Resettable circuit protection devices can be used to help provide overtemperature, overcurrent and overvoltage protection for the electric motors and fans, controllers, touch-pads, displays and interface circuitry required by sophisticated appliances.
The motors, controllers and electronic components found in household and professional-grade appliances can benefit from the application of coordinated circuit protection. Polymeric positive temperature coefficient (PPTC) overcurrent protection devices offer low resistance and are compatibly sized with fuse solutions. Like traditional fuses, they limit the flow of dangerously high current during fault conditions. The PPTC device, however, resets itself after the fault is removed and power to the circuit is cycled, obviating the need to replace a blown fuse.
Protecting increasingly sophisticated and complex control boards from misconnection, power surges or short circuit damage is of particular concern to the equipment manufacturer. Although appliance transformers, their enclosures and connections are capable of withstanding higher voltage transients, the use of sensitive solid-state devices on the board necessitates improved overcurrent, overtemperature and overvoltage control.
Coordinating overcurrent and overvoltage protection can also help designers comply with safety agency requirements, minimize component count and improve equipment reliability. A metal oxide varistor (MOV) overvoltage protection device, used in a coordinated circuit protection strategy with a line-voltage rated PPTC overcurrent device helps manufacturers meet IEC 6100-4-5, the global standard for voltage and current test conditions for equipment connected to AC Mains.
Traditionally, single-use fuse technology has been used to protect electronic circuits from damage caused by overcurrent events. With this approach, the fuse blows when a wiring fault or part failure creates a condition in which excessive currents can flow, therefore breaking the electrical connection and helping prevent the potential for more widespread damage or fire hazards. The problem with this technology is that a failure in one system component can disable other components downstream and throughout the system. When this happens, the fuse must be accessed and replaced on all the affected components before the system can be made operational again.
In comparison, controllers and remote devices that utilize resettable fault protection technology can help minimize the impact that failure has on the system, reduce the number of system components affected, and shorten repair time. In many industrial controller applications, replacing single-use fuses with PPTC devices allows designers to maintain the same level of overcurrent protection on the critical interfaces, while generally eliminating the need for fuse replacement or service when an external fault condition causes high current conditions in the system.
In addition to controllers, any remote sensor, indicator, or actuator that requires a power, analog, or communications bus interface can benefit from the use of PPTC devices (Figure 1). These system components are subject to damage caused by mis-wiring, power cross, or loose neutral connections on AC Mains inputs.
From small countertop appliances to professional grade ovens, increasing complexity and functionality are driving the industry toward circuit integration and board size reduction. Protecting sensitive electronic components from voltage transients, short circuits, and customer misuse is of primary concern to manufacturers.
Electrical equipment can be exposed to potential damage from large voltage or power transients on the AC Mains inputs caused by lightning strikes or power station load-switching transients.
Coordinating overcurrent and overvoltage protection at the AC Mains input can help designers comply with safety agency requirements and minimize component count and cost.
Figure 2 shows how an MOV is used in combination with a PPTC device to help improve equipment reliability in the harsh AC environment, and helps fulfill the IEC-61000 test requirements.
The MOV device's high current-handling and energy absorption capability, fast response and low cost make it suitable for overvoltage protection in power supplies, control board transformers and electric motors. The PPTC overcurrent protection device is also rated at 240 VAC, permitting maximum intermittent voltages of up to 265 VAC, and it can be installed with the MOV device in the AC Mains input lines
Unlike a single-use current fuse, the resettable PPTC device helps protect against damage from conditions where faults may cause a rise in temperature with only a slight increase in current draw. When installed on the primary side of the circuit, in proximity to potential heat-generating components such as magnetics, field-effect transistors (FET), or power resistors, the PPTC device helps provide both overcurrent and overtemperature protection with a single installed component.
Certain Mains overload conditions may cause the MOV device to remain in a clamped state where it will continue to conduct current. This may eventually result in an overtemperature failure of the device. While not directly applicable to passing IEC 61000-4-5 tests, placing the PPTC device in close thermal proximity to the MOV device can help protect the MOV device in extended overload conditions. It does this by transferring heat to the PPTC device, which causes the PPTC device to trip faster, limiting the current through the MOV device.
The PPTC and MOV devices chosen for a particular application depend on the IEC 61000-4-5 class rating for the equipment as well as the operating conditions of the equipment itself. When selecting a PPTC device, the primary consideration is to match the hold current rating of the device to the primary current drawn by the electrical equipment under normal operating conditions.
Liquid crystal displays (LCDs) are used in a wide range of appliance applications and may be subjected to significant temperature variations. Because LCDs perform poorly at low temperatures, heaters are often employed to raise the display's temperature and improve functionality. Typically, the heaters incorporate temperature sensors connected to a microprocessor-controlled switch that modulates the heater, as well as a high-temperature shut off function that turns the heater off if the LCD temperature exceeds a specified limit.
The disadvantage of this approach is that the overheat control mechanism relies on the same microprocessor that is controlling the heater element. Consequently, if the heater control malfunction is due to microprocessor failure or some other functional control component, the high-temperature shut-off function may also be disabled. If this control circuitry fails, the current flowing through the heater element can increase and may lead to thermal runaway.
Installing a PPTC device, independent of the main heater controller, helps protect the LCD and the heater control circuitry from overtemperature damage. As shown in Figure 3, the PPTC device is typically placed in line between the power supply and the heater, in a thermally conductive relationship with the heated LCD panel. In this way, heat emanating from the LCD is transmitted to the PPTC device. When the LCD reaches a specified shut-off temperature the PPTC device "trips" and reduces the current flowing through the heater element. Once the fault is removed and the power is cycled, the circuit will reset to normal operating conditions.
Coordinating overcurrent, overtemperature and overvoltage protection can help designers minimize component count and reduce warranty returns resulting from failed motors and control board transformers. The low resistance, fast time-to-trip, low profile, and resettable functionality of the PPTC device helps circuit designers provide a safe and dependable product and comply with regulatory agency requirements.