Patent ID: 8440942

Claim:
A circuit arrangement for protecting from overheating a heating element whose resistance value is a function of its temperature, the circuit arrangement comprising: a switch means; a power supply coupled to the heating element for supplying a current to the heating element by means of the switch means; a switch control circuit with an output for controlling the switch means such that the switch means is switched to a conducting state when the switch control circuit is in a first state and the switch means is switched to a non-conducting state when the switch control circuit is in a second state; a current sensor means coupled to the heating element, the output of the current sensor means providing a signal proportional to the current flowing through the heating element; a first scaling means whose output provides a signal proportional to a supply voltage of the heating element; a first detector means having inputs each coupled to an output of a respective one of the current sensor means and the first scaling means, the first detector means having an output that provides a difference signal formed from the signals of the current sensor means and the scaling means; and an evaluation circuit operable to compare the difference signal determined by the first detector means with a reference signal, wherein the evaluation circuit comprises: second detector means having a first input, a second input, and an output, wherein the output of the second detector means forms the output of the evaluation circuit, the first input coupled to the output of the first detector means; and second scaling means for providing the reference signal, the second scaling means including a voltage divider having a first resistor and a second resistor, wherein an output node of the second scaling means is between the first resistor and the second resistor and the output node is coupled to the second input of the second detector means for providing the reference signal to the second input of the second detector means, the switch control circuit being operatively connected to the evaluation circuit such that the switch control circuit can be switched from the first state into the second state by the evaluation circuit.