Patent ID: 7256371

Claim:
A controller adapted to control the temperature of a heating element within a selected temperature range wherein the instantaneous electrical resistance of the element is measured to determine the instantaneous temperature of the element, comprising: a heating element of a type where the electrical resistance of the element increases with increasing element temperature; a switch adapted to electrically couple a sinusoidal alternating current power source to the heating element when a switch controller is in an ON state, and to decouple the sinusoidal alternating current power source from the heating element when the switch controller is in an OFF state; a current sensor coupled in series with the heating element and adapted to provide as an output a voltage, wherein the current sensor comprises a sense resistor, wherein said voltage is dependent on the magnitude of a electrical current passing through the heating element and said sense resistor; a zero set point controller adapted to supply a first voltage proportional to a supply voltage of the sinusoidal alternating mains current power source; a difference sensor with a first input connected to the current sensor, and a second input connected to the zero set point controller adapted to produce an output signal which is dependent on the difference between voltage signals received at the first and second inputs, an operational set point controller adapted to supply a second voltage proportional to the supply voltage of the sinusoidal alternating mains current power source, the value of the second voltage being dependent on a selected desired temperature of operation of the heating element; a timer adapted to supply timing pulses of a selected pulse duration and period; a threshold detector electrically connected to receive the output signal of the difference sensor and the output signal of the operational set point controller and adapted to provide a selected output when an extent of magnitude and direction of difference between the said signals is greater than a selected extent; and logic circuitry adapted to set the switch controller to an ON state in response to a said timing pulse, and to reset the switch controller to an OFF state in response to the selected output from the threshold detector.