SMD Resistors: Codes, Size, Testing, Tolerance and Selection

Abstract

SMD Resistor or Chip Fixed Resistor is one of the metal glass glaze resistors. It is a resistor made by mixing metal powder and glass glaze powder and printing on the substrate by the screen printing method. It is resistant to humidity and high temperature with a low-temperature coefficient. SMD Resistor can greatly save the cost of circuit space and make the design more refined. SMD is the abbreviation of Surface Mounted Devices, which is a special kind of SMT (Surface Mount Technology) element device. SMD resistors are usually called chip resistors.

Catalog

I How to identify SMD resistors codes?

1. Digital cable nominal method (generally used for rectangular chip resistors)

SMD resistor

The digital cable nominal method is to mark the resistance with digits on the resistor. Its first digit and second digit are significant digits, and the third digit represents the number of "0" added after the significant digit. No letters appear in this one. For example: "472 '" means "4700Ω"; "151" means "150".

The resistance value of the SMD resistor is usually directly marked on the surface of the resistor in digital form, so the resistance value of the reading resistor can be directly seen by the number on the resistor surface. There are generally three representation methods:

(1) Composed of three numbers, indicating that the tolerance of resistance is ± 5%. The first two digits are significant digits, the third digit represents the multiplying multiplier by zero, and the basic unit is Ω. For example, 103, 1, and 0 are valid numbers, just write them down, 2 means multiplying by zero, which is the power of 10 (in short, the third digit is the power of 10). So the resistance represented by 103 is the power of 10 × 10 = 10 × 1000 = 10000Ω = 10KΩ

(2) Composed of four numbers, indicating that the tolerance of resistance is ± 1%. The first three digits are significant digits, and the fourth digit represents a multiplier by zero (that is, the number means the power of 10). For example, 1502, 150 is a significant number, write it down directly, 2 represents the power of 10. So the resistance of 1502 is the square of 150 × 10 = 150 × 100 = 15000Ω = 15KΩ

(3) Composed of numbers and letters, such as 5R6, R16, etc. Here only need to replace R with a decimal point.

5R6 = 5.6R = 5.6Ω R16 = 0.16R = 0.16Ω

It should be noted here that "R" is the expression of resistance and "Ω" is the expression of resistance unit. In daily life, we may not mix the two, but in industrial production, the boundary between the two is very vague.

Here you can use Utmel's resistor code calculator to quickly determine the resistance value of an SMD resistor using the markings found on the resistor.

2.Nominal color ring method (generally used for cylindrical fixed resistors)

SMD resistors are the same as general resistors, and most of them use four rings (sometimes three rings) to indicate their resistance. The first ring and the second ring are significant digits, and the third ring is the magnification (color ring codes are shown in Table 1). For example: "Brown Green Black" means "15Ω"; "Blue Gray Orange Silver" means "68kΩ" with tolerance ± 10%.

3.E96 digital code and letter mixed nominal method

The mixed nominal method of digital codes and letters also uses three digits to indicate the resistance value, that is, "two digits plus one letter", where two digits represent the E96 series resistance code. Its third digit is the magnification expressed by letter code (shown in Table). For example: "51D" means "332 × 103; 332kΩ"; "249Y" means "249 × 10-2; 2.49Ω".

II SMD resistors size

Surface-mount resistors are standardized in shape and size. Most manufacturers use the JEDEC standard. The size of the SMD resistor is represented by a digital code, such as 0603. This code contains the width and height of the package. Therefore, in the example of the 0603 Imperial code, this means that the length is 0.060 "and the width is 0.030". This code can be given in English or metric units, usually using English codes to indicate package size more frequently. In contrast, in modern PCB design, metric units (mm) are more commonly used, which may cause confusion. In general, you can assume that the code is in English units, but the size unit used is mm. The size of the SMD resistor depends mainly on the required power rating. The following table lists the dimensions and specifications of common surface-mount packages.

Package and size table

III SMD resistors testing method

1. Grounding resistance test requirements: a. AC working grounding, resistance should not be greater than 4Ω; b. Safe working grounding, resistance should not exceed 4Ω; c. DC working grounding, resistance should be determined according to the specific requirements of the computer system; The patch resistance of the lightning protection ground should not be greater than 10Ω; e. If the shielding system uses joint grounding, the grounding resistance should not be greater than 1Ω.

2. SMD resistor tester

ZC-8 ground resistance tester is suitable for measuring the resistance value of various power systems, electrical equipment, lightning rods, and other grounding devices. It can also measure the resistance value and soil resistivity of low-resistance conductors.

ZC-8 ground resistance tester

3. The work of this instrument is composed of a hand-cranked generator, current transformer, slide wire resistor, and galvanometer. All the mechanisms are installed in the plastic shell, and the outer shell is easy to carry. The accessories include auxiliary probe wires, etc., which are installed in the accessory bag. Its working principle uses the reference voltage comparison formula.

4. Check whether the tester is complete before use. The tester includes the following devices: 1. One ZC-8 grounding resistance tester 2. Two auxiliary grounding rods 3. Three wires, each of  which is 5m, 20m, and 40m

5. Use and operation

(1) When measuring the resistance of an SMD resistor, the E terminal button on the instrument is connected with a 5m wire, the P terminal button is connected with a 20m wire, and the C terminal button is connected with a 40m wire. The other end of the wire is connected to the ground electrode E’, potential probe P’ and current probe C’, and E’, P’, C’ should be kept in a straight line with a distance of 20m.

When the wiring diagram when the chip resistance is greater than or equal to 1Ω, connect the two E-terminal buttons on the meter together. Related pictures on this topic are as follows:

The wiring diagram when the chip resistance is greater than or equal to 1Ω;

When the chip resistance is less than 1Ω, connect the two E-terminal button wires on the instrument to the ground body under test to eliminate the additional error introduced by the resistance of the connecting wire during the measurement.

The wiring diagram when the chip resistance is less than 1Ω

(2) Operation steps

1) All wiring at the instrument end should be correct.

2) The connection between the instrument and the ground electrode E’, potential probe P’, and current probe C’ should be in firm contact.

3) After the meter is placed horizontally, adjust the mechanical zero position of the galvanometer, and return to zero.

4) Set the "Magnification Switch" to the maximum magnification and gradually increase the speed of the crank handle to 150r / min. When the galvanometer pointer deflects in a certain direction, turn the dial to restore the galvanometer pointer to the "0" point. At this time, the reading on the dial multiplied by the magnification scale is the measured resistance value.

5) If the dial reading is less than 1, the galvanometer pointer is still not balanced, and the magnification switch can be set to the next lower magnification until it is adjusted to full balance.

6) If the pointer of the meter galvanometer is found to be jittery, the speed of the crank can be changed to eliminate the jitter.

Circuit and physical diagrams

IV Tolerance

What is a precision SMD resistor? The precision SMD resistor means that the tolerance of the chip resistor is relatively small. It is generally called tolerance of 1%. The minimum error can reach 0.01%. The temperature coefficient is as low as ± 5ppm / °C, which is rarely achieved by the industry: It can be applied to precision instruments, communication electronic products, and portable electronic products. So many people will ask if the chip resistance is so small, can it be distinguished if 5% and 1% are not tested? So below we compare the difference between 5% and 1% chip resistors.

The 5% series SMD resistors are represented by 3 characters: in this method, the first two digits represent the effective digits of the resistance value, and the third digit represents the number of "0" that should be added after the effective number. When the resistance is less than 10Ω, R is used to indicate the position of the decimal point of the resistance value in the resistor code. This notation is usually used in a resistance series with a resistance value error of 5%. For example, 330 means 33Ω instead of 330Ω; 221 means 220Ω; 683 means 68000Ω or 68kΩ; 105 means 1MΩ; 6R2 means 6.2Ω.

The 1% series precision SMD resistors are represented by 4 characters: the first 3 digits of this notation represent the effective digits of the resistance value, and the fourth digit represents the number of 0s that should be added after the effective digits. When the resistance is less than 10Ω, R is still used in the code to indicate the position of the decimal point of the resistance value. This representation method is generally used in the precision resistance series with a resistance error of 1%. For example: 0100 means 10Ω; 1000 means 100Ω; 4992 means 49900Ω, or 49.9kΩ; 1473 means 147000Ω or 147kΩ; 0R56 means 0.56Ω.

The surface of the SMD resistors is engraved with letters. If there are only three digits, the error is 5%. If there are four digits, the error is 1%.

V Selection of SMD resistors

The application of surface assembly technology (SMT) is very common, and the proportion of electronic products assembled by SMT has exceeded 90%. With the development of small-scale SMT production equipment, the application scope of SMT is further expanded, and aerospace, aerospace, instrumentation, machine tools, and other fields are also using SMT to produce various small-scale electronic products or components.

Electronic product developers often use SMD devices to develop new products. In recent years, maintenance personnel has also begun to repair a large number of electronic products assembled by SMT technology.

The model of the SMD resistor is not uniform and is set by each manufacturer, and the model is particularly long (composed of more than a dozen letters and numbers). If the various parameters and specifications of the SMD resistor can be correctly presented when purchasing, then the required resistor can be easily purchased (or ordered).

There are 5 parameters for SMD resistors, namely size, resistance, tolerance, temperature coefficient, and packaging.

1. Size

SMD resistors generally have 7 sizes, which are expressed by two size codes. A size code is an EIA (American Electronics Industry Association) code represented by 4 digits. The first two digits and the last two digits indicate the length and width of the resistor in inches respectively. The other is the metric code, which is also represented by 4 digits in millimeters. Different size resistors have different power ratings.

2. Resistance

The nominal resistance is determined by the series. Each series is divided by the tolerance of the resistance (the smaller the tolerance, the more the resistance value is divided), and the most commonly used is E-24 (the tolerance of the resistance value is ± 5%).

On the surface of the SMD resistor, three digits are used to represent the resistance value, in which the first and second digits are valid numbers, and the third digit represents the number followed by zero. When there is a decimal point, use "R" to represent, and occupy one significant digit.

3. Tolerance

The tolerance of SMD resistor (carbon film resistor) has 4 levels, namely F level, ±1%; G level, ±2%; J level, ±5%; K level, ±10%.

4. Temperature coefficient

The temperature coefficient of the SMD resistor has two levels, namely w level, ±;200ppm / ℃; X level, ±100ppm / ℃. Only resistors with tolerance class F are grade x, while resistors with tolerances of other grades are generally class w.

5. There are mainly two kinds of packaging: bulk and ribbon roll.

The working temperature range of the SMD resistors is -55-+125 ℃. The maximum working voltage is related to the size: 0201 is the lowest, 0402, and 0603 are 50V, 0805 is 150V, and other sizes are 200V.

The digits on the surface of the SMD resistor are used to represent the resistance characters arranged horizontally and are specified to be represented by three digits, where the first two digits are valid digits and the third digit is an exponent of 10. For example: 473 means 47 × 103 = 47 kΩ. If the second character on the surface of the resistor used to indicate the resistance value is the letter R, it represents the decimal point, for example, 5R1 means the resistance value is 5.1 Ω.

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