| 摘要 |
PURPOSE:To enable a top plate to detect occurrence of electrostatic shield and crack by forming an rear side of the top plate a wide-sensing resistant film. CONSTITUTION:A negative characteristic heat-sensing film 5 is joined onto almost entire surface of rear side of a top plate 4 consisting of reinforcing ceramic and arranged in the vicinity of the top of a heating coil 3, and one end of this is connected to low potential side of a power source circuit 8 and the other side is connected to a contact 9 of a resistant R2 and a capacitor C2. In this mechanism, AC electrostatic induction voltage goes to low potential terminal and potential of the heat-sensing resistant film 5 is always kept at low potential level, and at the same time, if a crack should occur on the top plate 4 because of shock, etc., during heating operation, the heat-sensing resistant film 5 is disconnected, base current of a transistor Tr3 stops travelling, collector potential turns into low level reversing an output of a NAND gate 10 to high voltage level, stopping operation of a switching transistor Tr1 through conduction and non-conduction of the transistors Tr4 and Tr2, and therefore, the heating operation is stopped. |
