| 摘要 |
PURPOSE:To prevent the electric spark when a metallic piece to be heated contacts with an energizing terminal by detecting that the energizing terminal is short-circuited by the metallic piece and driving a delay circuit for starting the energization. CONSTITUTION:An auxiliary DC power source 2 is operated by a switch 7 to short-circuit terminals 9, 10 and the metallic piece 11. Then slight current flows through a resistor 3, the metallic piece 11 and a diode 4 and a breakdown voltage is generated at both terminals of the diode 4. A transistor 5 is therefore operated and a delay type relay 6 is excited upon lapse of the prescribed time. A contact point 8 is closed by the excitation and a main DC power source 1 is turned on by which the large current is passed and the metallic piece 11 is heated instantaneously. Since a short-circuit detecting circuit for the energizing terminal and the delay circuit for starting the electrical heating are provided, the electric spark in the stage of starting the energization is prevented and the electric spark in the stage of reheating is prevented when the metallic piece and the terminal are detached during the energization.
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