| 摘要 |
A heating conductor 14 for an electric blanket is wound concentrically with a sensing conductor 22 but separated from it by a temperature- sensitive insulating layer 28 to form a composite element 29. In the event of local or general overheat, the insulation of the layer 28 drops significantly and a signal is generated at terminal III of a 3- terminal device 42. Device 42 may be a transistor or a thyristor for example. The device responds to the signal by switching ON so as to provide a virtual short-circuit between its terminals I and II, thus causing open-circuiting of the overcurrent fuse 40. The device 42 is sufficiently sensitive to be switched ON by a short circuit occurring at virtually any position along the element 29, that is, even when the short-circuit occurs so close to one end as to produce a very low potential difference across the fault. In an alternative embodiment (fig. 6, not shown) a heating resistor in series with the thyristor trips the circuit by blowing a thermal fuse. <IMAGE> |
