Printed circuit board (p.c.b)

(Part of a 1983 Sinclair ZX Spectrum computer board; a populated PCB, showing the conductive traces, vias (the through-hole paths to the other surface), and some mounted electrical components)

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).


PCBs are inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Much of the electronics industry's PCB design, assembly, and quality control needs are set by standards that are published by the IPC organization



History

The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.


Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap or turret board can be more efficient. Predating the printed circuit invention, and similar in spirit, was John Sargrove's 1936-1947 Electronic Circuit Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could produce 3 radios per minute.


Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.


In recent years, the use of surface mount parts has gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

Ohm

This article is about the SI (Omega) derived unit. For other meanings, see Ohm (disambiguation).

A multimeter can be used to measure resistance in ohms. It can also be used to measure capacitance, voltage, current, and other electrical characteristics.

Several resistors. Their resistance, in ohms, is marked using a color code.

The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance, named after Georg Simon Ohm.



Definition

The ohm is defined as a resistance between 2 points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.[1]

In many cases the resistance of a conductor in ohms is approximately constant within a certain range of voltages, temperatures, and other parameters; one speaks of linear resistors. In other cases resistance varies (e.g., thermistors).

Commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, and megohm.[2]





Use of the Ω symbol in electronic documents

Care should be taken when preparing documents (including HTML documents) which make use of the symbol Ω. Some document editing software will attempt to use the symbol typeface to render the character. Where the font is not supported, a W is displayed instead. As this represents the SI unit of power, not resistance, this can lead to confusion.

Unicode encodes an ohm symbol distinct from Greek omega among Letterlike Symbols.

Volt

The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage".[1] It is also the unit for the related but slightly different quantity electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery.

Definition

The volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor.[2] It can be written in terms of SI base units as: m2 • kg • s−3 • A−1. It is also equal to one joule of energy per coulomb of charge, J/C.

Josephson junction definition

Since 1990 the volt has been maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as:

K{J-90} = 2e/h = 0.4835979 GHz/µV.

This is typically used with an array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design).[ The relationship KJ = 2e/h is apparently exact, with no correction terms required in a practical implementation.


Common voltages


• Nerve cell resting potential: around −75 mV[5]


• Single-cell, rechargeable NiMH or NiCd battery: 1.2 V


• Mercury battery: 1.355 V


• Single-cell, non-rechargeable alkaline battery (e.g., AAA, AA, C and D cells): 1.5 V


• LiFePO4 rechargeable battery: 3.3 V


• Lithium polymer rechargeable battery: 3.75 V (see Rechargeable battery#Table of rechargeable battery technologies)


• Transistor-transistor logic/CMOS (TTL) power supply: 5 V


• PP3 battery: 9 V


• Automobile electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running).


• Household mains electricity: 230 V RMS in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see List of countries with mains power plugs, voltages and frequencies)


• Commercial and Military Jet aircraft: 400 V AC, 28 V DC[citation needed]


• Trucks/lorries: 24 V DC


• Rapid transit third rail: 600–750 V (see List of current systems for electric rail traction)


• High speed train overhead power lines: 25 kV RMS at 50 Hz, but see the list of current systems for electric rail traction and 25 kV at 60 Hz for exceptions.


• High voltage electric power transmission lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005[citation needed])


• Lightning: Varies greatly, often around 100 MV.


Note: Where RMS (root mean square) is stated above, the peak voltage is times greater than the RMS voltage for a sinusoidal signal centered around zero voltage.

History of the volt

In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.


The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.

Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972.

This SI unit is named after Alessandro Volta. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (V). When an SI unit is spelled out in English, it should always begin with a lowercase letter (volt), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

—Based on The International System of Units,