Patent ID: 7180037

Claim:
In combination: a heater wire, which comprises: a heating conductor having a predetermined resistance; a sensing conductor; a polymer layer interposed between the heating conductor and the sensing conductor; and an outer insulative layer covering the heating conductor, polymer layer and sensing conductor; and a control circuit for controlling an AC (alternating current) power signal applied to the heater wire, the control circuit comprising: a capacitor, the capacitor being coupleable to one polarity of the AC power signal; a resistor connected in series with the capacitor and to an opposite polarity of the AC power signal, the resistor being connected to the capacitor to define a junction thereat, one end of the sensing conductor being coupled to the junction of the resistor and capacitor, the junction exhibiting a phase shifted AC power signal when power is applied to the heater wire and control circuit; a first zero cross detector which detects the zero crossing of the AC power signal and generates a first zero cross signal in response thereto; a second zero cross detector which detects the zero crossing of the phase shifted AC power signal exhibited at the junction of the resistor and capacitor and which generates a second zero cross signal in response thereto; a time difference determinator circuit responsive to the first zero cross signal and the second zero cross signal and which determines the time difference between the zero crossing of the AC power signal and the zero crossing of the phase shifted AC power signal and generates a control signal in response thereto; and a switch responsive to the control signal which controls the duty cycle of the AC power signal provided to the heater wire in response thereto.