A CIRCUIT ARRANGEMENT FOR CONTROLLING THE TEMPERATURE OF A HEATING ELEMENT

摘要

The temperature of a heating element (3), which heating element has a temperature dependent resistance, is measured by comparing the current through the heating element (3) with a user adjustable reference value. The AC voltage over a current sensing resistor (7) in series with the heating element (3) is compared in a comparator (9) with a reference AC voltage generated a tap (6) of a voltage divider (8). The phase of the logic output signal of the comparator (9) indicates a first state in which the heating element (3) is too hot or is not conducting, or a second state in which the heating element (3) is too cold and conducting. A second comparator (16) detects whether or not the heating element (3) is conducting. A logic gate (18) and a latch (17) switch on the current through the heating element (3) in response to timed pulses from a timing circuit (15) and switch off the current through the heating element (3) when the heating element (3) is conducting and too hot.