Medical Magnetics

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Winatic Medical Magnetics

Magnetics are classified as a passive electronic component and are comprised of either inductorscoils, or chokes. They are manufactured by winding a copper wire, called magnet wire, around some type of core. These core materials can be made of plastic, iron, or even air itself. The latter is called a self supporting coil or air coil.

These type components are used extensively in all types of medical equipment, from defibrillators to X-ray equipment. As you might imagine, the integrity of any component used in any type of medical device has to be flawless. So, for that reason high reliability components are required in the manufacture of these types of instruments.

All these parts used in medical applications need to be inspected 100%. Whereas, an AQL or sampling method may be adequate for consumer electronics, it will not be stringent enough for the medical equipment industry, where every application can be quite literally a life or death situation.

Winatic Corp. is a company that has been in the magnetics business for well over forty years and specializes in these type of high reliability components. They are currently working on their ISO 13485 certification which was specifically created for the medical device industry. They will be approved within the next twelve months.

There are many changes waiting in the wings for the medical industry that may or may not transpire. One thing that will not change, however, is the integrity of the machines that monitor our health and the reliability of the components within these devices.

Passive Electronic Components

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Variety of Film Caps by Arizona Capacitors

The term Passive electronic components is a broad category. They are used in all types of electronic circuitry, but are often overlooked in their importance to the operation of the circuit. The passive electronic component stands ready to perform in a circuit when provided an external voltage or current stimulus. The active component, on the other hand, is often part of the power supply providing the stimulus for the passive circuit. Semiconductors are probably the most commonly used active electronic component.

Passive components, like resistors and transformers, are often constructed using resistance wire for resistors and magnet wire for transformers. Various metal foils are often used in wound film capacitor construction.

Another construction method used for metal film resistors and tantalum capacitors is to deposit either metal or dielectric film using vacuum thin film sputtering techniques. Vacuum sputtering is a precise and accurate means of construction. No wire or foil windings are necessary with this type of resistor or capacitor construction.

When ceramic substrates are used and RF or microwave frequency performance is desired, thick film construction of the metal conductors becomes a cost effective method for products like attenuators and terminations.

Thin film construction, while more costly, provides the best performance for RF/microwave resistors since the thin film morphology can be made very homogeneous. The created homogeneous resistor surface then distributes the applied power very uniformly eliminating hot spots that can often lead to poor MTBF’s and early product failures. Since thin film resistors are created using optical masking techniques, very small and accurate resistors can be constructed.

Sputtering Techniques

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Sputtering Machine

There are essentially three different ways to sputter material onto a substrate. One is called Direct Current (DC) Sputtering, the second is called Radio Frequency (RF) Sputtering, and the third is referred to as Reactive Sputtering.

Sputtering is an arc process where there is a thin film that is deposited by etching a material from a source onto a substrate. The sputtered atoms are ejected from a target. It is often referred to as thin-film deposition. This process is used extensively in the manufacture of both active and passive electronic components.

Direct Current (DC) Sputtering is the simplest sputtering process. It cannot be used to deposit dielectric or inorganic materials like oxides, but it can be used to deposit almost any metallic material.

Radio Frequency (RF) Sputtering is more adaptable than DC Sputtering. It is not limited to electrically conductive targets like DC Sputtering. It can use targets such as silicon oxides and polymers.

These sputtering processes are commonly used to manufacture thin film resistors and rod resistors.Both of these resistors operate at the RF to microwave frequencies.

The Reactive Sputtering is the most complex of the three processes and is where a reactive gas is used along with inert argon to form a plasma. The reactive gas becomes activated and chemically combines with target atoms to form a compound. Two widely used reactive gases are oxygen and nitrogen. This process is used widely for the deposition of dielectrics, resistors, and semiconductors.

Three Types of Capacitors

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Various Types of Caps by Arizona Capacitors

Capacitors fall into a category of electronics called passive electronic components and they are the second most common component in the average circuit, next to resistors. There are various types of capacitors available to the design engineer, depending upon what he needs.

There are three different types or families of capacitors that will be discussed in this Hub; wound film capacitors, tantalum & electrolytic capacitors, and ultra-capacitors. Only the first one of these is wound. All of the others are manufactured by laying down a film via sputtering or some other method. Whereas, the wound film capacitor is produced via winding a metal film onto a core, as the name implies.

The tantalum and electrolytic capacitors have their own niches, such as higher frequency applications. The wound film capacitor, on the other hand, has been the workhorse for the electronics industry for a number of years, this is because, in no small part, of its reliability. There are a number of different types of wound film capacitors. These are mostly categorized by the type of film they are wound upon.

There are many wound film capacitors. The polycarbonate capacitor is a typical example of a wound film capacitor. It is often used in the instrumentation, filtering, and switching power supplies fields. They are also preferred for many applications because of their precision and stability.

The tantalum capacitor is a typical electrolytic capacitor. It is one that is used extensively in audio applications. It is manufactured from a tantalum powder. The dielectric is thin making a high capacitance achievable with a relatively small size, but they can also be more expensive as well. The electrolytic capacitor is one that used an electrolyte as one of its plates. They are able to create the highest capacitance’s of any capacitors. The other type of electrolytic capacitor is aluminium.

Lastly, we have the ultra-capacitor or super-capacitor. It is an electromechanical capacitor that has a high density level. They are also know as electric double layered capacitors (EDLC) and they can have capacitance up to 5,000 farads. They do not have the dielectric found in most other capacitors, but have a double layer of separation giving them a rating for higher voltages than the other more common types of capacitors.

Types of Transformers

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Various I-Tech Transformers

There are six different types of transformers we will address in this hub and none of them will be related to a toy or a movie. Transformers are a type of passive electronic component that are used to step up or step down the voltage in a circuit or system.

The voltage transformer is similar to the more common power transformer. It is also referred to as a potential transformer and is used for metering and protection in high voltage circuits. It is designed in order to achieve an accurate voltage ratio over the range of the load and is often used to step up low voltages or to step down high ones.

The power transformer is the most commonly used transformer. One thing to remember is that a transformer does not actually create power, but transfers it from one coil winding to another. The power transformer is actually a type of voltage transformer and it is used in many different power type applications. Some of these transformers are immersed in oil in order to keep them cooler and stop them from overheating.

The current transformer is also known as a series transformer. It is often placed in series with a high current circuit. It is used for the measurement of electric circuits and they are often used in metering and protective relays.

As the name implies, the impedance transformer was designed to ensure accurate impedance transformation. For example, a transformer may be used to “match” the impedance of an amplifier to a speaker. In fact, they are often used in low-frequency amplifiers.

The Isolation transformer has no direct connection between windings, but they are connected via the magnetic flux in the core. In most cases, the winding ratio is a one to one. In other words, it is neither a step up, nor a step down but a means of isolating the circuit from the power supply.

Lastly, the auto transformer in unique in its tapped windings. The primary is normal, but the secondary has at least three taps where electrical connections can be made and these different taps result in different voltages. They are often used in applications where it is needed to interconnect systems operating at different voltages.

Oil-Filled Capacitors

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Arizona Capacitors- Oil-Filled Cap
Source: Arizona Capacitors, Inc

Oil-filled capacitors are generally high power and/or high voltage. They are often of the wound film capacitor variety and their size is usually fairly large because of the energy they can store.

The oil in the capacitor has more than one purpose. Firstly, it is used to remove any air voids and secondly it helps to cool or remove the heat away from the capacitor windings. These windings tend to generate an excessive amount of heat when they reach their discharge capacity. The oil works to offset these higher temperatures so that the capacitor can remain working optimally.

For metalized film capacitors, the oil has another benefit, in that it helps prevent arcing from between the two plates. If, however, arcing were to occur the oil would reseal the hole caused by the arcing. This is why, even though oil-filled metalized film capacitors are not a very common application, these type capacitors are often referred to as self healing capacitors.

The type of oil used is not what one would find in a automobile engine or the lawn mower sitting in the garage. Arizona Capacitors, Inc., for example, uses a mineral oil in most cases, but a silicon oil may be used as well. Both of these oils are chosen because they are non-hazardous to people and the environment, but the mineral oil is the most economical of the two.

Prior to the mid 1970’s some of the oil-filled capacitors contained PCBs (polychlorinated biphenyls). These were desirable because of their stability and low flammability. The United States Congress banned all domestic production of PCB material in 1979.

Rod Resistors

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Res-net Microwave Rod Resistors

Rod resistors are a cylindrical type resistor that are used in both RF and microwave applications. They are thin film in nature and can be made of alumina, beryllium oxide, or aluminum nitride.

Because of their thin film construction they can achieve very high frequencies. These frequencies can be as high as 26.5 GHz or even higher in certain applications. Res-net Microwave Inc. is one of the few companies that offer rod resistors in alumina, beryllium oxide, and aluminum nitride materials.

The resistance range for rod resistors is generally from 10 ohms up to 500 ohms, and the resistive tolerances are generally 1%, 2%, or 5%. The temperature range is -65 Degrees Celsius to +175 Degrees Celsius. The resistors are covered with a protective coating of high temperature epoxy in order to insulate them from these temperature extremes.

In addition to these classic thin film rod resistors, they can also be supplied using pyrolytic carbon. These carbon rod resistors also perform very well at microwave frequencies and have the added advantage of being able to work to the temperature of +200 Degrees Celsius. The sizes of the carbon resistors mimic those for the other thin film types. These carbon rod resistors will have a negative temperature coefficient as compared to the other resistors which will have either a plus or minus 100 ppm TC.

The power ratings can go from 0.050 watts all the way up to 75 watts, depending upon the material used. They are used extensively in the RF and microwave industries especially in the manufacture of other components such as, SMA terminations.

Carbon Film Resistors

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Vamistor Carbon Film Resistors
Source: Tepro of Florida, Inc.

Carbon film resistors are designed for both high temperature and high voltage applications. They are actually made in a vacuum by breaking down hydrocarbon gases at high temperatures. This in turn forms a carbon deposit on the substrate. The process is accomplished by heating methane or propane gas in a process called high temperature pyrolysis. When the heat is applied, molecular condensation results and hydrogen is released. The end result is carbon.

The main advantage and reason for using the carbon film resistor is that these components are designed to withstand high voltages as well as high temperatures. The highest voltage the devices can operate up to is 15kV with a nominal temperature of 350°C. They are available with tolerances of 2 percent, 5 percent, 10 percent, and 20 percent. Cutting a helical grove in the carbon film creates the resistive tolerances. The desired resistance value is achieved by regulating the pitch of the helix. The thinner the carbon layer, the finer the pitch and the higher the resistance value. After the helixing or spiraling is completed special alloy contact caps and tinned electrolytic copper connecting wires are pressed onto the ends of the resistor body. The final step involves coating several layers of tan lacquer or using a glass film as a shield. This is done primarily for electrical protection, but also for shielding from the climate.

Carbon film resistors are a good choice because they have a small size for such high ratings and they have a wide resistance range as well. The resistance can be anywhere from 500 Ohms to 100M Ohms. Another plus is that they have no outgassing. This means they will not deposit any vapor into the environment during operation. Outgassing is a condition that causes high material vapor pressure in the device to emit contamination deposits into the air. It corresponds directly to the temperature.

These deposits can cause problems with the surrounding equipment. Some of the other advantages are that they have less stray capacitance and inductance, so they are better at high frequencies. Also, they have a high stability of performance when compared to carbon composition resistors and, of course, they are flame proof.

Electrical noise is another factor when choosing a resistor. There are two main types of electrical noise, thermal and shot. Thermal noise is the product of the Brownian motion of ionized molecules. This noise cannot be eliminated because it is fundamental to resistance.

Carbon film resistors have less of this thermal noise than the carbon composition resistor. The lower values tend to be noise free while increasing with the higher values. Shot noise usually results from the flow of electrons through a highly charged field. It is more prevalent in solid state devices. It is not significant in carbon film resistors and until recently was not even known to exist, with respect to resistors.

There are also some disadvantages when using carbon film resistors. In the first place, they are limited to about 1 percent accuracy. Secondly, they exhibit drift with temperature and vibration. The resistors can have a TCR (temperature coefficient of resistance) range of -250 to -1000 ppm/°C, depending on the resistance value. The advantages, however, seem to outweigh most of the disadvantages, especially in specific applications.

Surface Mount Components

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Chip Resistors
Source: Res-net Microwave – Res-net Microwave, Inc.

Electronic surface mount components are attached directly to the printed circuit board (PCB). They are also known as SMT components (surface mount technology) or SMD (surface mount devices). These components can be either active or passive electronic components. Some of the types of components made in these surface mount configurations include: capacitors, inductors, resistors, semiconductors, and thermistors,

The actual technology was developed in the 1960’s, but was not fully implemented or widely used until the 1980’s. This technology helped to allow for the progression of smaller and smaller electronic devices such as calculators and computers. It also helped to open the door to an entirely new industry, electronic contract manufacturing.

In this process, the components are first put on reels using a tape and reel machine. Then the boards are populated with the various SMD components usually using a pick and place machine. Next, the printed circuit boards are run through a re-flow soldering oven. This solders the components to the board itself with a minimal amount of hand labor.

Their smaller size is not without its drawbacks. Most of the surface mount components cannot carry as much power as the older thru-hole technology because of their smaller size. Sometimes the transformer or inductor must be mounted in a separate (usually by hand) process because they are too large to be tape and reeled. Hence these boards cannot be entirely populated by a pick and place machine.

All in all, however, the surface mount technology is how most components will be manufactured in the future. The call for power is decreasing, with the exception of specific applications, so the smaller size is not an issues for most computers, tablets, and cell phones. Going forward most components will be surface mount devices, if they are not already.

Disk Resistors

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Res-net Microwave Disk Resistors

A resistor is a commonly used two terminal passive electronic component. It produces a voltage across it’s terminals that is proportional to the current flowing through it. Resistors are used to limit or regulate the flow of electrical current in an electronic circuit. They are often used within a circuit for current control or to turn a current signal into a voltage signal. Resistors can be manufactured in two different ways, one by winding a resistive wire on a core (wirewound) or a second by using a film via a sputtering process or some other method (metal film).

Disk resistors, however, are not a very common component. They are a specific type of resistor that is physically small and circular. They can be either thick film or thin film and can therefore operate at higher frequencies. They can be used in lower frequency SMA terminations in lieu of a rod resistor. Since they have a symmetrical geometry there is essentially no magnetic field, which would normally create more inductance. Therefore, with the absence of this field, they are ideal for applications that require lower inductance’s. They are also often used for pulsed power systems.

A typical disk resistor, such as those produced by Res-net Microwave, are the thick film type. They have a resistance range of 5 ohms to 500 ohms. They also have a nominal power of 10 watts.