| 摘要 |
<p>In a circuit arrangement for controlling the operating temperature of the heating coil of an electric soldering iron, the resistance variation of the heating coil itself is to be evaluated at lowest possible cost of circuitry and doing away with a thermocouple sensor that used to be arranged in the region of the soldering bit or of the heating coil, at high mechanical cost. To this end, a precision resistor is inserted in the heating circuit and a TRIAC switching element connected between the latter and the heating element, which is switched open by a comparator circuit as a function of the voltage drop at the precision resistor or which lets through only the pulses of a pulse generator circuit required for measuring the switch-on threshold, which pulse generator circuit is synchronized via a null detection circuit for null passage of the AC voltage. The voltage drop is further compared with a preset voltage and when it is reached an integration capacitor is charged, the voltage of which controls the comparator in proportion to a reference voltage. The TRIAC is switched on as long as the voltage at the output of the integration exceeds the reference voltage, and is switched off as long as this voltage is lower than the reference voltage. In the latter case the integration capacitor forms a time function element for the reheating of the heating coil.</p> |
