Learn more about resistors: basic concepts, standards and principles

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Learn more about resistors: basic concepts, standards and principles

Posted Date: 2024-01-13

Resistors, like capacitors and inductors, are the most basic components and are widely used in various electrical or electronic equipment. It may be too common for people who work in electrical fields to ignore it, but without resistors, electrical or electronic circuits cannot be built. Resistors are such an important component.

The role of resistance

Resistors are passive components with a certain resistance. The function of resistance is based on Ohm's law "Voltage (V) = Current (I) × Resistance (R)". The main functions are current control, voltage division, current detection and bias (providing bias voltage).

【Current Control】

Resistors control the current flow in electronic circuits below rated values. For example, in an LED circuit, by connecting a resistor in series with the LED and controlling the current below the rated value, the LED can be prevented from burning out. '

[Voltage division (voltage distribution)]

When two or more resistors are connected in series, any voltage can be distributed that is proportional to the value of the connected resistor.

【Current Detection】

When current flows through a resistor, a voltage converted by the current is generated across it.

Voltage measures the current flowing into a circuit.

[Bias (providing bias voltage)]

Applying a voltage to make a semiconductor such as a transistor operate is called "biasing."

This bias (bias voltage) requires applying different voltages to each terminal (emitter, collector, base) of the transistor.

In addition to the above functions, it is also used as a damping resistor, terminal resistor, pull-up/pull-down resistor, etc.

The principle of resistance

The value of the resistor depends on the resistivity of the resistive material and its cross-sectional area and length.

As shown in the formula, the resistance value is obtained by multiplying the resistivity ρ (Ω·cm) by the length L of the resistor and dividing it by the cross-sectional area S. The resistivity of the metal is given here for reference.

terminology for resistance

Resistors have parameters that indicate technical specifications and ratings. Their terminology is summarized here. The four items in bold are the basic parameters of the resistor.

Relevant standards for resistance value and resistance value tolerance marking

As mentioned in the term "Resistance Value", resistance values ​​and tolerances are marked according to the following standards.

IEC 60062: Marking codes for resistors and capacitors

IEC 60063: Preferred number series for resistors and capacitors

The resistance value is standardized and serialized according to the above standards. The resistance value is not an integer like 1Ω, 2Ω, or 3Ω, but a decimal number like 2.2Ω or 4.7Ω. This is because the resistance value is based on the standard number (E series). The "E" of the series is the E of Exponent, and the following number, such as 24, is the division number. That is, E24 is divided from 1 to 10 by a geometric series (the 24th root of 10). Resistors are often used in ratio or proportion in actual use. Compared with integers, in most cases this standard serialized value is more convenient to use.

Resistor Structure

The following is the basic structure of a typical resistor. They are used depending on their application, but in recent years chip resistors have become mainstream in small equipment.

Resistor selection

The selection of resistors in the design must be based on the size requirements of the circuit board, the installation method, and the performance required.

[Select according to installation method]

Surface mount, or lead insertion/fixed mount

Surface mounting has become more common in recent years, but resistors such as lead insertion and screw fixing must be selected according to the circuit size or specifications. Basically, the research is based on the mounting specifications of the entire circuit board. The ideal approach is to use one method of mounting on one type of board.

[Select based on performance, characteristics, and size requirements]

Ratings, accuracy, temperature characteristics, functions, environmental resistance, heat dissipation, size, height, etc.

In addition to the installation method, each resistor has requirements related to its performance, characteristics, and dimensions. In order to meet individual requirements, for example, even if you want to surface mount, some surface mount type resistors cannot be used. In addition, in some cases, there are compromises such as more restrictions and fewer options to cope with harsh environmental conditions. When selecting a resistor, it is necessary to study it from a global perspective such as performance, characteristics, dimensions, and installation methods.

[Selection of chip resistors]

When selecting a chip resistor, you must meet inherent requirements such as performance and characteristics. Generally speaking, following the following steps will result in getting twice the result with half the effort.

1) Choose a single chip resistor or a composite chip resistor.

2-1) When using a single chip resistor, choose a thick film chip resistor or a thin film chip resistor.

2-2) When combining chip resistors, choose multiple chip resistors (independent circuit) or chip type network (parallel circuit).

3) Select the shape according to the voltage (power) used.

4) When there are duplicate types, select them based on other properties.

Review Editor: Huang Fei

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