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How Terminal Block Contacts Work

Quick-connect type terminal block contacts consist of a flat blade or a simple tab with a design that accepts a push-on connector holding the end of a wire. A force-fit metal sleeve pushed over the tab makes the contact. Such quick-connect type terminal blocks are meant for thin wires up to AWG 12.

Tubular contacts are a length of rectangular metal tubing, with screws threaded through the top of the tube at both ends. In tubular screw contacts, the flat bottom of the screws secures the inserted wires by providing pressure on them. Tubular clamp contacts use a pressure plate between the screws and the wire to apply pressure on the wires. The screws usually hold the pressure plates captive, which makes tubular clamp contacts useful for fine stranded wires.

Feed-through contacts have mounting surfaces with studs going through them. Feed-through contacts are useful for wire leads passing through a wall under a block directly or very close to the wall. Stud contacts or strap-screws secure the wires for connection.

For installations that must stay connected even during shock and vibration, dead-front connectors with two-part plug-ins are very useful. For multiple points of contact on posts, quality connectors typically use socket-mating springs. Wire protectors made of beryllium-copper protect both single and multi-stranded wire terminations. Flush-mount designs for boards are best for minimizing stress on solder joints when tightening screw terminals.

Flat metal strap with screws through each end make up strap-clamp contacts. The screw heads usually have a wire-clamping element under them for exerting pressure on the wires. These contacts require the insertion of bare wires within the pressure contact.

Strap-screw contacts are similar in construction to the strap-clamp contacts but without the wire-clamping elements. The user simply loops the wires over the screws or may attach a ring or spade lug on them. Tightening the screws on the loop, ring, or spade lug serves to secure and connect them.

Fuse blocks usually consist of a fuse in series with a circuit. Usually, there are contacts at each end, like that of standard blocks. It is possible to insert a cartridge-fuse pug into clips connected to each contact. Apart from circuit identification, this arrangement facilitates easy fuse changing.

Plane and rigid insulating members can mount multiple one-piece blocks for connecting one or more circuits. There may be open barriers facilitating easy contact accessibility. Other variants may offer contact protection through closed barriers or dad fronts. A single base may hold standard units of 2, 4, 6, 8, 10, or 12 circuits.

Terminal block contacts may also be of the short-circuit type, like the one-piece blocks. Short-circuit contacts require a screw short-circuiting two strips, allowing current flow into the desired circuit by direct connection or by shunting out other circuits.

Section blocks usually come with individual molded units containing contacts. It is possible to form any desired number of circuits by assembling them together with end barriers in between. Formation of preassembled lengths requires snapping off or adding sections in the desired number of groups of contact sections to form a sectional terminal block assembly.

How do Surge Trap SPDs Work?

Surge trap Surge Protection Devices or SPDs are protection devices to absorb high-energy power spikes that could damage sensitive electronic equipment such as process controllers, instrumentation, and computers. They divert high-energy power away from an appliance by providing a low-impedance path to the common point earth ground. Frequently, panel boards use several metal oxide varistors or MOVs, connected in parallel, to act as surge suppressors or traps.

AC surge suppressors most commonly use an MOV comprising solid-state zinc oxide having multiple junctions. When conducting, MOVs offer a low impedance path, and come in packages for specific voltage and current handling capacities. Surge suppression devices found in DC applications mostly are single junction diodes and/or gas discharge tubes that ionize at preset voltages.

Installation of most AC surge traps are typically at the entrance to a utility service for protecting the entire facility, in distribution switchboards and panel boards for the protection of sensitive loads downstream, and/or in wall outlets for protecting an individual and specific piece of equipment such as a solid-state controller or a computer.

NEMA standards define the surge current capacity of a surge trap as the maximum level of current it can withstand for single transient event. The level indicates the protection capacity of the surge suppressor.

The suppressed voltage rating (SVR) or the clamping voltage of a surge trap is the voltage it permits passing on to the attached load during a transient event. The ability of the surge trap to attenuate a transient is its performance measurement and the clamping voltage provides this. The Underwriters Laboratories or UL confirms the clamping voltage during tests it conducts while evaluating surge traps.

The short circuit rating and the surge current capacity of a surge trap are the criteria a user should consider while selecting a device for its performance and safety. The user should make sure the surge device they have selected is not fuse limited, as many manufacturers use fuse limiting in front of the device for passing UL testing conditions.

Installing a surge trap SPD is always in parallel with the load. The surge trap SPD remains idle and does not conduct when the operating voltage is within the normal levels. The SPD turns on whenever the system experiences an overvoltage and conducts the extra current to the ground, while allowing the load to experience the correct voltage. The performance of a pressure relief valve in a steam system offers an operational similarity.

It is easy to retrofit an existing panel with a surge trap SPD, provided the panel has adequate space. Typical control panels in industries have a mains disconnect feeding a power distribution block, which then connects to individual loads. Users can mount the surge trap SPDs on the standard 35-mm DIN-rail that the panels typically use.

Manufacturers recommend mounting surge trap SPDs as close as possible to the power distribution block with #8-#14 AWG wires, not exceeding a length of 20 inches. Users must make sure of not twisting any wires together, and of not forming any loops, as these can result in higher voltages that the SPD let through.

ElectroSmash Pedal for the Raspberry Pi

Guitarists favor expensive gear. For instance, they hold online discussions about the best types of wire for guitar pickups. They even go to great lengths while selecting the type of transistors that will give them the best fuzz tone. They hold extensive discussions about the merits of the pentode rectifier over the tetrode type. While the geeks in the electronics world share several common characteristics with the guitar geeks, the ElectroSmash Pedal Pi would interest both.

Both teams are already familiar with the single board computer, the Raspberry Pi (RBPi). ElectroSmash provides a pedal that works with the RBPi Zero and allows the user to program the effects. The brains behind the project are in the code that the user has to download and compile on the RBPi.

Although it is possible to write the code afresh, but downloading the sample provided by ElectroSmash is more sensible, and gets you started faster. The community behind the Pedal Pi has contributed the code, and the user has the complete freedom to use it as it is, or to modify the parameters. ElectroSmash provides the Pedal Pi in a kit form, which means owners have to assemble it first.

Instructions for the assembly are available from the ElectroSmash website. The kit comes with all components neatly labeled, which makes the kit easy and straightforward to put together. One does not need extensive soldering experience for the assembly.

The kit has two ICs, the first an op-amp, and the other an analog to digital converter chip. Follow the instructions on the ElectroSmash site to place them on the board the right way around.

Typically, the RBPi Zero comes with the header pins not soldered to the board, and the user has to do the placement and soldering. However, one can get around this problem by using the RBPi ZWH variant, as this board comes with the header pins soldered in place.

Once you have assembled the pedal, you may find it is not as robust as the regular guitar pedals available on the market. According to ElectroSmash, the aim of the Pedal Pi project is to offer learning about guitar pedals and having fun with them. As an electronics kit, the ElectroSmash Pedal Pi kit certainly lives up to its claim.

Although the kit may seem slightly expensive, comparing it with other guitar pedals shows its true value. For instance, the distortion pedal from Ibanez, the classic Tube Screamer, costs almost twice the full kit. Although the ElectroSmash kit has about ten other effects built in, the user can add many more—in fact, only the programmer’s ingenuity, imagination, and programming skills limit the range of effects that the kit can handle.

Following the code sample that ElectroSmash provides is simplicity itself. They list the code sample in order of increasing complexity, ranging from the simple tone to the looping effect. The user can have fun playing with different types of distortion and use a processed quality on the fuzz, bit-crusher effect, and distortion. The effects are all available in the file fuzz.c and one can change a few numbers to give a new effect.

Which are Better – Round Cables or Flat Cables?

Both types of cables are available in the market—round ones and flats, and people use them according to the requirements of the application. As round cables were the first to arrive on the market, the industry has been using them as standard for long, in applications ranging from renewable energy to automation and manufacturing in general.

Flat cables arrived late on the scene, and offer a niche solution presently. However, they are gaining ground steadily for applications within the civil-aircraft markets, semiconductor industry, medical field, and for supplying data and power to machines. Flat cables are also called festoon cables, and the overhead crane companies actively use them for applications where winding cables around spools is difficult.

Comparison of Electrical Performance

The protection for internal EMI depends heavily on the construction of the cable. In general, flat cables do not transfer data very well. Individual shielded pairs within flat cables are necessary to provide coupling and crosstalk protection from pair to pair.

Most shielding materials to not hold a flat format and tends to become round. This makes it difficult to place a shield on the flat cable overall. This also makes it difficult to protect and shield a flat cable from the effects of external EMI. The naturally round shielding tendency provides greater protection against influences of external EMI on round cables.

The length of a cable, its quality of insulation, and the resistance of its conductors determines the voltage drop or attenuation on a power cable and this is immaterial whether the cable is round or flat. In both cases, higher quality of insulation and proper positioning of the ground wire improves the attenuation. Certain industries demand very high-performance (low attenuation and crosstalk) flat cables. With proper shielding, it is possible to transmit both power and signals through the same cable.

Comparison of Mechanical Performance

Cables in the industry face mechanical stresses of four main types—S-bend, rolling flex, tic-toc, and torsion. The natural capability of being able to move in multiple axes at the same time makes round cables capable of withstanding all the stresses. For instance, round cables can flex 30 million times in certain applications. On the other hand, flat cables can withstand only rolling flex, as the movement is only in one linear axis.

Movements in several axes such as during torsion can lead to flat cables binding, or twisting beyond a certain point. When under torsional loads, flat cables can spool and twist over a certain length. Preventing this requires every component of a flat cable to be integrated at the right position and twist. It also requires the cable to be embedded or wrapped with a PTFE (Teflon) tape for minimizing the frictional forces during torsion.


Round cables can maximally utilize the space inside the smallest required cross-sectional area. Drilling a round hole is easier than cutting a rectangle. Therefore, most machine or panel openings use round cables where using a flat cable may be more difficult, as it has an elongated cross-section. However, it is possible to stack flat cables to make them fit together in a smaller space than it is with round cables.

How do you select a Tactile Switch?

We find tactile switches almost everywhere – on keyboards, on mice, beside the monitor, on TV sets, on set-top boxes, on toys and on mobile phones. These tiny switches give a distinctive feeling when pressed. We are so used to using tactile switches; we press them a dozen times a day and never think twice about them – that is, as long as they work. However, tactile switches can also stop working, and engineers must select tactile switches with great care so they last long. After all, most feel that a bad or nonfunctioning switch equals a bad device.

Therefore, to avoid the possibility of a quality black eye, you must essentially select the right switch. Deciding what it is that exactly makes a tactile switch right of the job, may depend on a host of factors, of which two are most important. One is the actuation force and deflection characteristics necessary to meet the requirements of the application. The other is the reliability with which the switch must work during the life of the host electronic gadget.

Thinking of switches as commodity items selected straight off a datasheet, is an expensive mistake that many engineers do make. In reality, picking a durable switch with the right feel does require somewhat more than a mere glance at its specifications. Here is what you should be looking for.

Click ratio

The click ratio of a switch expresses the relationship of its actuation and contact forces. A higher click ratio is indicative of a snappier or crisper switch feel. The deflection or travel distance of a pressed switch also contributes to its overall feel.

A typical datasheet holds the force and travel specifications and these can be a starting point for selecting a switch that feels just right in its intended application. However, the ideal switch depends on the application – an important thing to remember.

For example, users of portable consumer electronic devices prefer crisp tactile switches that have a relatively high click ratio and shorter travel distances. On the other hand, tactile switches for the automotive industry need lower click ratios and longer travel distances. This prevents accidental actuation while driving. Therefore, each electronic application needs to reach a unique balance between the travel distance and the actuation forces.


Consumer electronics and medical applications need tactile switches that are protected against ingress of liquids and other contaminants – IP 67. Usually, these sealed tactile switches reach their maximum lifecycle, because of the sealing.

Manufacturers have traditionally used a bonded silicone membrane to seal the innards of a tactile switch. Now, technologically improved IP67 rated tactile switches use a patented laser welding process that seals the switch with a thin nylon film. This goes over the actuator rather than under it, giving a better seal. The seal not only preserves the crisp feel, but also protects the switch against side loads.


Protecting the switch with the nylon film improves its inherent reliability by not allowing ingress of contaminants. The best switches will typically offer a life expectancy of above one million press-and-release cycles.