Power transmission wires
Information transmission wires
Robots use electrical wiring for one of two purposes: transmission of power,
or transmission of information. Occasionally a line may serve both
purposes (e.g. USB cables) but generally the functionality is distinct.
There is good reason for this. Power transmission lines need to be sized
according to the amount of power they carry. If too small they can overheat,
melt, catch fire, and generally produce all sorts of mayhem.
Signal lines, on the other hand carry little power, and size is generally
not a critical issue. However nearby lines and components can produce
interference through electromagnetic coupling, so attention needs to be
paid to routing and shielding.
Electrical wire is usually made of copper. Very heavy gauge power lines are sometimes aluminum, and very fine signal wires connecting silicon chips to their packaging are sometimes gold (to resist corrosion), but the amateur robot constructor will deal almost exclusively with copper wire.
The gauge of a wire refers to the size of the conducting material inside. The most common system in the US is AWG, which refers to American Wire Guage. The larger the gauge number, the smaller the wire. The metric system specifies the cross-sectional area of the conductor in square mm. There are published tables for determining how much current a given gauge can carry in various situations. Exceeding these ratings creates a risk of fire or conductor failure, and should NEVER be allowed to happen. Preventing such occurrences is why power circuits always have fuses or circuit breakers.
Gauges of AWG 16 and lower (i.e. larger) are generally used for power transmission. Lamp cords are usually AWG 16 or 14. Household wiring ranges from AWG 14 to 10. Power lines may be AWG 0, 00, or larger. Small motors can often be powered by 18 down to 22 gauge wire. Smaller gauges are generally used for signal purposes, or for magnetic windings in motors, coils, or transformers.
Wire Gauge Table from Powerstream
Almost all commercial wire comes encased in some sort of insulation to
prevent electricity from going where it isn't wanted.
This is most often a plastic coating, often printed with information
about the wire. The most common material is flexible polyviny chloride,
but other materials such as high-density polyethylene, polyester,
rubber, and teflon are available for wire that
must tolerate abrasive or harsh chemical environments.
Coatings are available in a huge variety of
colors and color patterns to facilitate indentification.
Plastic insulation does not adhere strongly to the wire, and is easily
removed with wire strippers (or sometimes teeth or fingenails) when
connections need to be made.
The copper inside the insulation is sometimes coated with tin (shiny silver
appearance) which makes it easier to solder.
Electrical wire is classified as solid or stranded. Solid wire consists of a single metal conductor. It is robust, resistant to corrosion, easy to strip without damage, and holds its shape when bent. Stranded wire consists of several individual strands twisted together inside the insulation. Common numbers of strands are 7, 19, 26, and 34, though all sorts of numbers can be found. Stranded wire of a given gauge has the same total cross-sectional area (and thus current-carrying capacity) as solid wire of the same gauge, and is composed of several strands of smaller gauge wire. A specification such as 16 AWG 26/30 specifies a 16 gauge conductor composed of 26 strands of 30 gauge wire.
Stranded wire is more flexible than solid wire, and can be bent (within limits) without taking a set and thus without accumulating fatigue that would result in eventual failure. For a given gauge, the more strands the wire has, the more flexible it is. Wires that connect moving robot parts, circuit components that need to be repositioned moved during assembly or replacement, or that need to be moved themselves (e.g. machine power cords) should always be stranded. Stranded wire is more difficult to strip than solid wire - it is easy to accidently break strands when pulling off insulation. Losing one strand in a 7-strand wire is usually tolerable though not ideal. Any more, and you should redo it.
Single-conductor insulated wire whether solid or stranded is popularly known as hookup wire. It is available in gauges from AWG 26 to 10 (at least) and with insulation in many colors and patterns. There are no universal standards as to what colors mean. In household electrical wiring, black and red are used to indicate hot conductors, white to indicate neutral, and green (or bare copper) to indicate ground. However, these standards are not always followed, especially by amateur electricians, so never trust color in an unknown circuit unless you have traced the wire and understand its function completely.
So-called "magnet wire" is used for winding motors, transformers, and other magnetic circuits. It sometimes looks like bare copper, but it is covered with a very thin, very tough coating (often referred to as lacquer or enamel, but actually various highly engineered, multi-layer plastics), which can generally withstand hundreds (and sometime thousands) of volts, and temperatures near 100 (and sometime above 200) degrees C. The reason for the high tech is that the efficiency of motors and transformers depends on how densely wiring can be packed, so designers want as little volume as possible filled by insulation. Magnet wire insulation adheres very strongly to the metal, and cannot be removed with wire strippers. It can be removed by burning and washing with a solvent (alcohol or acetone) or by careful scraping or sanding (really only effective for larger sizes).
Wiring that contains multiple separately insulated
conductors is known generically as multi-conductor cable.
Products range from electrical power cable with 2 to 4 (or sometimes more)
conductors, to ribbon cables consisting of
several insulated signal conductors attached side by side,
to multiple twisted pairs of insulated
signal wires contained in a flexible, protective jacket along with foil
shielding and high-strength tension fibers to permit pulling through
walls or conduit without breaking.
Some of these cables are highly engineered to optimally resist the stresses
encountered during installation and use.
A familiar example is residential power cable, which contains
two or three insulated, usually solid, conductors plus a bare copper
ground, all inside an insulating sleeve.
Another is Cat 6 network cable, which contains 4 twisted pair signal channels
totaling 8 stranded wires, plus separating buffers and tension fibers.
Wire strippers and clippers
To function as intended, wires need good electrical attachment to each other and to the components they connect. They often need robust mechanical attachment as well. Problems with electrical systems are caused by poor or failed connections far more often than by poor or failed components. The aspiring robot constructor should make EVERY connection, temporary or not, using a reliable technique. Specifically, wires and electronic components should never be connected by twisting leads together.
As a general rule, you should try to minimize the number of connections in a circuit and in a robot. Every connection is a potential point of failure. This is particularly true for connections that are intended to be made and unmade repeatedly. That said, every robot is made of multiple electrical components that must be hooked together, and there are compelling reasons for using modular components that are easy to swap out or replace. So connections and connectors cannot be avoided.
The best way of connecting small to medium wires to components and to each other is soldering. A properly soldered connection is stable, mechanically strong, and introduces essentially zero additional resistance into a circuit. In fact, extended solder connections are sometimes used to reduce resistance in a circuit. Electrical solder should be solid or rosin-core, never acid core, which can cause corrosion of circuit elements.
Proper soldering procedure involves heating the connection with the point of a soldering iron (or gun for large connections), and melting the solder by touching it to the joint being soldered, NOT directly to the iron. Sometimes touching the solder to the angle between the iron and the metal is helpful. In order for the iron to make a good initial thermal connection to the joint, it needs to be "tinned", which means it is wetted with a thin film of liquid solder. A properly tinned tip is bright, silverly-shiny. The iron should be as hot as needed to make connections efficiently, but no hotter. The exact setting depends on the iron, the solder, and the nature of the connections being made, and usually requires some experimentation (or experience) to determine.
In a properly soldered joint, the solder "wets" the leads, displaying a concave surface where it meets underlying metal. Solder that "beads" up or displays convex interfaces indicates failure to connect. This is usually due to the metal being soldered not being hot enough before solder was applied, but sometimes it is caused by surface contamination, a dirty or untinned tip, or the wrong kind of solder. Larger wires need a higher power soldering tool because the additional metal conducts heat away more rapidly. A standard 40W electronic iron easily solders wires up to 22 gauge or so; 12 gauge power conductor needs a 200+ watt soldering gun.
Not every metal can be soldered. Tinned copper and factory-prepared printed circuit traces solder most easily. Clean, bare copper is also easy to solder, as long as you can get it hot enough. Tinned steel sheet and some brass can be soldered, but special techniques may be required. Bare iron and steel is difficult or impossible to solder with ordinary equipment, and aluminum and stainless cannot be soldered at all. Generally this is not a problem, since brass and steel are rarely used for electrical leads. Some large capacitors have aluminum connectors, and must be attached mechanically, as must aluminum wiring if you are unfortunate enough to encounter it.
The "assembly" chapter of this manual contains additional information about soldering.
wikiHow soldering tutorial
Various crimp connectors
Unfortunately, not every connection can be soldered. It is often necessary or desirable for components to be easily removable or replaceable, and unsoldering and resoldering connections is not easy. Connections involving large power conductors or aluminum wires are also not suitable for soldering.
A huge variety of specialty connectors is available for different applications. In many cases, connectors are provided in mating pairs, (male and female) which are attached by soldering or crimping procedures (special tool often needed) to the wires themselves. Mating pairs are engineered to meet certain specifications regarding electrical resistance, corrosion resistance, and number of times they can be connected and disconnected (tens to thousands). They may be made using special springy alloys (e.g. beryllium copper), and coated with gold, palladium, or other highly conductive, corrosion resistant metals.
Important: When male/female plug connectors are used in situations where there is a "hot" side to the circuit, the female (socket) connector is always used on that side. This is cruicial with high-voltage or high-amperage power circuits, where an exposed "hot" male prong would present a serious electrical shock and short-circuit fire hazard.
The following is a list of a few of the more common mechanical connector types.
Wirenuts are mostly used in household wiring of AWG 10 to 16. They are intended to be permanant, and should not be disconnected and reconnected more than a few times. They are also intended to be contained in an enclosure or junction box which provides additional protection. They bulky and ugly, and should probably be used in robots only for high-amperage power connections.
Wires under the moon
Wikipedia on copper wire
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