A linear circuit is one in which, for a sinusoidal input voltage of frequency f, any steady-state output of the circuit (the current through any component, or the voltage between any two points) is also sinusoidal with frequency f. Note that the output need not be in phase with the input.
Many different methods of connecting components have been used over the years. Early electronics often used point to point wiring, or terminal strips. Most modern day electronics now use printed circuit boards (PCBs) made of materials such as FR4, or the cheaper (and less hard-wearing) Synthetic Resin Bonded Paper (SRBP, also known as Paxoline/Paxolin (trade marks) and FR2). Health and environmental concerns associated with electronics assembly have gained increased attention in recent years, especially for products destined to the European Union, with its Restriction of Hazardous Substances Directive (RoHS) and Waste Electrical and Electronic Equipment Directive (WEEE).
Enclosure design considerations
An engineer or designer must balance many objectives and practical considerations when selecting enclosures and protective features of the product itself and its components.
- Hazards to be protected against: mechanical damage, exposure to weather and dirt, electromagnetic interference, etc.
- Heat dissipation requirements
- Tradeoffs between tooling capital cost and per-unit cost
- Tradeoffs between time to first delivery and production rate
- Availability and capability of suppliers
- User interface design and convenience
- Ease of access to internal parts when required for maintenance
- Product safety, and compliance with regulatory standards
- Aesthetics, and other marketing considerations
- Service life and reliability
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