| 摘要 |
An electrical system which comprises (1) a temperature-sensitive device comprising a PTC or NTC element, (2) a load in series with the temperature-sensitive device and (3) a control device in parallel with the load and the temperature-sensitive device. The control device is a device whose resistance can be changed from a high value (which may be infinite) to a low value or vice versa, by a change in the electrical or physical environment thereof, e.g. a diode, triac, thyristor, PTC device or NTC device. The system is connected to a power supply so that, under normal conditions of operation, either (a) the temperature-sensitive device has a low resistance and the control device has a high resistance, whereby the load is activated, or (b) the temperature-sensitive device has a high resistance and the control device has a low resistance, whereby the load is isolated. By an appropriate change in the power source, e.g. its size or direction, or in the physical environment of the control device, e.g. its temperature, the relative resistances of the temperature-sensitive device and the control device can be reversed, thereby isolating or activating the load. The system is particularly useful for isolating a remote load, e.g. a telephone, while retaining a continuous circuit e.g. through the legs of a telephone circuit, for example for test purposes. |
