Medical Electronics
suited to handle high current pulse while thick film is better equipped to endure high voltage. The question then remains which is easier to modify. Wirewound resistors can handle high
current pulses 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. Thick film resistors are not as ideally suited for current pulse, because they are smaller, with less overall surface area to work from. To equip an alumina substrate resistor for high current pulse, electrical designers cannot make the resistive deposit much thicker. Instead, they need to make it very wide in order to increase surface area. The more current that must pass through, the wider the square will get. Ohmite has developed a new series of thick film resistors that incorporates this design feature.
Ohmite’s TP Series can handle up to 100 joules of energy in a 1% tolerance, feature higher pulse currents than standard parts, and were designed with more area for the current to flow through.
In contrast, achieving high voltage handling in wirewound resistors can become difficult relatively quickly. For voltages of up 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 this new voltage. However, most defibrillators will endure a
voltage of around 2500V. At that point, the diameters required can become restrictive.
In short, it is generally easier to design an alumina resistor to handle the current pulse than it is to design a wirewound resistor to handle the extreme voltages experienced by defibrillators and their monitoring equipment.
Small size and dependability That same small size that can cause high current pulse handling concerns can also serve as an asset for alumina substrate- based resistors. In fact, 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 use by a layperson outside of a hospital environment. With
the advent of this technology, victims of cardiac dysrhythmias, ventricular fibrillation, or pulseless ventricular tachycardia no longer have to wait until arriving at the hospital for treatment, and this reduction in wait time is life-saving. In order for these AEDs to be viable, however, they need to be miniaturised to a small portable device and withstand harsher temperatures and humidity than their counterparts in hospitals. Alumina substrate based resistors like the high voltage Slim-Mox have provided the miniaturisation necessary to make this possible. As mentioned earlier, with alumina substrate, manufacturers can print resistors directly onto 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 utilisation. While components such as transistors and diodes must still be surface mounted on alumina substrate, the space saved on resistors adds up to big benefits. Alumina’s ceramic properties also contribute to its compactness. With wirewound resistors, the entire resistor is wrapped in conductive wire, and so designers must be careful about placing other elements too close to the resistor. If a conductive component is placed too close to the wirewound resistor, it may arc or cause a fault. Alumina, on the other hand, will be conductive on one side (the side where the conductive and resistive pastes were deposited) and non- conductive on the other. 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, the failure mode of an alumina resistor is still an open circuit. In summary, alumina substrate technology offers a variety of benefits to medical device designers and engineers, especially for the optimisation of defibrillators and their monitoring equipment. As one of the most commonly used resistors in the industry, it offers distinct advantages that include high resistance values, excellent high voltage handling, small size, and tight tolerances.
Ohmite |
www.ohmite.com
Obsolete Chip?
E-tec offers a range of competitively priced package adaptors to solve obsolete package problems. These high quality adaptors are typically produced in 1.6mm thickness material or where the upper footprint interferes with the lower footprint then 2.4mm material is used. Here the pins are fitted into blind holes from below and the combination of a press fit and a solder fillet around the pin shoulder provides a reliable and robust connection.
Daughter boards can be created to customer requirements and raised pins are available to raise the complete assemble off the pcb to the required level.
E-tec welcomes special adaptor enquiries, send the chip package details for the top and bottom parts and we will respond promptly with a quotation.
t: 01892 530260 f: 01892 515560 e:
info@e-tec.co.uk
www.cieonline.co.uk
www.e-tec.co.uk Components in Electronics
October 2013 41
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