Which PCB connectors are best? - Embedded.com

Which PCB connectors are best?

At the outset of this blog, let me make full disclosure. I used to work for one of the companies that I mention in this article. Also, my current employer was spun off from, and still has a business arrangement with, that company. This arrangement has kept me occupied for 22 years. As a result, I have greater access to Weidmuller versions of the terminals I discuss than to others, so this blog is likely to appear biased in that regard. Although I will try and stay balanced, it must be said that the contents of this (and all my other blogs) are my opinions alone, and not those of my employers — current or previous.

It seems to me that the connection requirements for industrial electronics are different than the other market sectors, with the possible exception of hobbyist electronics. In the industrial market, integrators take a bunch of functional modules and combine them with Programmable Logic Controllers (PLCs) and sensors to make a complete system whose various modules are connected by long lengths of cables. Often, some portions of the system are installed long before the others and they have to be hooked up on site. Changes in specifications and plant upgrades also mean that there must be some flexibility in the interconnections.

Industrial products do not approach the high volumes of the automotive and consumer market places and are much less cost-sensitive, trading this for interface flexibility. Mostly, the whole industrial package is concentrated and housed in cabinets. They are typically mounted on rails, and the European TS35 and TS32 rails seem to me (sitting on the fringes of the market) to be the most dominant. There are many variations of terminal blocks that mount on these rails (I may write another blog on those in the future).

There are many approaches to flexible (by which I mean versatile) connections, but it seems to me the European offerings appear to be slowly taking over the market, despite established approaches like barrier strips in the North American industry. There are also other common approaches, such as the 0.1″ or 0.156″ headers made by Molex, Amp, and many others which require crimp tools. Then there are spade terminations and their ilk, and you may even remember the low-end Fahnestock clips.

The first issue in making field connections is the service personnel typically have only three tools, a BFS, a BFH, and a BFW (a screwdriver, a hammer and a wrench — the “B” stands for “big” and I leave the “F” to your imagination). The bigger the connectors used, the happier the installers are. The second issue is terminating the wire, which may also require specialized crimping tools. This is part of what the nascent industry (in the 1950s and 60s) was addressing, but — as we shall see — things have come full circle.

One of the problems is getting high density connections off a board and connected to the field wiring. An approach was to bring out connections from the PCB via a standard high density cable, and to provide interposing external terminals to connect to as you can see in Figure 1.

Figure 1. A 37-way D-subminiature to screw terminal adapter. The connectors are double stacked to improve the density, but are not pluggable. (Click here to see a larger image.)

Figure 1. A 37-way D-subminiature to screw terminal adapter. The connectors are double stacked to improve the density, but are not pluggable.
(Click here to see a larger image.)

Variations also allow for industry standard flat IDC (insulation displacement connector) cable to screw terminal. This kind of cable can be inflexible; it only comes in fixed lengths, and it needs specialized crimping/soldering equipment. Since it mounts horizontally on the rail, it also uses up rail space, which is often an issue as a result of under-design. The biggest issue is the isolation and current capabilities of the cables.

From my biased perch, the lion's share of the PCB screw terminal market appears to be owned by two German companies, which are, in no particular order, Weidmuller (Weidmueller in the USA) and Phoenix Contact (if you disagree, please feel free to contradict me in the comments below). There are a few more German companies (Wago and Weco, for example) and some US and Asian (try Degson Electronics) manufacturers as well.

It used to be easy to tell who made a terminal by its color. Weidmuller's offerings were orange or black, while and Phoenix's were green. More recently, Phoenix has introduced some in black, and there are now many mimics of the original shades. I should also mention that there are some higher-end housings that include connectors that are integral to the package as you can see in Figure 2.

Figure 2. A housing with specialized pluggable connectors. (Click here to see a larger image.)

Figure 2. A housing with specialized pluggable connectors.
(Click here to see a larger image.)

There is an ever-expanding range of PCB terminals. They started out with non-pluggable screw terminals (see Figure 1 and Figure 3), and today they can be purchased with different numbers of poles. Furthermore, some can even be joined together to make terminal blocks of any length.


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