The specifications, models and parameters of various components are indicated on the circuit diagram, which is the basis for the selection of electronic components. For the finalized products, the components marked on the schematic diagram are put into production after design, development and trial production, and the parameters are determined according to the method of "qualitative analysis, quantitative estimation and test adjustment". Generally, whether the selected components are allowed to be replaced. However, for the developers, amateurs and maintenance personnel of electronic products, due to the influence of many factors such as objective conditions, under the condition of meeting the technical requirements and specifications, components can also be selected flexibly because of less consumption.

In some specific cases, even if there is a schematic diagram, due to the incomplete marked parameters of some components, such as electrolytic capacitor only marked with electric capacity but not withstand voltage, it should be reconsidered in the power supply circuit; If the on-site conditions of the product are inconsistent with the technical data, some components can be adjusted to adapt to the actual situation; If some components cannot be purchased locally, they can be replaced by qualified components; If it is found that some components are unreasonable in the maintenance process, it is necessary to replace them with appropriate components.

Electronic components are basic circuit units that perform predetermined functions and cannot be disassembled, such as resistors, semiconductor discrete devices, semiconductor integrated circuits, microwave components' relays' magnetic components, switches, electrical connectors, filters, sensors, fiber optic devices, etc. Practice has proved that 44% ~ 67% of the total failure of components in electronic equipment is caused by improper selection, while the failure rate caused by the quality of components accounts for 33% ~ 46%. Therefore, component selection plays an important role in circuit design, and designers must attach great importance to it.

2、 Selection principle of components

1. The standard components of manufacturers with stable quality, high reliability, promising development and good reputation shall be selected, and the obsolete or inferior components shall not be selected.

2. The technical performance, quality grade and service conditions of components shall meet the requirements of circuit design.

3. When the performance parameters are met, components with low power consumption, low thermal resistance, low loss, high power gain and high efficiency shall be selected.

4. Optimization of domestic components. It is preferred to select certified components that comply with national standards; Components that meet the requirements and can be supplied stably after use test; Other components with successful application experience and meeting the requirements.

5. Imported components. Components in ppl (list of preferred products) and QPL (list of qualified products) of foreign authorities; Highly reliable components strictly screened in the production process; High quality components that meet the requirements after domestic use assessment.

6. The selection shall follow the principle of standardization and generalization.

3、 Selection of components

The components are preferred and shall comply with the product preference manual or the list of preferred products (PPL) published by foreign authorities. The designer shall formulate accurate and clear technical specifications for purchasing components. In order to ensure the reliability requirements, the specifications shall specify the measures and methods of screening (including secondary screening) and quality consistency inspection. Meanwhile, the procurement list of qualified components shall be prepared according to the model regulations.

The factors affecting the reliability and quality of components must be specified in the procurement list, such as quality grade, environmental conditions, failure efficiency, technical standards, packaging form, special requirements (antistatic characteristics, chip protection process, etc.), manufacturer, etc. The procurement specification shall be implemented after being approved as required. After the application of components in the product is clear, its reliability shall be estimated, and whether it meets the requirements of the circuit for the reliability of components shall be considered.