Kent Hou researched options for preventing overcurrent when designing electronic products.

For the overcurrent protection of electronic equipment, fuses have long been a standard solution. They have a variety of ratings and installation methods, making them suitable for almost any application. When they are turned on, they completely stop the current flow, which may be the expected reaction; the equipment or circuit cannot operate, which will draw the user's attention to the cause of the overload situation so that corrective measures can be taken.

However, in some situations and circuits, it is necessary to automatically recover from a temporary overload without user intervention. A positive temperature coefficient (PTC) thermistor-also known as a polymerized positive temperature coefficient device (PPTC) or resettable fuse-is an excellent way to achieve this type of protection.

Although ceramic PTC has different operating characteristics, including greater internal resistance, higher environmental heat resistance, and higher rated voltage, it is also widely used. Because they are generally used in high ambient heat areas, including heating equipment applications that are not common for many electronic products, they are not in the scope of this article.

PTC consists of a polymer material filled with conductive particles (usually carbon black). At room temperature, the polymer is in a semi-crystalline state and the conductive particles contact each other, forming multiple conductive paths and providing low resistance (usually about twice that of a fuse with the same rating).

When current passes through the PTC, it will dissipate power (P = I2R) and the temperature will rise. As long as the current is less than its rated holding current IHOLD, the PTC will maintain a low resistance state and the circuit will operate normally. When the current exceeds the rated trip current ITRIP, the PTC will suddenly heat up. The polymer becomes amorphous and expands, breaking the connection between the conductive particles. This will cause the resistance to increase rapidly by several orders of magnitude and reduce the current to a low (leakage) value just enough to keep the PTC in a high resistance state-usually from about tens of milliamps to hundreds of milliamps at rated voltage (Vmax ). When the power is turned off, the device cools down and returns to a low resistance state.

Like a fuse, the rating of a PTC is the maximum short-circuit current (IMAX) that it can break at the rated voltage. The IMAX of a typical PTC is 40A, and it may reach 100A. The breaking ratings of fuses of various sizes that can be used in the various applications we consider here range from 35 to 10,000 amperes at rated voltage.

The rated voltage of PTC is limited. Generally, the rated working voltage of PTC does not exceed 60V (PTC for telecommunications applications has 250V and 600V breaking voltages, but the working voltage is still 60V); it provides surface mount and small box fuses with ratings from 32V to 250V or higher.

PTC's rated operating current range is as high as about 9A, and the maximum level of the fuse type considered here may exceed 20A, and some can reach 60A.

The upper limit of useful temperature of PTC is usually 85°C, while the maximum operating temperature of thin film surface mount fuses is 90°C, and the maximum operating temperature of small box-type fuses is 125°C. Both the PTC and the fuse require a temperature derating above 20°C, although the PTC is more sensitive to temperature (Figure 2). When designing any over-current protection device, be sure to consider factors that may affect its operating temperature, including the effect on lead/wire heat dissipation, any air flow, and the proximity to the heat source. The response speed of PTC is similar to that of time delay fuse.

Most of the design work for personal computers and peripherals is strongly influenced by the Microsoft and Intel system design guidelines, which states that "it is unacceptable to replace the fuse every time an overcurrent condition occurs." The SCSI (Small Computer System Interface) standard for this large market includes a statement that "...a positive temperature coefficient device must be used instead of a fuse to limit the maximum current source."

PTC is used to provide secondary overcurrent protection for telephone central office equipment and customer premises equipment, alarm systems, set-top boxes, voice over IP (VOIP) equipment, and subscriber line interface circuits (SLIC). They provide primary protection for battery packs, battery chargers, car door locks, USB ports, speakers, and Power over Ethernet.

SCSI plug-and-play applications that benefit from PTC include motherboards and many peripheral devices that can be frequently connected to and disconnected from computer ports. The mouse, keyboard, printer, modem, and monitor ports represent opportunities for incorrect connection and connection of faulty units or damage to cables. The ability to reset after the fault is corrected is particularly attractive.

PTC can protect disk drives from potentially damaging overcurrents caused by excessive currents caused by power failures.

PTC can protect the power supply from overload; a separate PTC can be placed in the output circuit to protect each load with multiple loads or circuits.

Motor overcurrent will generate excessive heat, which may damage the winding insulation. For small motors, it may even cause a failure of the winding wire with a very small diameter. PTC usually does not trip under normal motor starting current, but it will play a role in preventing damage caused by continuous overload.

The transformer will be damaged by the overcurrent caused by the circuit fault, and the current limiting function of the PTC can provide protection. The PTC is located on the load side of the transformer.

The following procedures will help to select and apply the correct components. Equipment suppliers also provide assistance. To get fair advice, it is wise to find a company that provides both fuse and PTC technology.

1. Define circuit operating parameters. Consider the following: normal operation; ampere current; normal operating voltage in volts; maximum interruption current; ambient temperature/re-rating; typical overload current; turn-on time required for a specific overload; expected transient pulse; Set or one-time; agency approval; installation type/shape; typical resistance (in the circuit). 2. Select the desired circuit protection components.

3. Determine the opening time of the fault. Review the time-current (TC) curve to determine if the selected component will operate within the limits of the application.

4. Verify the environmental operating parameters. Ensure that the applied voltage is less than or equal to the rated voltage of the device, and the operating temperature is limited to the range specified by the device

5. Verify the size of the equipment. Compare the maximum size of the device with the available space in the app.

6. Test the selected product. Independently test and evaluate suitability and performance in actual applications.

Kent Hou is the global product manager for Littelfuse in Des Plaines, Illinois, USA. www.littlefuse.com

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