Circuit arrangement for controlling the temperature of a heating element

A circuit arrangement for controlling the temperature of a heating element, the resistance of which heating element is a function of its temperature, the circuit comprising:

• means for coupling an alternating current mains supply to the heating element for supplying an alternating current to the heating element via a switching means,
• a switch control circuit having an output to control the switching means, wherein the switching means conducts the alternating current to the heating element when the switch control circuit is in an ON state and does not conduct when the switch control circuit is in an OFF state,
• a current sensor means coupled to the heating element, the current sensor means supplying at an output thereof an alternating voltage, which is dependent on the alternating current passing through the heating element and representative of the temperature of the heating element,
• a scaling means having means for a user to select a desired temperature of the heating element, the scaling means adapted to supply to an output thereof an alternating voltage, which is dependent on a temperature selected by the user,
• a timing circuit for supplying timing pulses, the timing pulses having a predetermined pulse duration and pulse period,
• a first detector means having respective inputs coupled to the outputs of the current sensor means and the scaling means and adapted to supply a first logic control signal when the temperature of the heating element exceeds the temperature set by the user,
• a second detector means for supplying a second logic control signal when said alternating current is supplied to the heating element,
• a logic means adapted to set the switch control circuit to the ON state always in response to the timing pulses and to reset the switch control circuit to the OFF state in response to the logic AND combination of the first and second logic control signals, and
• wherein a Wheatstone bridae arrangement is created which has a balance point equivalent to said temperature selected by the user, and which has a first branch comprising the heating element, a second branch comprising the current sensor means, a third branch comprising the means for a user to select a desired temperature, and a fourth branch comprising a resistor which is part of the scaling means and wherein one input of said first detector means is coupled to the junction of the third and fourth branches and the other input of the first detector means is coupled to the junction of the first and second branches.