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March 2017 • The PCB Magazine 27 terrupts electric current when heated to a specific temperature. These devices may be for one-time use, or they can be reset manually or automati- cally. In the case of a heater that is being misun- derstood by the monitoring device, as needing more power, the TCO will heat up and eventu- ally open like a switch, thus cutting off all power and preventing an unacceptable event such as a heater catching fire in an operating room. Connection to Power Source and Monitoring Device Once the heater is designed for power, monitoring and safety, it must have a method of attachment. Several methods are commonly used: Bare wires that may be soldered, a cable with connector that may be plugged, or a zero- insertion force (ZIF) connector that also may be plugged. These methods have all been used CRUCIAL CONSIDERATIONS FOR BUILDING FLEXIBLE HEATERS used, the concept remains the same. Monitor the temperature and adjust the power source to keep it in the desired range. Fusing We've mentioned earlier that some applica- tions are very critical. In those cases, redundant safety features may be prudent. In applications where the heater is being continually monitored, it is intuitive to believe that the monitoring de- vice will detect any issues with the heater run- ning too hot or too cold. However, the device, which is controlled by a software program, might not detect these issues. Software programs often cannot prepare 100% for scenarios in practice. As well, a defective component that fails at an inopportune moment can cause the monitoring device to think all is well, when in fact it is not. In these cases, a redundant safety measure is designed into the heater. A common method is to attach a thermal cut-off device (TCO). A ther- mal cut-off is an electrical safety device that in- Figure 3: Thermistor temp sensing device on a flexible heater. Figure 4: Thermal cut-off device on a flexible heater.