Medical Electronics
Providing the essentials
Thick film on alumina technology is able to offer precision and high resistance values in defibrillators, as Ohmite explains to CIE
S
urge resistors are a crucial component in many electrical devices, providing protection and
reliable operation in applications ranging from industrial power supplies to telecommunications. While this reliability is important in every industry, it is arguably most critical for medical equipment, where dependable operation can literally constitute the difference between life and death.
Medical equipment manufacturers have used a variety of resistor types to provide this essential surge protection, including thick film and solid composition-type resistors, but the two most prevalent are solid composition-type and wirewound for common surge applications. Thick film resistors, consisting of conductive, resistive, and insulating pastes deposited and
technology has been around for decades, but has enjoyed something of a renaissance in recent years, as the use of microcircuits and hybrid microcircuits has increased.
As opposed to competing technologies, such as PC Boards, alumina substrate technology allows resistors and other passive components to be printed directly on the ceramic base, saving space and opening the door for creative component configurations. In addition, as a ceramic, alumina also offers high thermal conductivity and excellent heat dissipation characteristics. Manufacturers can therefore proceed without worrying about error due to hot-spotting or other irregularities.
One example of an alumina-based fused onto a
ceramic substrate base, have proven popular in medical applications because of the precision and high resistance values they offer. One of the most effective and often-used substrate bases is alumina (Al2O3). Alumina substrate
40 October 2013
resistor is the TFS Series of resistors, with values from 100Ω to 100 KΩ , power rating of 0.5 to 2 W, and voltage rating of 3.0 to 11.0 Kv. Using thick-film-on- alumina substrate technology, these resistors can be provided in the precision tolerances, high voltage ratings, and high resistance values that this technology is known for. Wirewound resistors, most often composed of a rod-shaped ceramic or fibre-glass core wrapped in metallic wire, also offer distinct advantages for safety- critical medical equipment. Wirewound resistors typically do not need to specify a maximum voltage in the way that thick film resistors do, because there is less concern about potential conduction from typically non-conductive traces. They are also better suited to manage excessively high current pulses, due to the surface area afforded by the wrapped wire. For this reason, some manufacturers may prefer to use wirewound resistors, such as Ohmite’s 30 Series high energy wirewound resistors, in medical device applications such as defibrillators. However, although wirewound
resistors can offer some advantages in handling current pulse, resistors constructed from thick film on alumina substrate can provide wider, more encompassing benefits in the design of medical device equipment including high
Components in Electronics
resistance values, high voltage handling, small size, and resilience.
High resistance values
One of the chief advantages of thick film on alumina substrate resistors is their high resistance values. As mentioned above, with this technology, resistivity is achieved by depositing and fusing a resistive paste to the alumina substrate base. These pastes can range in strength from 10Ω/square to 10MΩ/square, and different degrees of resistivity can be achieved by selecting the appropriate strength paste. The shift to a higher or lower resistance value is therefore relatively easy from a design perspective. All it involves is a change in paste selection rather than a change in geometry, size, or another more involved aspect of the resistor. Achieving high resistance values is a bit
more involved when dealing with wirewound resistors. In order to achieve the desired resistivity, manufacturers need to fabricate a smaller and smaller wire diameter for higher and higher values. In addition, high resistance values are inherently less valuable in wirewound construction due to the fine wire involved. Occasionally, other issues can also evolve, such as potential circuit interference caused by inductively wound wire. However, although there are some
difficulties surrounding high resistance values, wirewound resistors often offer easier handling of high current pulse than thick film on alumina resistors.
Pulse handling High-stress medical devices like defibrillators and their accompanying monitoring equipment need to withstand both high pulse and high voltage in the course of operation. The defibrillator itself endures a great deal of electrical strain when administering shock to the patients, with some modern machines capable of putting out nearly 2,000 V. (While this capacity is rarely exercised on patients, it is relevant for designers.) In addition, the monitoring equipment faces its own voltage and current challenges. These machines must absorb any pulse before it travels into the reading system, or the strong current will burn up the system.
Pulses in medical equipment can vary
greatly from manufacturer to manufacturer. For example, defibrillators can experience current pulses as low as 20 J or as large as 300 J. For this reason, pulse is of varying concern for different applications. When designing for demanding circumstances, the choice between wirewound and alumina can end up seeming like a simple trade-off. In general, wirewound is more naturally
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