Abstract: A protection circuit (1) for input comprises two transistors (11, 12) connected in series between a first voltage supply (V.sub.cc) and a second voltage supply (GND), and an intermediate junction point is used as an input terminal and an output terminal. When a surge voltage is applied to the input terminal, since terminals (51, 53) of the two transistors (11, 12) are connected to predetermined junction points in such a way that the transistors can operate as bipolar transistors or cause punch through phenomenon (without causing breakdown operation of a low response speed to surge voltage), the surge voltage can be absorbed at high speed, thus increasing anti-ESD (electro static discharge) rate. Further, a protection circuit for power supply comprises two transistors (31, 32) connected in parallel to each other between a first voltage supply (V.sub.cc) and a second voltage supply (GND).

Abstract: A method and apparatus for measuring the oxygen activity of the slag in molten iron or molten steel with high accuracy. A conventional measuring method in which an oxygen concentration cell produced by winding a platinum contact electrode around the outer surface of a solid electrolyte of one end closed tube type is disposed directly in the molten slag involves a fear of producing an error in measurement due to the difficulty in positioning in the fluid slag and the defective contact with the contact electrode caused by the densification by the heat of the slag. In the present invention, the oxygen activity in the slag is not directly measured but a specific metal between which and the slag an oxygen equilibrium is established when it comes into contact with the slag is charged in a container together with the slag. The oxygen activity of the specific metal is measured so as to indirectly measure the oxygen activity in the slag.

Abstract: An oscillation circuit having a clocked inverter operating during a predetermined period of time after the start of the oscillation. A feedback circuit is formed by the clocked inverter and another inverter at the start time of the oscillation. The oscillation circuit is therefore driven by a large amount of current and thus the oscillation start time can be shortened and the oscillation start voltage can be lowered. On the other hand, the oscillation circuit is driven only by the other inverter after starting the oscillation. Since a constant current source is inserted serially in the path of the power source of the other inverter. A constant operating current always flows through the other inverter without being affected by the variations of the threshold voltage of the transistors and the variations of the power source voltages. As a result, a low consumption current characteristic of the oscillation circuit can be obtained under low power source voltage.