SUPPLEMENT ENCLOSURES & FITTINGS
HOW TO SELECT THE CORRECT PCB TERMINAL BLOCK
CamdenBossdiscusses the main requirements users need to consider when selecting the best type of terminal block to use and describes some simple methods to help navigate through the choices
O
ne of the simplest yet most effective and flexible methods of connecting
wires to a printed circuit board (PCB) is the humble terminal block. There are, however, an apparently bewildering number of types and options making selection of the correct solution a seemingly confusing process. The parameters of any application vary
widely but, principally, the choices come down to the following: Fixed terminal or connector (two-part) Wiring clamping method Access to wiring (position of cable entry and screw)
Current rating and cable entry size of the terminal
Voltage rating and pitch between poles Approvals (UL, VDE etc.) Cost
The first task is to choose the type of
terminal block, either a fixed block or a two-part connector. For applications with a large number of wires terminating on a PCB, the question of repair or replacement of a failed assembly will be a factor. The increased cost of a two-part connector compared to a fixed terminal is then weighed against the shorter down time of the equipment as the use of connectors greatly reduces a repair technician’s time. In addition, the connector option virtually eliminates any possibility of the board being wired incorrectly. PCB terminal blocks also feature choices in the method of clamping the wires. The simplest and lowest cost is the wire protector type. These have a copper alloy or steel leaf spring between the tip of the clamping screw and the installed wire. This, as its name suggests, prevents the screw from cutting into or, indeed, breaking through the wire. (Fig 1). The main disadvantage of this type is that
the wire protector can deform when the screw is tightened. This can sometimes cause the wire to jam in the block when the screw is released. When the wire is then removed, the wire protector can be pulled out of the block. For this reason, these blocks should be regarded as having a very limited number of wire insertions. The other disadvantage is that, if the screw is over- tightened, the torque can cause the insert to
S4 SEPTEMBER 2016 | ELECTRONICS
rotate within the moulding. This can cause a cracked solder joint that can often remain hidden. To avoid such difficulties and for a little extra cost, a rising clamp type can be used. This mechanism, as its name implies, has a box clamp that rises as the screw is tightened. (Fig 2). This method gives an increased number of
Figure 1:
insertions, if required, and greater mechanical strength as the screw torque is not directly applied to the pin. A further option is the screwless block. These have the advantage of being able to be wired at speed as no tool is required. Usually used with solid core wires, a screwless terminal block clamps the wire by opening a spring gate which then closes on the wire preventing its removal. The gate is opened by either pushing the wire in or using an actuator on the block. To remove the wire, the actuator is pushed again or, if no actuator is present, a screwdriver is pushed into the actuator slot. (Fig 3). Screwless terminals are typically used in the
Figure 2:
lighting industry where high speed wiring is the norm due to the high product volumes. All the above types are commonly vertical
screw (or actuator) with a horizontal cable entry. Additional options are vertical or 45o cable entry. Terminal blocks are usually available as 2- and 3-pole interlocking types or as a solid block. Connector type terminal blocks are usually
Figure 3:
rising clamp but screwless types are gaining in popularity. The PCB headers usually feature a shrouded pin. These are the highest cost solution although wire protector versions offer a lower cost alternative. The most popular type is shown in figure 4. This is a horizontal cable entry socket with a horizontal header. There is a simple friction latch on both mouldings which assists in keeping the assembly together.
Figure 4:
Other options available are vertical cable entry parts in two different orientations of cable entry, vertical headers, headers with or without end walls, and pin cable entry parts with socket headers. For higher density circuits, two row headers are available. Screw flange types offer a solution in environments where vibration may cause the connectors to separate.
GETTING TO GRIPS WITH APPROVALS, CABLE ENTRY SIZES AND RATINGS Current rating and cable entry size are linked. It may appear confusing that, depending on differing approvals, the same part can have two different ratings. This is because the approval bodies, principally UL and VDE, have different test methods and parameters. As an example, the UL rating of a part may be 14A, 30-12 AWG with VDE being 24A, 0.5-2.5mm2. The voltage rating and pitch (distance
between adjacent poles) are similarly linked. For example, with the part above, the 5mm pitch version has a UL rating of 300V and a VDE rating of 250V. The 7.5mm pitch version of the same part has a UL rating of 300V (no change) but the VDE rating is 750V. As a rule of thumb, for lower voltages much closer pitches can be used. It is obviously important to select the correct rating and size for the application so users need to ensure that all electrical specifications are taken into consideration. If there is any doubt, suppliers like CamdenBoss are ready to help out with advice on all aspects of selecting and specifying PCB terminal blocks. CamdenBoss manufacture and supply PCB
Terminal Blocks, Terminal Blocks, Rising Clamp Terminal Blocks, Wire Protector Terminal Blocks, 3.5mm Pitch Terminal Blocks, 3.81mm Pitch Terminal Blocks, 5mm Pitch Terminal Blocks, 5.08mm Pitch Terminal Blocks, 6.35mm Pitch Terminal Blocks, 7.5mm Pitch Terminal Blocks, 7.62mm Pitch Terminal Blocks, 9.52mm Pitch Terminal Blocks, 10mm Pitch Terminal Blocks and 10.16mm Pitch Terminal Blocks.
CamdenBoss
www.camdenboss.com T: 01638 716101
/ ELECTRONICS
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