface of the chip resistor is used to indicate the horizontal arrangement of the resistance characters, and is specified by three digits, where the first two digits are significant digits and the third digit is an index of 10 in ohms. For example: 473 means 47 × 103 = 47 kΩ. If the second character used to indicate the resistance on the surface of the resistor is the letter R, it represents a decimal point. For example: 5R1 means the resistance is 5.1 Ω.