June, 2013
www.us-tech.com
Alumina-Based Resistors Aid Medical Applications
Saff, Ohmite Mfg. Co. M
edical 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 med- ical applications these components can spell a mat- ter of life or death. Medical equipment manufactur- ers 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 tech-
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High Resistance One of these benefits is the high resistance
High energy TP810 series mounts on edge for maximum heat dissipation.
values possible with thick-film resistors on alumi- na. Thick-film resistivity is achieved by depositing and fusing a resistive paste to the alumina sub- strate base. Resistive pastes range in strength from as little as 10W/square to as much as 10MW/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.
Continued on page 59
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TFS Series surge capable thick film resistor.
nology allows resistors and other pas- sive components to be printed direct- ly on the ceramic base, saving space and promoting creative circuit config- urations. Alumina also exhibits high thermal conductivity with excellent capability to dissipate heat, minimiz- ing concerns about circuit “hotspots” due to heat buildup. Examples of alumina-based
resistors are the TFS Series of resis- tors from Ohmite Manufacturing Co. The TFS Series includes resistors with resistance values from 100W to 100 kW, 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 toler- ances, and the high voltage ratings required for critical medical electron- ic 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 volt- age 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 applica- tions such as in defibrillators. Still, thick-film resistors on alumina sub- strates can offer more benefits than wirewound resistors in critical med- ical device applications.
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