Portable handheld medical equipment can improve patient care no matter in medical institutions or in patients' homes. Proper use of electronic circuit protection devices is an important part of medical equipment design to ensure reliability and safety. Circuit protection is an important design element Portable medical instruments, such as blood glucose meters, sphygmomanometers, and oxygen measuring instruments, can be designed with communication functions to provide continuous communication between patients and caregivers in almost any location. This will hopefully improve patient care at a lower cost, but requires equipment designers to pay closer attention to reliability and safety issues in their circuit design and component selection. The use of overvoltage and overcurrent protection components is a key design consideration. The main subsystems that must be considered include communication interfaces, DC input / charging circuits, battery packs, sensors, LCD displays, keyboards, and buttons. Figure 1 is a simplified circuit diagram of a general-purpose handheld instrument. The green text box indicates circuits that may be subject to overvoltage or overcurrent conditions. The remainder of this article will introduce electrical threats and typical solutions. Since this product will be frequently used by users, the main electrical threat is electrostatic discharge, which is easily generated when the user walks across the carpet. Electrostatic discharge (ESD) can transfer excessive voltages and currents to internal circuitry. Fortunately, a variety of circuit protection devices are available to address overvoltage (electrostatic discharge and lightning surge) and overcurrent (short circuit and overload) conditions. For example, adding a self-resetting positive temperature coefficient (PTC) thermistor to a USB port commonly used to charge batteries can provide overload current protection on portable instruments. These devices can limit the current when an overload occurs, and then "reset" to a low resistance value after the overload condition disappears. This eliminates the inconvenience of having to replace the fuse. For overvoltage protection, the handheld device will not be directly connected to the AC grid, so it will most likely encounter low-level (residual) surge events and electrostatic discharge shocks. For these events, it is possible to use space-saving devices based on semiconductor technology. These discrete and array diodes provide low clamping voltages and different form factors to provide multiple design options for protection. This low clamping voltage is an important feature of semiconductor-based protection because it makes it possible to protect today's cutting-edge and vulnerable chipsets. Examples of circuit protection schemes Communication interface protection is a high-priority issue, especially for instrument manufacturers who are striving to meet the Continua Health Alliance ’s guidelines for interconnecting transmitted data. The first edition of Continua ’s standard published in 2008 defined specific versions of Bluetooth and USB for wireless and wired transmission to connect home health care equipment with caregivers. Some examples are provided below to illustrate how these protection devices are applied. As shown in Figure 1, the wireless (radio frequency) interface of a handheld instrument may be harmed by an electrostatic discharge surge caused by its antenna. The circuit in Figure 2 illustrates an electrostatic discharge solution based on semiconductor technology that protects the RF amplifier input module of the Bluetooth circuit from the threat of electrostatic discharge. This protection device is a 0.5pF (to maintain signal integrity) discrete diode that provides a low clamping voltage to protect the sensitive RF front end.
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