Power Sources/Circuit Protection

ASI Controls uses resettable PPTC devices to protect power inputs, communications bus and analog interfaces on their ASIC/1-8655 controller board.



Improvements in size, cost and performance have resulted in widespread acceptance of polymeric positive temperature coefficient (PPTC) devices in industrial, computer, battery, consumer electronics, telecom and automotive applications. These resettable circuit protection devices are used to protect against harmful overcurrent surges and over temperature faults. Like traditional fuses, they limit the flow of dangerously high current during fault conditions. The PPTC device, however, resets after the fault is cleared and power to the circuit is removed. This helps reduce warranty and service costs because the protected equipment remains fully functional, and there is no need for service personnel to physically replace a fuse.





Circuit Protection Challenges in Commercial Building Environments

The modern commercial building depends on a variety of automated systems to control its environment, provide security, minimize energy usage and protect against fire. These systems are typically located in parts of the building that are difficult or inconvenient to access, and electrical failures can result in costly service calls and disrupt the activities of building tenants. Developing products that use resettable fault protection technology can help minimize the impact that any failure has on the system, reduce the number of system components affected and shorten repair time.



Traditionally, one-shot fuses are used to protect electronic circuits. With this technology, when a wiring fault or part failure creates a condition in which excessive currents can flow, the fuse blows — breaking the electrical connection and preventing more widespread damage or fire hazards. The problem with this approach is that a failure in one system component can disable many others by creating high current conditions throughout the system. A service technician must access and replace fuses on all the affected components before the system can be made operational again — a far more time consuming task than simply repairing the original fault.





PPTC Device Implementation Enhances Reliability in Network Control System

In response to customer feedback regarding the difficultiesand expenses associated with servicing building control systems, ASI Controls, a building automation systems manufacturer, recently replaced the fuses on its controller product line with PPTC devices. The result is that the controllers maintain the same level of overcurrent protection on the critical interfaces but no longer require fuse replacement or service when an external fault condition causes high current conditions in the system.



“Incorporating resettable PPTC devices into our design enhances the reliability of the analog interfaces and RS-485 communications lines. This is critical in a networked control system,” states Bill Chapman, CEO of ASI Controls. “The improvements we’ve seen since we began using these devices on our ASIC/1-8655 interfaces has led us to include them on new designs and expand their use to our power inputs as well.”



This circuit protection scheme helps protect the controller electronics from any transient wiring or sensor faults that cause damaging overcurrent conditions. The PPTC device responds to overcurrent or overtemperature conditions by going from a low-resistance state to a highresistance state. This reduces to a minimal level the amount of current that can flow, and it protects the equipment in the circuit. Resetting the device is as simple as removing power and clearing the fault. This restores the controller to normal operation, and no technical service or device access is required.











ASI Controls is leveraging the performance advantages of PPTC devices to create more robust systems for its customers and to reduce service and warranty costs. “We have not registered a single failure of PPTC devices since we began using them to protect the RS-485 communications and analog signal inputs on the ASIC/1-8655 controller more than a year ago,” reports Chapman. “We are particularly pleased with the way the I/O interfaces have survived in environments subject to frequent lightning storms.”















Expanding the Resettable Protection Strategy to the Whole System

In addition to controllers, any remote sensor, indicator or actuator that requires an AC power, analog interface or communications bus interface can benefit from PPTC resettable circuit protection devices. These system components are subject to damage caused by miswiring, power cross, or loose neutral connections on AC Mains inputs.



An example of how PPTC devices can be used to help protect individual power, signal or communications interfaces is presented in Figure 1. Each PPTC device is combined with an overvoltage protection device, such as an MOV, to protect against overvoltage transients in addition to overcurrent faults. This circuit topology can be used to help equipment meet the UL requirements for Class 2 products, as well as IEC61000-4-5 surge immunity requirements.



PPTC devices also allow the system to operate when a single remote component fails. If there is a failure in a remote sensor that would normally short or disable a power or communications bus, applying a PPTC device at the remote sensor will help protect the failed component from creating a fire hazard itself, as well as act as a high impedance between the bus and the failed device. This helps maintain the functionality of the remainder of the system. Figure 2 presents a diagram of an architecture that would accomplish this in a power distribution bus application.





Conclusion

In many industrial controller applications, manufacturers are replacing one-shot fuses with PPTC devices. This approach allows designers to maintain the same level of overcurrent protection on the critical interfaces, but no longer necessitates fuse replacement or service when an external fault condition causes high current conditions in the system.



PPTC devices offer a resettable alternative to single-use fuse technologies, and they help protect valuable electronic systems, reduce warranty and services costs and improve user satisfaction. They have demonstrated their effectiveness in a wide range of circuit protection applications and have gained the safety approvals of UL, CSA and TUV.





About the Author

Matt Chamberlain is sensors and controls marketing manager, Tyco Electronics/Raychem Circuit Protection Group. For more information, contact Raychem Circuit Protection, 308 Constitution Dr., Menlo Park, CA 94025; (800) 227-7040;www.circuitprotection.com.