WIRING PRACTICES

Anyone who has attended a mobile robot competition has witnessed the entire gamut of designs, ranging from ugly "mobile rats nests" with wires dangling every which way, to designs employing custom made ciruit boards with modular connectors.

Aside from aesthetic considerations, neat wiring practices also greatly promote reliability, and in particular - debugging! Some practices worth investing in include:

If possible - try to use stranded wire! Many students use solid copper wire in their designs, as this is main type of hookup wire sold at Radio Shack! Stranded wire not only is much more flexible/pliable than solid wire, but is much easier to solder and much less prone to breakage. Soldering joints with solid core copper often results in "cold" solder joints with very poor continuity. Such joints are also prone to breakage.

Color coding - to facilitate debugging, obtain spools of stranded wire in many colors. It is customary to use RED wires for Vcc (e.g. +5V, +12V) power supply wires, and to use BLACK for ground wires. In addition, you may wish to, e.g., use YELLOW for signal wires, etc.. Just devise a consistent color coding scheme, and stick to it!

Avoid unnecessarily long wires! When routing power, e.g., from a battery to a microcontroller, consider using a twisted wire pair using color coded wires. Not only does the result look better, but the inductance of the wire pair will be decreased substantially. This in turn will generally reduce noise problems.

Be careful when routing power wires. Remember to use a wire size that is suitable for the current being carried! Wirewrap practices are ok for wiring up small prototype circuits, but one should consider breaking up their designs into functional modules, and eventually migrating their design onto custom made printed circuit boards. This will improve appearance and greatly enhance reliability. When designing such boards, board connectors should be integrated into the PCB design. Remember this- in a sloppily wired system that fails permenantly or intermittantly, the problem can most often be tracked down to an open or shorted connection!

Copper-Wire Table:

Wire Gauge	Diam.(mils)*	Ohms/1000 ft. 20 Deg. C	Current Cap.(Amps) @700 C.M./Amp
10	101.9	0.9989	14.8
12	80.8	1.588	9.33
14	64.1	2.525	5.87
16	50.8	4.016	3.69
18	40.3	6.385	2.32
20	32.0	10.15	1.46
22	25.3	16.14	0.918
24	20.1	25.67	0.577
26	15.9	40.81	0.363
28	12.6	64.9	0.228
30	10.0	103.2	0.144
32	8.0	164.1	0.090

*Note - a "mil" is 1/1000 of an inch.
Cross-sectional area may be calculated from the diameter(D) as: A = pD²/4 (mils²).

Resistance of a length of wire may be calculated as:
R = rL/A (Ohms) where r is the resistivity in ohm-mil²/foot, L is the
length in feet, and A is the cross-sectional area in mil².

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