June, 2013 Alumina-Based
Resistors... Continued from page 49
Achieving high resistance with a
wirewound resistor calls for a different construction approach. Smaller wire diameters are needed for higher resist- ance values, which require the han- dling of extremely fine and delicate wires. Other problems can arise with wirewound resistors at high resistance values, such as potential circuit inter- ference caused by the inductively wound wire. But in spite of their com- plexities at high resistance values, wirewound resistors are often superior to thick-film resistors on alumina sub- strates 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 puls- es and high voltages during opera- tion. A defibrillator survives tremen- dous electrical strain while adminis- tering shock to patients, with some systems capable of delivering volt- ages approaching 2000 V. The associ- ated monitoring equipment must also safely handle high levels of volt- age 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 suit- ed 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 han- dle the sudden increase in current. Thick-film resistors present
smaller areas with less area for cur- rent distribution. To equip an alumina substrate resistor for high current pulse, electrical designers cannot make the resistive deposit much thick- er. 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% toler- ance. They were designed with suffi- cient area for high current flow. Achieving high-voltage capabili-
ty in a wirewound resistor can be dif- ficult. 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 high- er voltages. However, most defibrilla- tors 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 con-
www.us-tech.com
cerns 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 environ- ment. For AEDs to be viable, 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-sub- strate-based resistors such as Ohmite’s high-voltage Slim-Mox devices. Using an alumina substrate, thick-film resis- tors can be formed directly on the sur- face 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. Manu - fac turers 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 mount- ed on an alumina substrate, a great deal of space can be saved because of the resistors. Alumina-based thick-film resis-
tors can simplify circuit design and component placement. Because a wirewound resistor is based on con- ductive wire, circuit designers must be careful about placing other elements too close to the resistor. A conductive component placed too close to a wire- wound resistor can result in arcing. In contrast, an alumina thick-film resis- tor is conductive on one side (where the conductive and resistive pastes
Page 59
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 worry- ing 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. 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 E-mail:
info@ohmite.com, Web:
www.ohmite.com
4000Plus Micro Materials Testing
The Nordson DAGE 4000Plus delivers a wide range of new features that enable it to be used for micro materials testing
The 4000Plus test platform with Paragon™ software builds on the strong Nordson DAGE tradition and reputation of handling and testing at low geometries.
The 4000Plus offers a wide range of optical solutions for positioning, viewing, recording tests and failure modes, making it the perfect choice where a more comprehensive materials test tool is required.
Application Specific Tooling Nordson DAGE has over 50 years of micro testing experience and continues to evolve test methods. An extensive range of standard and custom tools are available.
www.nordsondage.com |
globalsales@nordsondage.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92