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.
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.
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.
The use of fixed terminals means that -- when removing and re-installing a module -- you have to individually unscrew every terminal and then remember how to reconnect the wires. As a result, the trend is to pluggable connectors, but there are exceptions, especially when you are using connectors for very high currents like those in Figure 4.
Your choice of which pluggable connector to use is governed by the voltage and current of the signal. Often, connectors are interchangeable between manufacturers, sometimes down to the plug of one manufacturer fitting the socket of another. The highest capacity pluggable connector I know (Figure 5) is rated at 54 amps, but I don't imagine many of you are involved in that ballpark, so I will quickly move on, not the least that it also exceeds my comfort level by many orders of magnitude.
Weidmuller produces a series of pluggable connectors denoted as a BL/SL pair (BL for the socket; SL for the plug). There are four sets of spacings of the poles: 7.5mm, 5.08mm (0.2 inch), 5mm, and 3.5mm, with a large selection in the number of poles. Each manufacturer has variants on these, but the 5.08mm, 5mm, and 3.5mm seem to be common to all. Probably the most common pluggable connector is the 5.08mm, which -- I guess -- is in the sweet spot of the size-current-voltage tradeoff. As a result, there is a dizzying array of plugs (normally PCB mount) with variations in insertion angle (90°, 180°, and 135°), endplates, screw reinforcement, double-decker plugs, and even flying lead. The sockets can be just as varied, and some of the options of both plugs and sockets can be seen in Figure 6.
You can mix-and-match some plugs and connectors from Phoenix and Weidmuller, just make sure in advance. It is possible to polarize the connectors, but my experience is that the end result is finicky and flimsy, so I tend to vary the number of poles in each connector as a differentiator (you will see an example of this quite clearly in Figure 9).
As density has increased (and we will discuss this in a little while), the size of the screw hardware gets smaller, reaching its limit at about 3.5mm for plug-in connectors and 2.54mm for terminal blocks. This is leading to some growth in the market for tension camps and insulation displacement solutions. In the latter, forcing an insulated wire onto a bifurcated (look it up) pin pierces the insulation, also making for a quick connection. With the former, you use a small screwdriver or an affixed plastic lever to open a spring loaded terminal, insert the wire, and release making this a quick approach. There is one variation where you simply strip a solid core wire and push it into the tension clamp. Because of the BFS syndrome mentioned earlier in this blog, we have been known to add a suitably sized screwdriver to the BOM (bill of materials).
The next stage in density is the move to double-decker connections, like Weidmuller's B2L range as illustrated in Figure 8. The choices drop dramatically, only allowing for vertical or horizontal insertion angles. Also, the sockets are only available with tension-clamp terminations, although there is one variation with socket removal levers and another that allows you to screw the plug and socket together.
Phoenix have gone even smaller, down to 0.1 inch and 2.5mm in their COMBICON HD PTSM range, which employs a tension-clamp technique (Phoenix calls this a spring clamp) and solid wire. The problem with smaller connections is that the wire diameter, of necessity, becomes smaller as well. Also, the socket clamps become more suited to solid core wire, thereby losing some flexibility on the loom and causing increased problems because of breakage due to metal fatigue.
When you use a multi-stranded wire in a screw-tightened cable clamp, there is a tendency for the wire to work its way loose when the wire is flexed. The strands get rearranged, and the problem is compounded by an uneven distribution of the surface area of the wire. There is another issue when strands get caught during the insertion process and wave about in the breeze posing a danger of shorts or shocks. The preferred solution is to use a ferrule, which is a soft metal barrel (usually with some plastic insulation) that fits over the wire and is crimped. The resulting wire termination is then inserted into the socket housing (no loose ends) and screwed down. The soft metal deforms into the shape of the cable clamp, thereby making a connection that distributes the current more evenly and does not come apart. You can see a selection of ferrules and an associated crimping tool in Figure 10.
One more thing -- because of the mechanical strength required, most of the PCB plugs are through-hole technology (the smaller Combicon HD Phoenix range mentioned above are one of the "exceptions that prove the rule").
What PCB terminations do you use? Please post your comments and questions below. If you are aware of manufacturers other than those I mentioned in this article, links would be appreciated. And if you represent these other manufacturers, please feel free to chime in also.
