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Alumina-Based Resistors Aid Medical Applications
High energy TP810 series mounts on edge for maximum heat dissipation.
Medical electronic equipment, such as the defibrillator, depends on surge resistors for protection and reliable operation. In fact, many industries rely on such resistors, but in medical applications these components can spell a matter of life or death. Medical equipment manufacturers have used a variety of resistor types for surge protection, including thick-film and composition-type resistors. But the two most prevalent solutions for medical electronic equipment surge protection are solid-composition and wirewound resistors.

Thick-film resistors are popular for medical electronic equipment because they are available in high and precise resistance values. These resistors are formed of conductive, resistive, and insulating pastes deposited and fused onto a ceramic substrate base, with alumina (Al2O3) one of the most often-used and effective substrate bases for this purpose. In contrast to PC boards and their use of discrete components, alumina substrate technology allows resistors and other passive components to be printed directly on the ceramic base, saving space and promoting creative circuit configurations. Alumina also exhibits high thermal conductivity with excellent capability to dissipate heat, minimizing concerns about circuit “hotspots” due to heat buildup.

Examples of alumina-based resistors are the TFS Series of resistors from Ohmite Manufacturing Co. The TFS Series includes resistors with resistance values from 100{0} to 100 k{0}, power ratings from 0.5 to 2 W, and voltage ratings from 3.0 to 11.0 kV. Fabricated with thick-film-on-alumina substrate technology, the resistors are available with high resistance values, precision tolerances, and the high voltage ratings required for critical medical electronic applications.

Wirewound resistors are also commonly used in medical electronic applications. Such resistors are most often composed of a rod-shaped ceramic or fiberglass core wrapped in metallic wire. For medical equipment where safety is critical, wirewound resistors offer an advantage over thick-film resistors, since they need not be specified for a maximum voltage rating in the manner of thick-film resistors: there is less concern about potential conduction from typically non-conductive traces. Wirewound resistors, with the large surface areas occupied by their wrapped wire, can handle high current pulses. For this reason, many medical equipment manufacturers choose wirewound resistors, such as Ohmite’s 30 Series high energy wirewound resistors, in critical medical electronic applications such as in defibrillators. Still, thick-film resistors on alumina substrates can offer more benefits than wirewound resistors in critical medical device applications.

High Resistance
One of these benefits is the high resistance values possible with thick-film resistors on alumina. Thick-film resistivity is achieved by depositing and fusing a resistive paste to the alumina substrate base. Resistive pastes range in strength from as little as 10{0}/square to as much as 10M{0}/square, so a wide range of resistance values can be realized with a thick-film alumina resistor. Achieving different resistance values can be accomplished by a change in resistive paste rather than a change in geometry, size, or another more involved aspect of the resistor.
TFS Series surge capable thick film resistor.

Achieving high resistance with a wirewound resistor calls for a different construction approach. Smaller wire diameters are needed for higher resistance values, which require the handling of extremely fine and delicate wires. Other problems can arise with wirewound resistors at high resistance values, such as potential circuit interference caused by the inductively wound wire. But in spite of their complexities at high resistance values, wirewound resistors are often superior to thick-film resistors on alumina substrates at handling high current pulses.

High Current and Voltage
High-stress medical devices such as defibrillators and their accompanying monitoring equipment must often handle high current pulses and high voltages during operation. A defibrillator survives tremendous electrical strain while administering shock to patients, with some systems capable of delivering voltages approaching 2000 V. The associated monitoring equipment must also safely handle high levels of voltage and current. Pulses in medical equipment can vary greatly from manufacturer to manufacturer. Defibrillators may process current pulses as low as 20 J or as large as 300 J. As a result, the choice between wirewound resistors and thick-film on alumina resistors is not trivial or simply a matter of choosing one tradeoff versus another. In general, wirewound resistors are better suited for high current pulses while thick-film resistors are better equipped to endure high voltage. But when modifications are needed, which type is a better choice?

Wirewound resistors can handle high current pulses so well because of the way they are constructed. The wire wrapped around the core, though fine and thin, provides a great deal of surface area to absorb the pulse. When the current jumps, the lengths of wrapped wire can handle the sudden increase in current.

Thick-film resistors present smaller areas with less area for current distribution. To equip an alumina substrate resistor for high current pulse, electrical designers cannot make the resistive deposit much thicker. It can be made wider for increased surface area, which is an approach used by the Ohmite’s TP Series of thick-film resistors, which can handle as much as 100 J energy at 1% tolerance. They were designed with sufficient area for high current flow.

Achieving high-voltage capability in a wirewound resistor can be difficult. For voltages to 150 V, it is easy and inexpensive enough to simply decrease the diameter of the wire so that it is fine enough to handle higher voltages. However, most defibrillators are rated to endure a voltage of around 2500 V. At that point, the required wire diameters can become restrictive. It is generally easier to design a thick-film alumina resistor for a high current pulse than to form a wirewound resistor to handle the extreme voltages experienced by defibrillators and their monitoring equipment.

Small and Dependable
That same small size that can cause high current pulse handling concerns can also serve as an asset for alumina-substrate-based thick-film resistors. Their small size and resiliency have actually contributed to advancements in the medical field. Public access defibrillators, also known as Automated External Defibrillators (AEDs), are designed for users outside of a hospital environment. With the advent of this technology, victims of cardiac dysrhythmias, ventricular fibrillation, or pulseless ventricular tachycardia need not wait until arrival at a hospital for crucial treatment. For AEDs to be viable, however, they must be suitably small and portable, and withstand harsher temperatures and humidity than their counterparts in hospitals.

That miniaturization is possible thanks to thick-film, alumina-substrate-based resistors such as Ohmite’s high-voltage Slim-Mox devices. Using an alumina substrate, thick-film resistors can be formed directly on the surface of the substrate. These resistors are very thin, approximately the same height as the copper conductive traces on a PC board, and can be fitted into a space-saving planar package. Manufacturers can also place components directly on top of these resistors while still making conductor traces between them, doubling the space utilization. While components such as transistors and diodes must still be surface mounted on an alumina substrate, a great deal of space can be saved because of the resistors.

Alumina-based thick-film resistors can simplify circuit design and component placement. Because a wirewound resistor is based on conductive wire, circuit designers must be careful about placing other elements too close to the resistor. A conductive component placed too close to a wirewound resistor can result in arcing. In contrast, an alumina thick-film resistor is conductive on one side (where the conductive and resistive pastes have been deposited) but not on the other side. As a result, designers can place other elements very close to the backside of the resistor without worrying about a fault or arc. Should a fault occur, an alumina thick-film resistor will fail as an open circuit.

Alumina substrates and thick-film resistor technology offer many benefits to designers and engineers of safety-critical medical electronic equipment, such as defibrillators and their monitoring equipment. These resistors offer the right combination of features for these applications, including high resistance values, excellent high voltage handling, small size, and tight tolerances.

Contact: Ohmite Mfg. Co., 85 West Algonquin Road, Suite 230, Arlington Heights, IL 60005, 866-9-OHMITE, 847-258-0300, fax: 847-574-7522, E-mail:, Web:

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