Pluggable terminal blocks
(21 232)Pluggable terminal blocks are used in many industries, e.g. in electrical engineering, automatics, mechatronics, and in countless electrical and electronic devices. They are commonly applied in microcontroller development boards, PLCs (programmable logic controllers), DAQ cards, and all kinds of drivers and similar devices. Often, you will find yourself using a socket, where a specific plug can be used with a pluggable terminal block that has corresponding pin headers. The possibility of choosing between these two solutions results from the different needs of different users. A plug with an integrated cable will be suitable for the final user, e.g. someone using the device, however, if you need to test a device, it is easier to use the terminal block, so that you can plug or switch the signal cables easily and quickly, without having to solder them to the corresponding pins.
There are different variants of pluggable terminal blocks (also known as electrical terminal blocks) and which one you choose can sometimes be very important, not only because of the size, number of ways, mechanical or electrical mounting, but also because of the manner of connecting the wires or spatial orientation.
Pluggable terminal blocks – parameters
One of the most important parameters when choosing the correct terminal block is the number of ways. This parameter is correlated to the electrical connections of the system or device, and it also depends on whether you want to lead the specific bundle of wires straight to the corresponding inputs/outputs, or to use fewer plugs and combine signal cables into a single wire bundle. The latter solution is definitely going to be faster when it comes to the final mounting, however, it can’t always be used, e.g. due to the signal interference. If you decide to go with the first solution, you must remember to use different terminal blocks, different configuration of the ways, or at least different colours. This will help you avoid accidental incorrect connection of the signals, which, in turn, is definitely going to result in malfunction of the device, or even in damaging the components. The number of ways for pluggable terminal blocks can vary from two up to thirty inputs, with different configurations, such as: single-row, e.g., 1x2, 1x24, double-row, e.g., 2x2 or 2x16, etc. The most common colours of the terminal blocks are black, grey, blue, orange, and green.
Electrical terminal blocks – mounting
The choice of the manner of mechanical mounting is just as important. The most common electrical terminal blocks are those that are mounted on PCBs (printed circuit boards), soldered to the dedicated holes. They usually have the shape of sockets, where the plugs are the so called “cable-mounted” terminal blocks/strips. Terminal blocks for panel mounting (with extra brackets, mounting holes or fasteners that make it possible to attach the block e.g. to the housing of the device), screw-mounted ones (with integrated fastening screws), those with mounting holes, and also with snap fasteners (that protect the plug from unplugging) are also available.
As mentioned before, these terminal blocks can be found as plugs and sockets, however, they are also available in the shape of pin headers, which are very similar to goldpins, which are very popular with electronics hobbyists.
We can also divide the pluggable terminal blocks according to the manner of electrical mounting. First of all, we can distinguish screw terminal blocks. The end of the wire is placed in the socket of the terminal block, and then a small screw is tightened in order to ensure proper connection, so that the wire doesn’t disconnect or doesn’t get accidentally pulled out. The optimum way is to have an insulated bootlace ferrule clamped on the uninsulated end of the wire. The bootlace ferrules protect multicore cables from splitting, reduce the danger of short-circuit, and make it possible to use the entire cross-section of the cable for conducting of current. Cage clamp is another type of screw terminal; however, spring clamp can also be used instead. It has the same functionality, but its use is way faster. The end of the wire should be plugged into the socket, and if you need to remove it, you just have to use a small, flat screwdriver to press the lever that will release the clamp. Many terminal blocks can be soldered directly to the PCB using THT (Through Hole Technology Mounting), but you can also find many that are going to be mechanically mounted, e.g. in the housing and that have pins to which the wires can be soldered. IDC and snap fastener terminal blocks are also available, and others that are usually variants of aforementioned solutions.
If you are going to connect only low-power signal cables to the pluggable terminal block, its rated voltage parameter, which is measured in volts, will almost always be retained. You should be particularly cautious if you are going to plug in cables that will conduct high voltage current (even if it’s just for a moment). If that’s the case, you need to pick a terminal block that can provide suitable level of insulation, which can range from 100V up to 1kV. Another related parameter is the current rating that the specific terminal block will be able to conduct through its pin headers safely, without the risk of the build-up of excessive heat that could lead to the burning of the terminal block. The current rating ranges from 4A to more than 70A.
Another important aspect of the mechanical compatibility that should be considered is the connectors’ pitch. This term is used to describe the distance between the axes of the electronic components’ pins, i.e. integrated circuits and pluggable terminal blocks, among others. The usual available values of the connectors’ pitch are: 2.5mm, 2.54mm, 3.5mm, 3.81mm, 5mm, 5.08mm, 7mm, 7.5mm, 7.62mm, 10.16mm, or 12mm. As you can notice, rounded values such as 2.5mm, and 5mm are available, but also not so obvious ones, such as 2.54mm, and 5.08mm. It stems from the use of the metric system (the SI) along with the imperial system (that’s commonly used in the UK and the USA), where the unit of measurement is an inch. So, 2.54mm equals to 1/10 of an inch, which is 100mils (a unit commonly used by electricians, e.g. while designing printed circuits), and 5.08mm equals to 1/5 of an inch.
Pluggable terminal blocks – characteristics
The most popular solution in the field of spatial orientation are 90° angled terminal blocks and straight ones. Vertical and 45° angled terminal blocks are also available. The choice will depend on the design of the device, but also on the housing, and space available for the wires, not to forget the individual preferences of the user or the engineer.
Pluggable terminal blocks – choice
Even though the pluggable terminal blocks usually don’t receive a lot of attention, they have some characteristics that should be considered when deciding which one to buy. The most important ones were mentioned above, but the components can be also picked according to the housing material (e.g. nylon 66, PA66, ABS, LCP, PBT, polyamide), connectors’ pitch, flammability class, compliance with standards, connector material (e.g., brass, bronze, copper), operating temperature, and many other parameters.
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