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PRINTED FLEXIBLE HEATERS
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Printed flexible heaters are constructed using several elements including
positive-temperature-coefficient (PTC) materials for delivering heat. Such
constructions can be designed to operate in a steady-state or limiting modes.
In the latter mode the final temperature is bounded by the limiting resistance
of the PTC material. Temperatures up to 80°C can be achieved by allowing
the heater to draw a small amount of current at a fixed potential. At the start
of the heating the current draw is typically a few microamperes, but as the
heater approaches equilibrium, the current requirement is diminished to a level
that is necessary to maintain the limiting temperature.
Critical parameters for heater construction include physical and chemical
characteristics of the electrodes and the applied voltage. PTC material can be
deposited using standard screen-printing techniques in a wide range of
thickness. As the deposit thickness increases, its resistance decreases and
the observed temperature decreases. Electrode spacing as small as 250 microns
(0.010") can be achieved. Typical spacings are in the range of 0.75- 1.5 mm.
Heating temperatures at a fixed potential increase as the electrode spacing
decreases. The temperature response as function of applied potential is always
positive. Applied voltages are usually in the range of 3-12 VDC.
Substrates are typically PET in the range of 0.125- 0.356 mm. More flexible
materials such as Tyvex® can be used for applications where extreme flexibility
or conformability is required.
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