1. Resistance

The resistance standard is developed by IEC (International Electrotechnical Commission), and the standard documents are IEC60063 and EN60115-2.

In order to facilitate the management and selection of component specifications, and to make the specifications of resistors not too much, electronic component manufacturers adopt unified standards for component values.

The nominal resistance of resistors is divided into E6, E12, E24, E48, E96, E192 six series, which are used in resistors with allowable deviations of ±20, ±10%, ±5%, ±2%, ±1%, and ±0.5%. Among them, the E24 and E96 series are the most commonly used.

"E" stands for "exponential spacing", which indicates that the resistance value is calculated by the formula.

The letter n refers to the values 24 and 96 in E24, E96 and other standards, and the value range of m is 0~n-1; In this way, E24 has 24 reference values, E96 has 96 reference values, and these reference values are multiplied by 10 to the x power to obtain a variety of resistance values.

2. Silk screen printing

The resistors above package 0603 (including 0603) are printed with silk screen printing on the surface.

Silk screen printing shows two layers of meaning: resistance size and precision.

Commonly used resistance screen printing generally has these situations:

1) Silk screen with three or four digits

2) Silk screen with the letter "R"

3) Silk screen with numbers and letters other than R

2.1 Silk screen with three or four digits

Three digits represent 5% precision, four digits represent 1% precision, the first few digits represent numeric values, and the last digit represents 10 to the power of x.

Example 1: If the silk screen is "103", then: R=10k, 5% accuracy

Example 2: If the silk screen is "1003", then: R=100k, 1% accuracy

2.2 Silk screen with the letter "R"

Resistors with the letter "R" generally have a small resistance value and an accuracy of 1%, but they are not absolute, and R can be regarded as a decimal point, and the number in front is an effective value.

Example: The silkscreen is "22R0", which treats R as a decimal point, and the preceding 22 represents the effective value, and the reading is 22.0Ω, which is a 1% precision resistor with an accuracy of 22Ω.

2.3 Silk screen with numbers and letters other than R

This resistor silk screen is more common in the 0603 package, with an accuracy of 1%, which corresponds to the standard E-96.

E-96 stipulates: use two digits plus a letter as a silk screen print, the actual resistance value can be obtained by looking up the table, the two digits indicate the resistance value, the letter indicates the x power of 10, and the lookup table is also required.

Example: The silk screen is "88A", from the following table, "88" represents 8.06, A represents 100, that is, the resistance value: 806.

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3. Accuracy

The accuracy of the resistance is generally represented by letters:

T：±0.01%

A：±0.05%

B：±0.1%

D：±0.5%

F：±1%

J：±5%

K：±10%

The most commonly used accuracy is 1% and 5%

5% accuracy is used in general occasions, and 1% resistors are used for precision requirements, such as DCDC, current sampling, and higher precision is selected according to the actual situation for special requirements.

4. Packaging

The package is named according to the actual size of the resistor—inches

Example:

0402 actual size: 1mm 0.5mm 0.04 inch 0.02 inch 0402

Commonly used resistance packages are:

01005、0201、0402、0603、0805、1206、1210、1218、2010、2512

At present, general electronic products are mainly packaged with 0402, 0603, 1206 is required for high power, and smaller packages are used for mobile phones or wearable devices, such as 01005, 0201, etc

1 inch = 2.54cm

5. The power of the resistor

5.1 Resistor power rating and package

The rated power of the resistor is mainly determined by the package, but it is not absolute, and it is also related to the process of the resistor (film or thick film), brand, resistance value, etc.

If you check the relationship between power and packaging on the Internet, some netizens will give a table of power and packaging, which is not always correct, you need to be careful when using the same package, different brands of resistance power may be different. The same package, different accuracy resistor power may be different. The same package, different values of resistance power may be different.

5.2 The relationship between resistance rated power and temperature

It should be noted that the power rating provided in the previous section is under the condition of 70 °C, and if the temperature exceeds 70 °C, its rated power will decrease.

In addition, the power ratings of R01005 and R0201 tend to decrease faster than other package resistors with increasing temperature.

The operating temperature range of R01005 and R0201 resistors is -55°C~~125°C, and the operating temperature range of R0402 and above is: -55°C~155°C.

The figure below shows the load (rated power) drop curve of the chip resistor.

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Note 1: Curve (1) is used for products 01005, 0201, and curve (2) is used for products 0402, 0603, 0805, 1206, 1210, 2010, 2512.

Note 2: When the ambient temperature of the resistor exceeds 70°C, its rated load (rated power) decreases according to the above curve.

6. The rated voltage of the resistor

The resistor has a rated withstand voltage value and cannot be used beyond the rated withstand voltage value.

1. The rated voltage of the same material (thick film) is not much different between brands.

2. The material is different, the rated voltage is different, and the film is lower than the thick film.

3. The larger the package, the higher the rated voltage.

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7. Temperature drift of resistance

7.1 Temperature coefficient of resistance TCR

Temperature coefficient of resistance (TCR) indicates the relative change of resistance value when the temperature changes by 1 degree Celsius, in ppm/°C, ppm (part per million).

Temperature coefficient = (R-Ra)/Ra ÷ (T-Ta) × 1,000,000

Ra: Resistance under reference temperature conditions

Ta: Reference temperature 20 ℃

R: Resistance at any temperature

T: any temperature

Generally, the range of commonly used resistance temperature coefficient is: -200~500ppm/°C

7.2 The influence of resistance temperature drift

Example: What is the resistance change rate of a chip resistor with a temperature coefficient of resistance of 100ppm/°C from a reference temperature of 20°C to 100°C?

Resistance change rate = temperature difference * temperature coefficient / 1 million = (100-20) * 100/1000000 = 0.8%

8, 0Ω resistance

8. The role of 1-0Ω resistance

1. Convenient for testing current

2. Compatible design, jumper

3. Analog ground, digital ground separated, single point grounding

4. Occupy a position (can be replaced with resistance of other values, can also be replaced with magnetic beads)

5. Do circuit protection and act as a low-cost fuse

What is the resistance value of 8.2-0Ω resistor

According to the resistance standard document EN60115-2, the actual maximum resistance of 0Ω resistor is 10mΩ, 20mΩ, 50mΩ is optional, the actual query of each manufacturer, the resistance value of ordinary 0Ω resistor can reach up to 50mΩ

8.3-0Ω resistor overcurrent capability

It should be noted that the overcurrent capability of 0Ω resistors varies from manufacturer to manufacturer, as can be seen from the following table: