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Eye On Electronics

Motor, Mar 2007 by Dale, Mike

Good connections are essential to proper electrical component function. You can do your part by selecting quality wire terminals and making professional crimps.

While location, location, location is important in real estate, it's connection, connection, connection that's important in automotive electrical systems. Most electronic devices are really very reliable. Statistics show that it's most often in the connections and the wiring where the trouble starts.

Connectors, terminals and wiring don't often get the respect they deserve. Alter all, how complicated could they be? Truth is, they're both simple and quite complicated. They don't look like much, but how you handle them and repair them is directly related to just how reliable they'll be.

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The AMP Corp. defines connector as "a device that provides a separable connection between two elements of an electronic system without unacceptable signal distortion or power loss." AMP has a lot it can tell us about how connections are supposed to work in its paper "Connector design/materials and reliability" posted on the company's website (www.amp.com). We'll give yon the gist of it here; check out the website if you want to dig deeper.

Automotive electrical connectors have several purposes. The first is that, purely from a practical standpoint, you couldn't build a vehicle without them. Wire harnesses and electrical components are serviced by installing or removing them in pieces. The use of connectors allows easy access for repair, easier installation of the wire harness during vehicle assembly, replacement and upgrading of snhassemblies and the possibility to access connections for diagnostic purposes.

Let's start with the basic wire used in todays vehicles. It's almost always stranded fine copper wire coated with insulation. The stranded wire has a gentle twist of one or two turns per foot, to allow for easier coating during the insulation process. It also adds to the ability of the wire to resist flexing, since it makes the wire act as a bundle rather than individual strands.

The most common materials used to cover the wire are PVC, irradiated PVC and crosslinked polyester. The insulation holds the wires together and insulates it from chassis ground. Almost as important, the insulation limits mechanical movement of the wire and prevents water and other contaminants from attacking the metal of the wire.

Differences in insulation are related to how much heat the wire can stand. Both the irradiated PVC and cross-linked polyester insulations can withstand temperatures in excess of 105°C. The plain PVC insulation is rated at 85°C or below.

Wire sizes are based on a couple of factors. One is the amount of current the wire needs to cany. The larger the cross section of the wire, the lower the resistance losses will be, and the higher the current-carrying capacity will be. The second is cost. The heavier the wire and the finer the stranding, the more it costs to make.

There's a limit to how small a wire can be. Even though computer signal wires don't carry much juice, if made too small, they become mechanically weak and subject to vibration or other types of damage, especially in automotive applications. Ribbon cables like those inside PCs bundle many wires together to give greater mechanical strength to very thin wires.

The wire itself can be plain copper or copper with tin or other coatings. The overcoating of insulation can create tiny spaces between the strands of wire and the insulation that can have a capillary effect, meaning that the wires can actually suck up water or flux inside the insulation itself. To prevent this, some makers impregnate the wire with anticapillary gel materials or plate it with tin so the wire cannot absorb flux from solder baths or saltwater that could result in corrosion beneath the insulation.

Wire terminals have two ends, one interfacing with the wire, the other with the mating terminal. Using a 1Ain. spade lug as an example, the wire side is a round ferrule. One of its purposes is to capture all of the wire strands. Stray wires not inside the ferrule are essentially not hooked up and detract from the current- or information-handling capacity of the wire. They also risk creating shorts to adjacent wires or terminals.

A properly shaped crimp is vital to the quality and reliability of a wireside connection. The goal of a good crimp is to form a mechanically solid connection that prevents any movement of the wire inside the ferrule. This also creates a line-on-line condition that forces the strands against each other and the terminal surfaces and minimizes contamination. The soft tin coating on the ferrule improves this contact interface.

The key to making a good crimp is to have a good, electrician-grade crimping tool. The jaws must be the same width as the ferrule and have the right curvature to fold the crimp back on itself. A proper crimp is like two letter Cs nested one inside another, with the wire caught between. One of the Cs is the original outside shape of the ferrule; the second is the opposite side of the ferrule formed backwards.

Automotive Industry

Eye On Electronics

Motor, Mar 2007 by Dale, Mike

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