Resistivity (Mod.3 Sect.2 Part 2) 1 of 2

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Caption: : The group of wires is attached to the terminal in a way that allows for temperature change.

From this information, you know that copper wire will expand when its temperature rises. Also, it will contract when its temperature is decreased. Although the amount of wire expansion and contraction may be very small, it’s important that you know about it. One application of this information is shown in Figure 16. It shows a number of wires attached to a terminal board. Observe that the wires aren’t stretched tightly between the cable and the connection. Wire layout such as this is sometimes called wire dressing. The loops in the wires are called stress reliefs. They allow for a change in the length of the wire with changes in temperature.

The Effects of Temperature on Wire

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Caption: : These are examples of stress relief when connecting resistors between terminals.

Wire Stress Relief

In both Figure A and B, a resistor is connected between two terminals by soldering. Here you can see that in Figure A, a tight connection isn’t used. Tight connections don’t allow for changes in dimension with changes in temperature. In the correct mounting, observe that the resistor is against the surface of the metal chassis, and its leads show a stress relief. That’s a requirement in many applications. When extreme temperature changes are a functional consideration, you should use this practice for mounting all electronic parts.

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Copper Wire vs. Aluminum Wire

Caption: : When copper wire is attached to a screw terminal, it com presses. The aluminum wire doesn’t compress, so the connection isn’t a good one.

When a material is malleable it’s capable of being easily shaped. this figure shows a piece of solid-copper conducting wire being connected to a screw terminal. When the screw is tightened, the wire becomes flat at the contact point. That makes a good connection. The same illustration shows a screw tightened onto aluminum wire. Aluminum wire is not compressible (malleable), so, it doesn’t become flat. This is not a good connection.

Fiber Optics

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Caption: : Construction of a Fiber-Optic Cable

This figure shows the construction of a fiber-optical cable. Two fiber optic strands can handle 672 two-way conversations. Compare this with two strands of copper wire that can carry only 24 two-way conversations. Most fiber-optic cables are made from very small threads of glass. (In a few applications, plastic threads can be used.) Glass is a very poor conductor of electricity, so power companies can’t deliver electrical energy to your house through fiber-optic cables. Fiber-optic cables do conduct light. They can be thought of as being light pipes. A transmitter turns the light ON and OFF in a code, and the fiber-optic cables carry information and conversations.

Wire Groups

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Caption: : Wire groups or bundles are often laced or tied together for ease of maintenance and inspection.

Wire groups or bundles are often used to carry signals and power be tween electronic boards and systems. These wire groups are usually laced or tied together for ease of installation, maintenance, and inspection. This figure shows several methods of tying wires together. In Figure A, lacing cord is used to make spot ties. In the spot-tying process, a group or bundle of wires is secured together with individual ties. The ties are spaced at intervals of 12 inches or less. Spot tying is often used when wiring panels. Figure B shows the cable lacing method. In cable lacing, a group or bundle of wires is secured together with a continuous cord that forms loops at regular intervals. The loops are also made at each point where a wire or wire group branches off. Figure C shows some different plastic ties and clips that are used to hold wires together and also to hold them to the panel. Prefabricated wire groups called harnesses are also available from some manufacturers.

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Computer Cables

Caption: : isn't this interesting

This figure shows some examples of plugs and sockets used for computer cables. The cables are connected to the peripherals (the other components in the computer system) and the computer with plugs and sockets. A few examples of input cables to the computer from peripherals are Keyboards that are used for typing information into the computer Scanners that deliver information to the computer Disk drives that deliver information stored on the disk There are also other, more sophisticated peripherals that can be connected to a computer by cables. A few examples of the output part of the system connected to the computer by cables include Color display monitors Printers Plotters Overhead projection

Coaxial Cables

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Caption: : Construction of a Coaxial Cable

This figure shows an important type of cable used in electronics. It’s called coaxial cable, or shielded cable and it has two conductors. The outer conductor is a shield made with braided wires. The inner conductor is a solid-copper wire. Some versions have two inner conductors. When the coaxial cable is connected between two points, the outer shield is grounded at one end. The shield is not grounded at both ends because a slight difference in potential between the grounding points would result in current flowing through the shield. If the slight difference in potential is AC, there will be an expanding and contracting magnetic field generated in the shield. That will produce an unwanted current in the center conductor.

Coaxial Cables (2)

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Caption: : This illustration shows how the shield is separated from the center conductor.

It’s sometimes necessary for a technician to attach connectors and/or probes to a coaxial cable or other shielded cable. The first step is to separate the shield from the center conductor. Then, the shield wire and the inner conductor are soldered to the connector or to special terminals. The shield and the inner conductor must never be allowed to touch each other. You’ll find coaxial cables and shielded wires in systems that operate at very high frequencies. They’re also used for connecting meter and oscilloscope probes. Shielded wires have many applications in audio systems. They’re an important part of many applications in electronic systems.

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Ribbon Cables

Caption: : Shown here is an example of a ribbon cable.

In cables that don’t have plugs or receptacles, the wires that make up the cable may have colored tracers. Tracers are colored threads that are visible when the insulation is removed. In other cases the wires have colored stripes or combinations of stripes on the insulation. Ribbon cables, like the one shown in this figure, are usually made with colored wire or wire with colored stripes. That makes it easy to perform a continuity test. It’s important to know that some ribbon cables have terminations called breakaway pins. They’re designed in such a way that they can be purposely snapped off. That allows the cable system to be keyed. In other words, by snapping off unused pins, the terminations can be mated only one way.