Category Archives: Guides

What is a Reed Relay?

A reed relay is basically a combination of a reed switch and a coil for creating a magnetic field. Users often add a diode for handling any back EMF from the coil, but this is optional. The entire arrangement is very low cost and a simple device to be manufactured.

The most complex construction in the reed relay is the reed switch. As the name suggests, the switch has two reed-shaped metal blades made of a ferromagnetic material. A glass envelope encloses the two blades, holding them in place facing each other, and providing a hermetic seal preventing entry of contaminants. Typically, reed switches have open contacts in a normal state, meaning the two metal blades do not touch when not energized.

The presence of a magnetic field along the axis of the reed switch induces the reeds to magnetize, which attracts them to each other. The reeds, therefore, bend to close the gap. If the applied field is strong enough, the blades bend to touch each other, thereby forming an electrical contact.

The only movement within the reed switch is the bending of the blades. The reed switch has no part that slides past another or pivot points. Therefore, it is safe to say the reed switch has no moving parts that may wear out mechanically. Moreover, an inert gas surrounds the contact area within the hermetically sealed glass tube. For high-voltage switches, a vacuum replaces the inert gas. With the switch area being enclosed against external contaminants, the reed switch has an exceptionally long working life.

The size of a reed switch is a design variable. In longer switches, in comparison with shorter switches, the reeds do not need to deflect much to close a given gap between the blades. To make the reeds in more miniature switches bend more easily, they need to be made of thinner material, and this has an impact on the switch’s current rating. However, small switches allow for more miniature reed relays, which are useful in tighter spaces. On the other hand, larger switches are mechanically more robust, can carry higher currents, and have a greater contact area (lower contact resistance).

A magnetic field, of adequate strength, is necessary to operate a reed relay. It is possible to operate a reed relay by bringing a permanent magnet close to it. However, in the field, a coil surrounding the reed relay typically generates the magnetic field. A control signal forces a current through the coil, which creates the axial magnetic field necessary for closing the reed contacts.

Different models of reed switches need different levels of the magnetic field to make them operate and close the contacts. Manufacturers specify this in ampere-turns or AT, which is the product of current flow and the number of turns in the coil. Therefore, there is a huge variation in the characteristics of the reed relays available. A higher voltage or power level is necessary for stiffer reed relays and those with larger contact gaps. These require higher AT levels to operate, as the coils require more power.

LDOs for Portables and Wearables

As electronic devices get increasingly smaller in form factor, they are also becoming more portable and relying more on battery power. These devices include security systems, fitness trackers, and Internet of Things or IoT devices. The design of such tiny devices demands high-efficiency power regulators that can make use of every milliwatt of power from each charge for extending the working life of the device. The efficiency of traditional linear regulators and switch-mode power regulators falls woefully short of the requirements. Moreover, transient voltages and noise in switch-mode power regulators are detrimental to their performance.

The most recent addition to switching and linear regulators is the LDO or the low-dropout voltage regulator. It lowers thermal dissipation while improving efficiency by operating with a very low voltage drop across the regulator. Low-to-medium power applications are well-served by various types of LDOs, as they are available in minuscule packages of 3 x 3 x 0.6 mm. In addition, there are LDOs with fixed or adjustable output voltages, including some versions with on-off control of the output.

A voltage regulator must maintain a constant output voltage even when the source or load voltages change. Traditional voltage regulator devices operate in one of two ways—linear or switched mode. While LDO regulators are linear regulators, they operate with a very low voltage difference between their output and input terminals. As with other linear voltage regulators, LDOs also function with feedback control.

This feedback control of the LDO functions via a resistive voltage divider that scales the output voltage. The scaled voltage enters an error amplifier that compares it to a reference voltage. The resulting output of the error amplifier drives the series pass element to maintain the output terminal with the desired voltage. The dropout voltage of the LDO is the difference between the input and output voltages, and this appears across the series pass element.

The series pass element of an LDO functions like a resistor whose value varies with the applied voltage from the error amplifier. LDO manufacturers use various devices for the series pass element. It can be a PMOS device, NMOS device, or a PNP bipolar transistor. While it is possible to drive into saturation the PMOS and PNP devices, the dropout voltage for PMOS-type FET devices depends on the drain-to-source on resistance. Although each of these devices has its own advantages and disadvantages, using PMOS devices for the series pass element has the lowest implementation cost. For instance, positive LDO regulators from Diodes Incorporated offer LDOs with PMOS pass devices featuring dropout voltages of about 300 mV, when their output voltage is 3.3 V and the load current is 1 A.

The output of the LDO must have an output capacitor. The inherent ESR or effective series resistance of the capacitor affects the stability of the circuit. That means the capacitor used must have an ESR of 10 ohms or lower for guaranteeing stability covering the entire operating temperature range. Typically, these capacitors are of the type multilayer ceramic, solid-state E-CAPs, or tantalum, with values upwards of 2.2 µF.

Thermal Interface Materials for Electronics

As the name suggests, TIMs are Thermal Interface Materials that the electronic industry typically uses between two mating surfaces. They help to conduct heat from one metal surface to another. TIMs are a great help in thermal management, especially when removing heat from a semiconductor device to a heat sink. By acting as a filler material between the two mating surfaces, TIMs improve the efficiency of the thermal management system.

There are various types of material that can act as TIMs, and there are important factors that designers must consider when selecting a specific material to act as a TIM for a unique application.

Every conductor has its own resistance which impedes the flow of electrical current through it. Impressing a voltage across a conductor starts the free electrons moving inside it. Moving electrons collide against other atomic particles within the conductor, giving rise to friction and thereby generating thermal energy or heat.

In electronic circuits, active devices or processing units like CPUs, TPUs, GPUs, and light-emitting diodes or LEDs generate copious amounts of heat when operating. Other passive devices like resistors and transformers also release high amounts of thermal energy. Increasing amounts of heat in components can lead to thermal runaway, ultimately leading to their failure or destruction.

Therefore, it is desirable to keep electronic components cool when operating, thereby ensuring better performance and reliability. This calls for thermal management to maintain the temperature of the device within its specified limits.

It is possible to use both passive and active cooling techniques for electronic components. It is typical for passive cooling methods to use natural conduction, convection, or radiation techniques for cooling down electronic devices. Active cooling methods, on the other hand, typically require the use of external energy for cooling down components or electronic devices.

Although active cooling can be more effective in comparison to passive cooling, it is more expensive to deploy. Using TIMs is an intermediate method to enhance the efficiency of passive cooling techniques, but without excessive expense.

Although the mating surfaces of the component and its heat sink may appear flat, in reality, they are not. They typically have tool marks and other imperfections such as pits and scratches. The presence of these imperfections prevents the two surfaces from forming close physical contact, leading to air filling the space between the two non-mating surfaces. Air, being a poor conductor of heat, introduces higher thermal resistance between the interfacing surfaces.

TIMs, being a soft material, fills a majority of the gaps between the mating surfaces, expelling the air from between them. In addition, TIMs have better thermal conductivity than air does, typically, 100 times better, and their use considerably improves the thermal management system. As such, many industrial and consumer electronic systems use TIMs widely for ensuring efficient heat dissipation and preventing electronic components from getting too hot.

The electronic industry uses different forms of TIMs. These can be thermal tapes, greases, gels, thermal adhesives, dielectric pads, or PCMs that change their phase. The industry also uses more advanced materials such as pyrolytic graphite, as these are thermally anisotropic.

New MEMS Switches Accelerate Testing

If you are using processor ICs from Advanced Digital, testing them may be costly and logistically challenging. This is because testing these ICs requires isolated DC parametric test equipment, including high-speed digital ATE or automatic testing equipment to assuring the quality. New MEMS switch technology from ADI, working at 34 GHz, offers both DC and high-speed digital testing, despite having a small form factor in the form of a 5x4x0.9 mm LGA package. They reduce the test costs and simplify the logistics necessary for testing RF/digital SoCs or systems on chips.

There are many high-speed chips on the market. These include high-density inter-chip communications for advanced processors. Such advanced processors are the norm for 5G modems, computer graphics systems, and other central processing units. Therefore, ATE designers constantly face the increase in demand and complexity for throughput while assuring quality. For instance, the greatest challenge comes from the increasing number of transmitter/receiver channels, and these require both DC parametric and high-speed digital testing. Not only does this increase the testing time, but it also increases the complexity of the load board, while reducing the test throughput. In turn, this drives up operational expenses, while reducing the productivity of modern ATE environments.

One way of solving such ATE challenges requires a switch that not only operates at DC conditions but also at high frequencies. The new MEMS switch ADGM1001 from ADI, while passing true 0 Hz DC signals, can also operate equally effectively at high-speed signals up to 64 Gbps. Therefore, testing with these new switches requires only one insertion for an efficient single test platform. It is possible to configure the test platform for both DC parameter testing and standards for high-speed digital communications.

High-volume manufacturing requiring HSIO or high-speed input-output testing is often a challenge. Testing strategies typically employ a high-speed test architecture as a common approach for validating HSIO interfaces. Such test equipment typically incorporates two test paths in one configuration—one for DC tests, and the other for high-speed tests.

Testers employ a few methods for performing tests at both DC and high speed on HSIOs or digital SoCs. They may use relays, MEMS switches, or different load boards—one for DC testing, and the other for high-speed testing, but this requires two insertions.

Use of relays for DC and high-speed testing can be challenging. This is primarily due to relays being unable to operate beyond 8 GHz. Therefore, users must compromise on test coverage and signal speed. Moreover, relays take up large areas on PCBs on account of their larger size, and this makes the load boards rather large. Another concern with relays is their limited life and reliability. Relays typically only last for about 10 million cycles, thereby limiting the lifetime and system uptime of the load board.

With its superior density and small form factor, the 34 GHz MEMS switch from ADI offers both DC testing and high-speed digital testing capabilities, overcoming the above challenges.

Batteries and Supercapacitors

In the past, only mission-critical devices had them. Now, a wide range of electronic applications demands backup power solutions. These applications include consumer, commercial, and industrial end-products. Of the several options available, the most energy-dense solution is that offered by supercapacitors, acting as energy reservoirs during interruptions of the main supply. Typically, this occurs during an outage of the mains power, or during swapping out batteries.

Although they are versatile, supercapacitors present challenges in design. This is due to their capacity to provide only 2.7 VDC. Potentially, this means adding multiple supercapacitors, along with the necessary cell-balancing circuitry, and voltage converters for step-up and step-down for supplying regulated power to the power rail operating at 5VDC. The solution is a nuanced and complex circuit, which not only takes up excessive board space but is also relatively expensive.

Comparing them with batteries can explain why supercapacitors offer many technical advantages for compact, low-voltage electronic applications. Supercapacitors help in designing simple, elegant solutions for powering a rail operating at 5VDC using only a single capacitor in combination with a buck/boost reversible voltage converter.

Modern electronic devices often need uninterruptible power as a critical element to provide a satisfactory user experience. The absence of a constant power source can not only stop the electronic product from operating, but it can also lead to vital information loss as well. For instance, a personal computer operating from mains power will lose the information contained in its volatile RAM during a power outage. Similarly, important blood glucose readings in the volatile memory of an insulin pump may be lost while replacing its batteries.

It is possible to prevent this from happening by including a backup battery. Not only will the battery store energy, but it can also release it during the failure of the main source of power. Currently, devices typically use lithium-ion batteries, as these are mature technology, offering very good energy density. This allows relatively compact devices to offer considerable backup power for relatively extended periods.

Irrespective of their base chemistries, batteries offer distinctive problematic characteristics under specific circumstances. Not only are they relatively heavy, but they also take relatively long times to recharge, which may be problematic in areas with frequent power outages. Moreover, it is possible to recharge the cells only a limited number of times, thereby increasing maintenance costs. In addition, batteries often include chemicals that can introduce environmental and safety hazards.

The supercapacitor, or ultracapacitor, offers an alternative solution. Technically, the supercapacitor is a capacitor with an electric double layer. Manufacturers construct supercapacitors using electrochemically stable, symmetric positive and negative carbon electrodes. They separate the electrodes by an ion-permeable separator that is insulating and use a container that they fill with an organic salt/solvent electrolyte.

Supercapacitor manufacturers design the electrolyte to maximize electrode wetting and iconic conductivity. The combination of the minuscule charge separation and high surface area of activated carbon electrodes results in the very high capacitance of the supercapacitor, as compared to the capacitance of regular capacitors.

The reliance on electrostatic mechanisms to store energy makes the electrical performance of supercapacitors more predictable than those of batteries.

Electronically Commuted Motors — Higher Efficiency

Restaurant owners have long been facing operational challenges. These include high energy costs, limited kitchen space, and equipment downtime. For addressing these challenges and improving restaurant productivity, the owners have turned to commercial kitchen equipment. Most of such kitchen equipment has an electric motor at heart, whose performance dramatically impacts how the equipment operates and how it mitigates the above challenges.

It is imperative that owners increase their productivity while reducing their costs, considering their profit margin usually falls between three and five percent. This requires a clear understanding of the connection between the motor and the equipment. Doing so not only reduces the operating costs but also ensures a smoother running operation.

Energy costs happen to be a major concern in the restaurant industry. Commercial kitchen equipment is uncommonly hard on the electricity bill, being typically robust and energy-intensive. According to the US Energy Information Administration, consumption in restaurants is typically three times more per square foot than any other comparative commercial enterprise. This is because restaurants use specialized equipment that has a high power demand, and they operate for extensive hours, thereby consuming huge amounts of energy.

Therefore, purchasing and using high-efficiency, higher energy star-rated restaurant equipment is one of the easiest ways to improve the bottom line. However, as a motor is at the heart of each piece of equipment, it offers a greater choice. In fact, restaurant operators can improve on this further by taking a proactive approach and selecting equipment that has an electronically commuted motor or ECM. They can even consider retrofitting existing equipment with ECMs for a more favorable option.

The reason for the above decision is that an ECM operates more efficiently as compared to what a traditional induction motor does when running restaurant equipment such as ovens, walk-in coolers, mixers, and fryers. Depending on the use cycle, equipment with ECM technology can save more than 30% in annual energy costs. This improves the bottom-line savings and improves the profitability of a restaurant.

A microprocessor and electronic control help to run an ECM. Compared to regular induction motors, this arrangement offers higher electrical efficiency. It also offers the possibility of programming the precise speed of the motor. Moreover, ECMs can maintain high efficiency across a wide range of operational speeds.

Apart from the higher efficiency, ECMs are precise and offer variable speeds, which in fans means an unlimited selection of airflow. A properly maintained airflow during changes in the static air pressure brings important benefits to the restaurant, especially for its hood exhausts and walk-in coolers. The higher efficiency of ECMs leads to reduced heat in the refrigerated space, thereby reducing the equipment runtime.

Forward-thinking original equipment manufacturers are re-engineering their designs and products to include ECMs for delivering smaller and more versatile equipment. Compact motors such as ECMs, are gaining wider recognition and appreciation as they improve the power density of their equipment. Compared to equipment with traditional induction motors, those using ECMs offer the same output, but with a much smaller footprint and lower weight.

Industrial Automation with Single-Pair Ethernet

Efficiency is the fundamental concern for the successful implementation of any factory automation solution. For this, it is necessary to implement control and power components that consume the least possible amount of energy over their lifetime. However, for the actual realization of those savings, it is necessary for proper installation of the system.

This is where the advantages of the SPE or Single Pair Ethernet technology really come across. The technology transfers power and data over the same thin-wire cable. Not only does this save installation costs up-front, but it takes much less to maintain and upgrade the system over time. Phoenix Contact offers their ONEPAIR series for standardized SPE solutions. The ONEPAIR series has two main types of connectors, and they each serve a specific application.

In numerous industries and fields, the IP20 connectors and patch cables enable effective data transmission. This includes building and factory automation, where it is common to achieve a transmission rate of 1 Gbps for a distance of 1000 meters.

The other is the M8 device connectors, rated at IP67. They can transmit power and data safely and quickly from the OT to the IT. This is a new standard in compact connections, which can withstand harsh environments.

SPE or single-power Ethernet is high-performance, parallel transmission of power and data via Ethernet over a single pair of wires. The technology typically carries data and power through PoDL or Power over Data Line starting from the sensor and carrying through right up to the cloud. For barrier-free networking of a wide range of connectors, cables, and components, it is necessary to deploy connectors with standardized pin patterns. For this, Phoenix Contact offers standard connectors, ranging from IP20 to IP6x.

Apart from being ideally suited for a wide range of applications, the SPE is the basis for all Ethernet-based communication. Not only does it enable smart device communication, but it also opens up newer fields of application. SPE has great transmission properties, can span long distances, and optimally supports future-proof network communications. With a trend for miniaturized, resource-conserving devices, SPE offers space-saving cables and electronics.

SPE brings many benefits to its users. It can provide transmission speeds of over 10 Gbps over a single pair of wires. This helps to reduce data cabling while avoiding media breakdowns and device failures, from the field to the cloud. The user has the freedom to establish networking with a consistent structure base of Ethernet, eliminating the need for gateways. With SPE, the cabling is easier and saves time, as the user needs to guide and connect only two wires. They can use the 10Base-TIL standard Ethernet cabling for ranges up to 1000 meters.

The IEEE 802.3 defines the SPE standards. Presently, there are five standards for different transmission speeds and distances. Further standards are under discussion. The IP20 compact male connector series from Phoenix Contact are in accordance with IEC 63171-2 and are ideally suited for building and control cabinet cabling. The M8 or IP67 contacts from Phoenix Contact are in accordance with IEC 63171-5, providing robust and industrial-grade connections.

What is I3C Interface Communication

I2C is a popular serial communication protocol, with I3C being an improved version. Embedded systems use this new protocol for achieving significantly higher data throughput and features that are more advanced than what I2C offers. Designers and engineers can use I3C for improving the functioning and performance of their designs while adding more features such as in-band interrupts, hot-join, and high data rate modes. With I3C being backward compatible, it can communicate with legacy targets using the present I2C protocol.

There are some major differences between I3C and I2C. While I2C works on bus speeds of 100 kHz, 400 kHz, or 1 MHz, I3C operates with bus speeds up to 12.5 MHz. The increase is due to I3C using push-pull outputs, which switch between push-pull drivers and open-drain outputs depending on the state of the bus. I3C uses open-drain driving during arbitration or initial addressing where multiple targets are controlling the line at the same time. I3C uses the push-pull driver for unidirectional communication, and no other device is expected to communicate simultaneously.

The voltage range of operation of I2C is between 3.3 and 5 VDC, and I3C operates with supply voltages of 1.2, 1.8, and 3.3 VDC, with the possibility of other voltages in between. Unlike 12C, I3C does not require external pull-up resistors, as the main controller on the bus provides these.

I2C uses static 7-bit and 10-bit addressing of target devices. On the other hand, I3C makes use of dynamic 7-bit addressing, where the active controller designates each target with an unambiguous address to prevent collisions with addressing. In contrast, I2C requires the designer to keep track of the current addresses to prevent assigning the same address to two or more devices. I3C assigns addresses dynamically during bus initialization.

I2C has no mechanism for a target to tell the controller that data is ready unless it uses an extra IO line. However, devices in I3C can signal an interrupt by using the serial data and serial clock lines, thereby making the protocol truly two-wire. I3C also uses this in-band signaling for implementing hot-join functionality. This allows new devices to join once the initial address assignment is over.

I2C allows multi-controller buses. Here, although multiple devices can operate as controllers, only one of them can actively communicate at a time. On the other hand, I3C can have only one active controller, while other capable devices can request to become active controllers on the bus. This device can then become the secondary controller. If the secondary controller is no longer acting as an active controller, it starts functioning as an I3C target.

I3C is backward compatible with I2C. However, for successful communication, the targets in the I2C protocol must have a 7-bit address, and must not use clock stretching. The new protocol suggests the I2C targets contain 50ns filters on their inputs. By meeting these requirements, I2C targets become compatible with the I3C bus. On the other hand, a few I3C devices may also operate as I2C targets, until they have been assigned a dynamic address. When working in the I2C mode, the I3C devices have static communication addresses.

Micro 3D Printing for Miniaturization

Engineers have been using additive manufacturing for prototyping for about 30 years now and are also using it for production. However, the biggest value addition from additive manufacturing comes from producing parts that other traditional manufacturing methods find difficult.

Fabricators use additive manufacturing as a valuable and important solution for producing parts such as those including complex design features like internal geometries and cavities that are impossible to achieve by regular machining. Additive manufacturing is helpful in producing structural elements that are too cumbersome or difficult to generate effectively by conventional means.

At present, engineers use 3D printers for printing large parts quickly. These parts may have resolutions around 50 µm and tolerances around 100 µm. However, sometimes, they also need to produce parts with sub-micron resolutions that are smaller than 5 um. Therefore, they needed a system for printing micro-sized parts at a reasonably high print speed.

Smaller parts require a more precise production process. For instance, cell phones and tablets, microfluidic devices for medical pumps, cardiovascular stents, MEMS, industrial sensors, and edge technology components require connectors with high resolution and accuracy. Most standard additive manufacturing machines cannot provide the resolution necessary for micro-sized parts.

BMF or Boston Micro Fabrication designs and manufactures the PµSL or Projection Micro Stereolithography technology-based printers. Using PµSL printers, it is possible to create 3D printed parts with 2 µm resolution at ±10 um scales. These 3D printers incorporate the benefits of both the SLA or stereolithography technologies and the DLP or digital light processing technologies.

Using a flash of ultraviolet light at microscale resolutions, these PµSL printers cause a rapid photopolymerization of an entire layer of resin. This takes place at ultra-high precision, accuracy, and resolution, not possible to achieve with other technologies.

For faster processing, the PµSL technology supports continuous exposure. Other design elements allow additional benefits to the user. For instance, in printers using the standard SLA technology, the bottom-up build method requires a support structure to hold the part to the base, while also supporting the overhanging structures. Conventional SLA systems can typically achieve resolutions of 50 µm, an overall tolerance of ±100 µm, and a minimum feature size of 150 µm. Similarly, standard DLP systems using a similar bottom-up build structure offer 25-50 µm resolution, an overall tolerance of ±75 µm, and a minimum feature size of 50-100 µm.

On the other hand, the PµSL uses a top-down build, thereby minimizing the need for a support structure. It also provides a way to reduce damage while removing bubbles with a transparent membrane. Comparatively, PµSL systems offer resolution down to 2 µm, dimensional tolerances as high as ±10 µm, and minimum feature sizes of 10 µm.

BMF provides this type of quality by properly employing every system component. This includes the resolution of the optics, controlling the exposure and resulting curing, the precision of mechanical components, and the interaction between parts and required support structures. It also depends on the ability to control tolerances across the build and the overall size of the part. Moreover, working with such diverse micro parts requires choosing the right material characteristics.

Using Ferrites in Wire Assemblies

The phenomenon of magnetism is prevalent all over the world, along with related concepts like the magnetic field, electromagnetism, and electromotive force. Although these are complex subjects at a higher level, they are easy to understand. However, these are principles on which electric motors operate, the earth’s magnetosphere shields life, and refrigerator doors remain closed.

The wonderful properties of magnetism also help products and applications like cable assemblies. There are well-known magnets like those made of neodymium, and these are permanent magnets with inherent magnetic properties. They comprise elements of Neodymium, Boron, and Iron. Neodymium magnets are among the most powerful permanent magnet types available. In comparison, there are non-permanent magnets also. Typically known as electromagnets, they derive their properties from the passage of an electrical current.

Other types of permanent magnets are also available. The most popular of these is the ferrite magnets, and industries use them for a lesser-known reason. Used in various forms like chokes, cores, and beads, these inexpensive devices greatly help filter electrical noise and get products to comply with EMI/EMC regulations. Countless design applications use them in different form factors and are available from numerous manufacturers. Ferrite magnets comprise a mixture of iron oxide and ceramic magnets. In doughnut-like shapes, they keep control over signal integrity within bundles of wire. For instance, a data cable carrying high-frequency data transmission,  when routed through the magnetic field of a ferrite, can eliminate unwanted electrical noise, as the ferrite acts as a passive EMI filter.

For a ferrite to be effective, the cable must pass through the center of the ferrite and its magnetic field. Looping and routing the wire multiple times through the ferrite helps incrementally improve the signal integrity. While a majority of cables have their wires passing through the ferrites only once, some designs require them to make as many as three loops to meet design objectives. Typically, there are two types of ferrites available that are suitable for cable assemblies—snap-on ferrites and doughnut ferrites.

Snap-on ferrites are the easiest to assemble. These are passive suppression devices with two halves. A plastic clamshell case holds the two halves as it snaps close around the wire. Available in a wide variety of sizes for different cable diameters and performance types, these are excellent devices that can mix and match various types of ferrite to help pass an aggressive test requirement. However, snap-on ferrites can be expensive and require accurate sizing to match the wire’s outer diameter to create an interference fit. As their design is like a clamshell, it is easy to remove snap-on ferrites.

Doughnut ferrites are simpler, being in the shape of a ring or a doughnut. The cable must pass through the center of the continuous circle of the ferrite before the wires terminate into a connector. The doughnut ferrite is therefore a permanent fixture, unlike the snap-on ferrite that the user can remove at any time. Overmolding the ferrite helps to fix its position on the cable while protecting the brittle ferrite magnet from damage.