Overheat protection
Heating conductors within existing seat heaters are controlled by a single thermostat. There is no doubt, that the "area of thermal detection" of a single thermostat is limited, particularly in the case of a localized overheating hot spot occurring at the greatest distance from the location of the thermostat.
For simple overheat protection, a temperature sensing polymer layer may be used which melts at a predetermined temperature—lower than the combustion temperature of the seat cloth. Conventionally a meltdown layer is used to create a dramatic decrease in resistance which will correspondingly draw a large current from the power supply and blow a fuse. However, to achieve the required resistances in a seat heating pad, two helically wound element wires are attached in parallel. In this situation, a fusible layer no longer causes an increase in current because the voltage between the two conductors is the same at each point on each conductor.
For a meltdown layer to be used effectively, the connection between the two conductors should be broken briefly to leave only one leg of the parallel resistor combination attached to the supply. If the current flowing through the combination is monitored during this period the current draw should be much lower than normal. In the event that the fusible layer has melted the current draw will still remain relatively high and therefore indicative of an overheat condition (see below).
Temperature Monitoring
Accurate temperature control can be achieved by the conductor also being a thermistor. Thus temperature sensing can be measured on the conductor itself. The image below shows a helically wound conductor with an NTC (negative temperature coefficient) thermistor layer.
This conductor has two heated circuits separated by a NTC thermistor plastic. The NTC layer has two functions:
An important consideration in the design of feedback sensor conductors is the long term stability of the thermistor polymer. It is essential that the control module's electronics receive repeatedly consistent signals from the conductor within a narrow band of tolerance. This consistency is achieved by controlled fabrication processing techniques of the thermistor polymer.
