Many of these issues apply to battery-powered units, not just those that are AC line powered. The function of many but not all protection devices is to suppress unacceptably large voltage transients. There are two major categories of transient suppressors: those that attenuate transients, thus preventing their propagation into the sensitive circuit; and those that divert transients away from sensitive loads and so limit the remaining voltage. It is critical to study device data sheets carefully for thermal and performance derating curves, as some are specified for transient protection of various durations bounded by defined voltage, current, and time limits rather than steady-state protection.
almost certainly be preferred or “standard” approaches, but there are also choices that must be judged, assessed, and made. Circuit protection options are many: choose wisely There are a variety of protection options. Each has a unique functionality and set of characteristics that makes it a suitable – or only – choice for implementing protection against specific classes of faults or unavoidable circuit characteristics. The main protection options are: ■ The traditional thermal fuse ■ Polymeric positive temperature coefficient (PPTC) devices ■ Metal oxide varistors (MOVs) ■ Multi-layer varistors (MLVs) ■ Transient voltage suppression (TVS) diodes ■ Diode arrays ■ Solid state relays (SSRs) ■ Temperature indicators ■ Gas discharge tubes (GDTs) The thermal fuse is simple in concept. It uses a conducting fusible link that is fabricated of carefully selected metals with precise dimensions. The flow of current beyond the design limit causes the link to heat up and melt, thus permanently breaking the current path. For standard fuses, the time to open circuit is on the order of several hundred milliseconds to several seconds, depending on the amount of overcurrent versus rated limit. In
various types of circuit and system protection components, using devices from Littelfuse, Inc. by way of example and examines the role and application of each. The role of protection in medical systems For most engineers the phrase “circuit protection” immediately brings to mind the classic thermal fuse, which has been in use for over 150 years. Its modern embodiment is largely due to the work of Edward V. Sundt, who in 1927 patented the first small, fast-acting protective fuse designed to prevent sensitive test meters from burning out (Reference 1). He then went on to found what eventually became Littelfuse, Inc. Since then, circuit protection options have expanded significantly in recognition of the many potential circuit failure modes. These can be: ■ Internal failures that may result in a cascade of damage to other components ■ Internal failures that may put the operator or patient at risk ■ Internal operational issues (voltage/current/thermal) that may stress other components and lead to their premature failure ■ Voltage/current transients and spikes which are an inherent and unavoidable part of the circuit’s functionality and must be carefully managed
How to select and apply the right components to protect medical devices, users, and patients
Among the many electrical parameters that must be
considered are clamp voltage, maximum current, breakdown voltage, reverse working maximum
to provide protection, and a typical system may need a dozen or more of these specialized protection devices. Protection devices are like insurance: while the latter may only be rarely or never needed, the cost of not having it far exceeds the cost of having it. This article looks at where protection is needed in such medical systems, including patient-facing signal/sensor I/O, power supply, communication ports, processing core, and user interfaces. It also discusses the
protection against multiple types of electrical issues that can harm the equipment, hospital staff, and patients. However, full circuit protection takes much more than just a thermal fuse, and implementing protection is not a matter of finding the single best device for a given design and application. Instead, it involves first understanding which circuits need protection and then determining the best mode of protection. In general, multiple passive components are needed
Written By Bill Schweber
or reverse stand-off voltage, peak pulse current, dynamic
resistance, and capacitance. It is also important to understand under what conditions each of these is defined and specified. Device size and number of channels or lines protected are also considerations. The choice of the best protection device to use in a given part of a circuit is a function of these factors, and there are often the inevitable trade-offs among the various parameters. There will
The use of non-laboratory, patient-contact diagnostic and life-sustaining medical equipment such as ventilators, defibrillators, ultrasound scanners, and electrocardiogram (EKG) units continues to increase. Reasons include an aging population, heightened care expectations among patients, and improvements in medical electronics technology which make such systems more practical. Such equipment needs
we get technical
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