Electronics/Cables

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Everything past this point is notes[edit]

Gage of wire. The larger the gage the smaller the wire if you are using AWG. Metric sizes go the other way.

Cables[edit]

Component video and audio.

Coaxial[edit]

Coaxial cable is an electrical cable consisting of a round, insulated conducting wire surrounded by a round, conducting sheath, usually surrounded by a final insulating layer.

The cable is designed to carry a high-frequency or broadband signal, usually at radio frequencies. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, such in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of copper. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Coaxial cables are usually terminated with RF connectors.

Electromagnetics[edit]

Open wire transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines are low loss, however they have undesirable characteristics. They cannot be bent, twisted or otherwised shaped without changing their characteristing impedance. They also cannot be run along or attached to anything conductive, as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line.

Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the cable, between the center conductor and the shield. The line itself forms a coaxial waveguide, and the tranmission of energy in the line occurs totally through the wave that propagates inside the cable between the conductors. Coaxial lines can therefore be bent and twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents on them.

Coaxial lines are filled with a dielectric material that maintains the spacing between the center conductor and shield. Unfortunately, all dielectrics have loss associated with them, which causes most coaxial lines to be lossier than open wire lines.

Important parameters[edit]

  • The surge impedance, also called the characteristic impedance, can be directly measured with an impedance meter in ohms (Ω) or calculated from the ratio of the inner and outer diameters (ø). Assuming the dielectric properties of the material inside the cable does not vary appreciably over the operating range of the cable, this impedance is frequency independent. The surge impedance does not depend on the length of the cable.
  • Capacitance, in farads per metre.
  • Resistance, in ohms per metre, can be directly measured with an ohmmeter.
  • Attenuation or loss, in decibels per metre. This is dependent on the loss in the dielectric material filling the cable, and resistive losses in the center conductor and shield. These losses are frequency dependent, the losses becoming higher as the frequency increases. In designing a system, engineers must consider not only the loss in the actual cable itself, but also the insertion loss in the connectors.
  • Outside diameter φ, which dictates which connectors must be used to terminate the cable.

Standard cable types[edit]

Most coaxial cables have a characteristic impedance of either 50 or 75 ohms. The RF industry uses standard type-names for coaxial cables. The U.S. military uses the RG-# or RG-#/U format (probably for "radio grade, universal", but other interpretations exist). For example:

  • RG-6/U: 75 Ω, low loss at high frequency for satellite television
  • RG-11:
  • RG-58: 50 Ω, φ = 0.2 in (5 mm)
  • RG-59/U: 75 Ω, φ = 0.25 in (6.5 mm)
  • RG-178:
  • RG-179: 75 Ω, φ = 2.8 mm

(φ = diameter, ΩΩ = ohms)

Uses of coaxial cable[edit]

Short coaxial cables are commonly used to connect home video equipment, or in ham radio setups.

Long distance coaxial cable is used to connect radio networks and television networks, though this has largely been superseded by other more high-tech methods (fibre optics, T1/E1, satellite).

In broadcasting and other forms of radio communication, hard line is a very heavy-duty coaxial cable, where the outside shielding is a rigid or semi-rigid pipe, rather than flexible and braided wire. Hard line is very thick, typically at least a half inch or 13mm and up to several times that, and has low loss even at high power. It is almost always used in the connection between a transmitter on the ground and the antenna or aerial on the tower. Hard lines are often made to be pressurised with nitrogen or desiccated air, which provide an excellent dielectric even at the high temperatures generated by thousands of watts of RF energy, especially during intense summer heat and sunshine. Physical separation between the inner conductor and outer shielding is maintained by spacers, usually made out of tough solid plastics like nylon.

Triaxial cable also exists, in which a third layer of insulation and sheathing is included. This allows a nearly perfect signal which is both shielded and balanced/differential to pass through. Multi-conductor coaxial cable is also used sometimes.

Biaxial cable or biax is a figure-8 configuration of two 50 ohm coaxial cables, used in some proprietry computer networks.

Timeline[edit]

  • 1884 - Coaxial cable patented in Germany by Ernst Werner von Siemens, but with no known application. [unverified: more details needed]
  • 1894 - Waveguide transmission demonstrated to the Royal Institution by Oliver Lodge.
  • 1929 - First practical coaxial cable patented by Lloyd Espenschied and Herman Affel of AT&T's Bell Telephone Laboratories.
  • 1934 - First transmission of TV pictures on coaxial cable, from the Berlin Olympic Games to Leipzig.
  • 1936 - AT&T installs experimental coaxial TV cable between New York and Philadelphia.
  • 1936 - Coaxial cable laid by the Post Office (now BT) between London and Birmingham, providing 40 telephone channels. [Source: archives at http://www.bt.com]
  • 1941 - First commercial use in USA by AT&T, between Minneapolis, Minnesota and Stevens Point, Wisconsin. L1 system with capacity of one TV channel or 480 telephone circuits.
  • 1956 - First transatlantic coaxial cable laid, TAT-1.

Copper

Fiberoptics

Cat 5

Cable Configurations[edit]

Twisted Pair[edit]

Twisted pair cabling is a common form of wiring where two conductors are wound around each other for the purposes of cancelling out electromagnetic interference known as crosstalk. The number of twists per meter make up part of the specification for a given type of cable. The greater the number of twists, the more crosstalk is reduced.

Shielded twisted pair (STP) has an outer conductive casing similar to coaxial cable and theoretically offers the best protection from interference. It was commonly used for token ring networks.

UTP or unshielded twisted pair is not surrounded by shielding. It is the primary wire type for telephone usage and is very common for computer networking.

UTP is standardized into various categories by number, which indicate signal integrity attributes. Category 5 cable is commonly used for Ethernet with 10BASE-T or 100BASE-TX.

In telephone applications, UTP is often grouped into sets of 25 pairs according to a standard 25 pair color code originally developed by AT&T. A typical subset of these colors (white/blue, blue/white, white/orange, orange/white) shows up in most UTP cables.

Audio cables that are twisted are often referred to as balanced.

Connectors[edit]

Registered Jack (RJ) - Untwisted and Twisted pair

  1. RJ11 = 6 Phone Line
  2. RJ14 = 6 Double Phone lines
  3. RJ45 = 8 Ethernet

GCSE Science/Electricity


Everyone knows mains is potentially dangerous. There are however safety features included in plugs. This module looks at how to correctly wire a plug.

The three pin plug[edit]

Nowadays most appliances are sold with moulded plugs already fitted. Nevertheless it is still important to understand the correct wiring of a plug because enough of the old plugs still exist. You are very likely to need to change a plug at some time in your life. In the UK mains electricity is 230V. if you were to touch the live wire a current would flow through your body to the ground. This current may be enough to kill you.

The cable from the appliance usually consist of three wires. The wires are made of copper surrounded by a plastic sheath. The sheath is made of plastic and is coloured:

  • The live wire is brown
  • The neutral wire is blue
  • The earth wire is green & yellow

The three wires are covered by an outer sheath made of plastic.

Q1)Use you knowledge of insulators and conductors to explain

Why are the wires made of copper
Why are the sheaths made of plastic


Plug diagram.png

The plug has the following features:

  • A cable grip, to grip the cable and prevent it being pulled out of the plug
  • Three pins made of brass. One of which is the earth pin.
  • A fuse
  • A case made of plastic

Q2) Why are the pins made of brass and why is the case plastic?

The purpose of the parts of a plug[edit]

The live and neutral wires[edit]

They carry the current around the circuit. Mains current is AC this means that it is going backward and forwards in cycles. (clockwise and anticlockwise around the circuit). The frequency of the cycle is 50 times a second (50 Hertz) This cycling of current is achieved by varying the voltage on the live wire from +230V to -230V and back again. The voltage on the neutral wire does not vary. It stays close to zero ( hence the term neutral). In contrast the voltage of a battery does not cycle. It says constant. This is known as D.C. (direct current)

The live and neutral pins of a plug often have the half nearest the case surrounded by a plastic shroud. This is to prevent your fingers from touching them if you curl them around the plug as you push the plug into the socket.

Q3)Why is only the part nearest the cases sheathed. Why not the whole pin?

The earth wire[edit]

This wire is there to protect you . Many appliances have metal cases. For example kettles, toasters, dishwashers and washing machines. If the live wire were to become loose inside the appliance and touch the case the whole case would become live. If you were then to touch it a current would flow through you to the earth. The earth wire is just a wire connected to the case of the appliance. It goes down the flex into the socket. In side the wiring of your house it travels down to the earth. It is often connected to a metal plumbing pipe. If the live wire were to touch the case a huge current would flow through the earth wire. This would probably blow the fuse and break the circuit {see next section} but even if the fuse doesn't blow the current would still prefer to flow through a nice low resistance wire than a high resistance human body.

The earth pin on a plug is longer than the live and neutral pins. This ensures that the earth pin always connects with the socket first.

The fuse[edit]

A fuse is simply a very thin wire. The wire has a low melting point. As current flows through the wire it heats up. If too large a current flows it melts. This breaks the circuit. Fuses are used to protect appliance. If too large a current flows through an appliance it may damage it. It may even cause a fire!

Fuses are rated according to how much current they can carry before melting. In plugs fuses are usually 3A , 5A, or 13A. The correct fuse for an appliance is one that is just above the normal working current for that appliance.

Q4) A table lamp usually carries a current of 0.5 A what fuse should be put in the plug 3 A, 5A, or 13A?


Q5) An iron usually carries a current of 5.2 A what fuse should be put in the plug 3 A, 5 A, or 13 A?

Q6) A kettle is protected by an earth wire and a 13 A fuse. The live wire comes loose and touches the side of the kettle. The fuse blows. Explain why. The unit is designed to break the circuit if the current spikes above 13A.

Summary[edit]

The live wire is brown, and has AC voltage of 230 V
The neutral wire is blue and has AC voltage of approximately zero
The earth wire is green & yellow and is a safety device designed to protect people from shocks.
The fuse is a device that melts and breaks the circuit when the current gets too high
The case is made of plastic (an insulator) for safety