Passive components are electronic components that can exhibit their characteristics without requiring an external power source in an electronic circuit. Their response to electrical signals is passive and compliant, allowing signals to pass through the electronic components only according to their original basic characteristics. Passive components are primarily used for transmitting, adjusting, and storing electrical signals, playing a crucial role in electronic circuits.
Some basic passive components include:
① Resistors
Components used to limit current, divide voltage, and generate heat. They fulfill their function by impeding the flow of current, with their resistance values typically measured in ohms (Ω).
The working principle of resistors is based on Ohm's Law, which states that the current through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor. Resistors impede the flow of current through their resistance values, thereby fulfilling functions such as limiting current, dividing voltage, and generating heat.
The main parameters of resistors include:
Nominal Resistance: The resistance value marked on the resistor, which determines the degree of impedance to current.
Tolerance — The permissible error range between the actual resistance and the nominal resistance.
Rated Power — The maximum power loss allowed for the resistor under normal operating conditions, exceeding which may cause the resistor to overheat and damage.
Maximum Operating Temperature — The highest temperature that the resistor can withstand during prolonged operation.
Resistors are indispensable components in electronic circuits, and their stability and reliability are crucial for the normal operation of the circuit.
② Capacitors
Electronic components capable of storing electrical charge, composed of two conductors (plates) positioned close to but not in contact with each other, separated by an insulating medium (dielectric). When a voltage is applied between the two plates of a capacitor, charge accumulates on the plates, forming an electric field and storing electrical energy. When electrical energy needs to be released, the capacitor can discharge its stored charge through an external circuit.
The main parameters of capacitors include:
Capacitance — This parameter represents the ability of a capacitor to store electrical charge, measured in Farads (F). Commonly used units for capacitance include μF, nF, and pF. A larger capacitance value indicates that the capacitor can store more charge.
Rated Voltage — The maximum voltage that a capacitor can safely operate at, measured in Volts (V). Exceeding the working voltage may damage or malfunction the capacitor.
Accuracy/Tolerance — The difference between the actual capacitance and the nominal capacitance of a capacitor is called tolerance. For example, a 10μF capacitor with a tolerance of ±10% may have an actual capacitance ranging from 9μF to 11μF.
Dielectric Material — The space between the two electrodes in a capacitor is usually filled with an insulating material called a dielectric or dielectric material. Different dielectrics have different dielectric constants, which affect the capacitance value and other characteristics of the capacitor.
ESR (Equivalent Series Resistance) — The equivalent series inductance of a capacitor refers to the inductive reactance of the capacitor to high-frequency signals, measured in ohms (Ω). A lower ESR value indicates better performance of the capacitor in high-frequency applications.
Dissipation Factor — The dissipation factor of a capacitor is the ratio of the energy converted into heat in one cycle to its average stored energy, usually expressed as a percentage. A lower dissipation factor indicates higher efficiency of the capacitor.
Frequency Response — Typically, as the frequency increases, the capacitance of a capacitor decreases. Therefore, when selecting a capacitor, its frequency response needs to be considered to ensure stable performance within the required frequency range.
Insulation Resistance — In an ideal capacitor, no current should flow through the capacitor when a DC voltage is applied. However, in reality, there is a small leakage current. The insulation resistance of a capacitor is the value obtained by dividing the DC voltage by the leakage current, reflecting the insulation performance of the capacitor.
Leakage Current — The dielectric of a capacitor has a strong impedance to DC current, but due to small defects or impurities in the dielectric, a small current may still flow, known as leakage current. The magnitude of leakage current is related to the structure of the capacitor, dielectric material, and operating conditions.
③ Inductor
An inductor is an electronic component mainly consisting of a coil wound from wire, used to produce inductance in a circuit. It is a physical quantity describing the relationship between current and magnetic field. When current passes through a wire, a magnetic field is generated around the wire, which in turn affects the current in the wire.
The basic parameters of an inductor include:
Inductance — A physical quantity representing the self-induction capability of an inductor. The inductance value mainly depends on factors such as the number of turns in the coil, the winding method, the presence or absence of a magnetic core, and the material of the magnetic core.
Quality Factor (Q-value) — The primary parameter for measuring the quality of an inductor, it refers to the ratio of the inductive reactance presented by the inductor when operating under an AC voltage of a certain frequency to its equivalent loss resistance. A higher Q-value indicates lower losses and higher efficiency for the inductor.
Distributed Capacitance — The capacitance that exists between turns of the coil, between the coil and the magnetic core, between the coil and ground, and between the coil and metal. A smaller distributed capacitance of the inductor indicates better stability.
Tolerance — The allowable error value between the nominal inductance value marked on the inductor and the actual inductance.
Rated Current — The maximum current value that the inductor can withstand under permissible operating conditions. If the operating current exceeds the rated current, the inductor may experience changes in performance parameters due to heating, and may even be burned out due to overcurrent.
④ Sensor
A sensor is a front-end detection device capable of sensing information from the measured object and converting this sensed information into electrical signals or other required signal forms according to a certain rule for output, to meet the requirements of signal transmission, processing, display, recording, storage, or control.
The operation of a sensor is mainly based on its internal sensing element and converting element.
The sensing element is responsible for directly sensing the measured quantity and outputting a physical quantity signal that has a definite relationship with the measured quantity. The converting element is responsible for converting the physical quantity signal output by the sensing element into an electrical signal, such as voltage or current. Additionally, the conversion circuit is responsible for amplifying and modulating the electrical signal output by the converting element to facilitate subsequent signal processing. Generally, the converting element and conversion circuit also require an auxiliary power supply.
When selecting a sensor for an electronic circuit, the following main parameters need to be checked:
Measuring Range — The range over which the sensor can accurately measure, which is the maximum and minimum values it can measure. The output signal of the sensor is accurate only when the measured quantity falls within this range.
Sensitivity — The ratio of the change in the sensor's output to the change in its input, which reflects the sensor's sensitivity to the measured quantity. High sensitivity usually means a high signal-to-noise ratio for the sensor, facilitating signal transmission, conditioning, and calculation.
Linearity — Also known as nonlinear error, it refers to the degree of linearity between the sensor's output and input. The ideal sensor input-output relationship is linear, but actual sensors deviate from this linear relationship to varying degrees.
Hysteresis — When the input quantity changes from small to large or from large to small, the resulting sensor output curves are usually not coincident. Hysteresis error reflects the inconsistency in the output value of the sensor during the forward and reverse strokes.
⑤ Diode
Although exhibiting nonlinear characteristics in some applications, diodes are generally considered passive components in most cases. Used for rectification, switching, voltage regulation, and other purposes, diodes are one of the important components in electronic circuits.
⑥ The main parameters of a diode include:
Forward Voltage (VF) — The measured voltage drop between the anode and cathode when the diode is biased. Under forward voltage, the diode conducts and allows current to pass. The magnitude of the forward voltage affects the diode's conduction state and power consumption.
Forward Current (IF) — The maximum forward current that the diode can handle, also known as the rated forward operating current. When the actual current exceeds this value, the diode may be damaged. Therefore, when selecting a diode, it is necessary to ensure that its forward current meets the circuit's requirements.
Reverse Breakdown Voltage (UBR) or Maximum Reverse Working Voltage (URM) — The maximum reverse voltage that the diode can withstand. When the reverse voltage exceeds this value, the diode may undergo breakdown, leading to damage. Therefore, when selecting a diode, it is necessary to ensure that its reverse breakdown voltage or maximum reverse working voltage is higher than the maximum reverse voltage in the circuit.
Reverse Current (IR) — The DC reverse leakage current through the diode at a given reverse bias. A smaller reverse current indicates better unidirectional conductivity of the diode. For circuits requiring high reverse blocking capability, a diode with a smaller reverse current should be selected.
Power Dissipation (PD) — The heat generated by the diode during operation. Power dissipation is related to forward voltage and forward current. When selecting a diode, it is necessary to ensure that its power dissipation does not exceed its maximum allowable value to prevent overheating and damage.
Junction Temperature and Storage Temperature — The junction temperature of a diode is its internal temperature during operation, while the storage temperature is the temperature of the diode when it is not conducting. These two parameters limit the operating environment and storage conditions of the diode. When selecting a diode, it is necessary to ensure that its junction temperature and storage temperature meet the circuit's requirements.
Thermal Resistance (Rth) — Indicates the diode's ability to conduct heat from the junction to the environment or casing. A smaller thermal resistance indicates better heat dissipation performance of the diode. When selecting a diode, models with smaller thermal resistance can be considered to improve its heat dissipation performance.
Reverse Recovery Time (trr) — The time required for the diode to recover its forward conduction capability when switched from reverse bias to forward bias. A longer reverse recovery time results in greater switching losses for the diode. For circuits requiring fast switching, a diode with a shorter reverse recovery time should be selected.
Frequency Characteristics — The performance of the diode at different frequencies. For circuits requiring the processing of high-frequency signals, a diode with high-frequency characteristics should be selected.
Optoelectronic devices, circuit connectors, connecting cables, transformers, and other components all belong to passive components. These elements play an indispensable role in electronic circuits, realizing various complex circuit functions through different combinations and connections.
Passive components are widely used in electronic circuits, covering multiple fields such as communications, computers, consumer electronics, industrial automation, and more. When selecting passive components, comprehensive consideration is needed based on circuit requirements and applications. Therefore, in addition to parameters, the following precautions also need to be known:
Choose the appropriate component type and specification according to circuit requirements — Different types of circuits have different requirements for passive components, which need to be selected based on circuit needs. For example, in high-frequency circuits, capacitors and inductors with high-frequency characteristics need to be selected. In circuits that require precise control of current and voltage, high-precision and high-stability resistors are needed.
Consider the package form and size of components — With the trend of miniaturization of electronic devices, the package forms and sizes of passive components are also getting smaller. When selecting, it is necessary to choose components of appropriate size according to the layout and space limitations of the circuit board. At the same time, the convenience of installation and soldering of the component's package form also needs to be considered.
Reliability and durability of components — In harsh environments or conditions of long-term operation, the reliability and durability of passive components are crucial to ensure the stability and reliability of the circuit.
Cost — Under the premise of meeting performance and quality requirements, it is necessary to select cost-effective components to reduce production costs. Comparisons can be made among the prices, performance, quality, and other factors of components from different brands and models to make a choice.
